Peter M Waller

Interests

Research

In the last few decades, my primary research interests were design and modeling of algae growth in outdoor raceways, and irrigation and modeling of guayule. My current focus is on modeling irrigation, evapotranspiration and soil moisture in common crops in Arizona. I have integrated these models in the online WINDS (Water-use, Irrigation, Nitrogen, Drainage, and Salinity) model. We also use the model to evaluate irrigation experiments.

Teaching

I teach four courses: BE170A2 Formation of a Planetary Biosystem; BE423/523 Biosystems Analysis and Design; BE/CE 455/555 and BE/CE 456/556, Irrigation Systems Design.

Courses

Scholarly Contributions

Books

• Kim, O., Waller, P. M., & coauthors, 7. o. (2019). Regional Algal Feedstock Testbed (RAFT) Final Report. DOE-UAZ-0006269.
• Ogden, K. L., Anderson, D. B., Simpson, S., Voorheis, W. V., Brown, J. K., Brown, J. K., Huesemann, M. H., Kacira, M., Kacira, M., Skaggs, R., Skaggs, R. L., Waller, P., Waller, P. M., & Voorheis, W. V. (2019). Regional Algal Feedstock Testebed (RAFT) Final Report. Office of Scientific and Technical Information (OSTI). doi:10.2172/1492217
• Waller, P. M., & Yitayew, M. (2016). Irrigation and Drainage Engineering. http://www.springer.com/us/book/9783319056982: Springer. doi:10.1007/978-3-319-05699-9
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  This textbook focuses on the combined topics of irrigation and drainageengineering. It emphasizes both basic concepts and practical applications of the latest technologies available. The design of irrigation, pumping, and drainage systems using Excel and Visual Basic for Applications programs are explained for both graduate and undergraduate students and practicing engineers. The book emphasizes environmental protection, economics, and engineering design processes. It includes detailed chapters on irrigation economics, soils, reference evapotranspiration, crop evapotranspiration, pipe flow, pumps, open-channel flow, groundwater, center pivots, turf and landscape, drip, orchards, wheel lines, hand lines, surfaces, greenhouse hydroponics, soil water movement, drainage systems design, drainage and wetlands contaminant fate and transport. It contains summaries, homework problems, and color photos. The book drawsfrom the fields of fluid mechanics, soil physics, hydrology, soil chemistry, economics, and plant sciences to present a broad interdisciplinary view of the fundamental concepts in irrigation and drainage systems design.

Chapters

• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Agricultural Drip Irrigation. In Irrigation and Drainage Engineering. Springer, Cham. doi:10.1007/978-3-319-05699-9_17
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  Subsurface drip irrigation saves water, improves crop yields and quality, and facilitates fertilizer application; however, system performance is dependent upon skilled management. Potential disadvantages include salt accumulation near plants, restricted root development, high system costs, and restricted crop rotation. The three primary hydraulic classifications of drip emitters are laminar, turbulent, and pressure compensating. Analysis of Reynolds equation shows the advantage of turbulent flow emitters over laminar flow emitters. Pressure compensating emitters have the best hydraulic performance and generally rely on diaphragms that reduce flow at high pressure. As with sprinkler irrigation laterals, drip irrigation laterals are multi-outlet systems. The hydraulics can be calculated with analytic equations if slope is uniform, or with spreadsheets for any field. Most drip laterals in agriculture are classified as in-line, with emitters manufactured into the tubing. Proper filtration is a key to successful drip irrigation system performance. Sand filters are a necessary pretreatment step when the water source is a pond or stream. The design of lateral length and diameter is evaluated by the emission uniformity, which is a function of number of emitters per plant, minimum and average pressure in the lateral, the emitter exponent, and the manufacturer’s coefficient of variation. The most popular drip system is subsurface drip irrigation with dual feed laterals.
• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Agricultural Sprinkler Irrigation. In Irrigation and Drainage Engineering. Springer, Cham. doi:10.1007/978-3-319-05699-9_14
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  Agricultural sprinkler systems include wheel-lines, hand-lines, undertree sprinklers, and microsprinklers. The design process includes selecting an appropriate sprinkler and sprinkler spacing, calculating the number and dimensions of zones, designing the pipe network, and selecting the pump. Constraints such as orchard tree spacing and premanufactured aluminum pipe lengths often limit the spacing options. Sources of nonuniformity in agricultural sprinkler systems include variability of application rates within a sprinkler wetted area and hydraulic variation of lateral pressure. Although not normally part of the design process, this chapter shows how to describe the variability of wheel-line application depth based on pressure variation and sprinkler application pattern. The instructor may decide not to include these details. An economic/environmental model is presented that optimizes seasonal application depth with respect to yield, energy and water cost, and environmental contamination.
• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Center Pivot Irrigation Systems. In Irrigation and Drainage Engineering. Springer, Cham. doi:10.1007/978-3-319-05699-9_12
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  A center pivot irrigation system is a movable pipe structure that rotates around a central pivot point connected to a water supply. Center pivot irrigation systems are the most popular sprinkler irrigation systems in the world because of their high efficiency, high uniformity, ability to irrigate uneven terrain, and low capital, maintenance, and management costs. The history of center pivot irrigation systems began in Nebraska in the 1950s, and there are now hundreds of thousands of center pivot irrigation systems in the world. Center pivots are “perhaps the most significant mechanical innovation in agriculture since the replacement of draft animals by the tractor” (Splitter, Scientific American). The systems move through the field by electrically powered tractor wheels. Sprinkler flow rates increase toward the outer end of the pivot because the end of the pivot travels faster. The primary design constraint is the prevention of runoff at the end of the pivot, where application rates are highest. This chapter covers center pivot pipeline and mainline design, selection of sprinklers, and optimization of the design with respect to yield, energy requirement, components, and economics.
• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Drip Irrigation System Design. In Irrigation and Drainage Engineering. Springer, Cham. doi:10.1007/978-3-319-05699-9_18
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  This chapter focuses on the design of a dual feed subsurface drip irrigation system, which is the most common agricultural drip system. The first example demonstrates the procedure for design and economic comparison of alternative mainline and submain designs for subsurface drip irrigation. Uniformity is calculated for an entire irrigation zone rather than an individual lateral. Submains are designed with lateral flow rate vs. pressure curves just as laterals are designed with emitter flow rate vs. pressure curves. The economic analysis includes water, energy, and pipe costs. One of the major advantages of a dual feed system is the automated flushing process; however, if the system will not maintain an adequate flow velocity in the laterals, then it is impossible to flush debris out of the laterals. The economic evaluation evaluates the sum of pipe and present value energy costs and selects the lowest cost alternative. The next examples focus on the economic comparison of several agricultural in-line drip products. The economic analysis includes frequency of replacement, cost, and degradation over time. Examples include the irrigation of a watermelon crop (high value with no yield reduction from overirrigation) and a cotton crop (lower value with yield penalty from overirrigation)
• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Irrigation Lateral Design. In Irrigation and Drainage Engineering. Springer International Publishing. doi:10.1007/978-3-319-05699-9_7
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  Water is conveyed to drip emitters and sprinklers through multioutlet pipelines called laterals. This chapter begins with a basic introduction to hydraulics and statistics. These introductory materials may be unnecessary for many students. Then these principles are used to simulate the distribution of water applied by an irrigation lateral. Hydraulics is used to determine the change in pressure and resultant application variability along the lateral. Normally, the design objective is high application uniformity, which is accomplished by keeping pressure variation along the lateral within an acceptable range. Using larger pipe reduces pressure loss due to friction, but it also increases capital cost. Irrigation system application uniformity is not only a function of hydraulics. Other causes of nonuniformity include emitter manufacturing variation and application rate nonuniformity. The manufacturer’s coefficient of variation or expected field application variability is used to add a statistical distribution to the hydraulic application rate distribution along the lateral. Monte Carlo analysis utilizes the random number generator in Excel to evaluate the effect of the coefficient of variation on application distribution and profit.
• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Landscape Irrigation Design and Management. In Irrigation and Drainage Engineering. Springer. doi:10.1007/978-3-319-05699-9_16
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  Landscape irrigation design begins with an owner interview, site survey, and determination of the municipal water pressure and flow rate. This chapter includes a landscape drip and bubbler irrigation design program. The procedure begins with the measurement of the canopy diameter and crop coefficient for each plant in the landscape. This is followed by a calculation of the water requirement of each plant. Based on the water requirement, the program calculates the number of emitters required by each plant. Based on the reference ET, the program recommends and irrigation schedule. The procedure is similar to the calculation of an agricultural schedule except that density of planting, variable sizes of plants, and exposure to heat sources such as roads and buildings are also considered. The program also estimates the percent wasted water due to nonuniformity of application. This chapter also includes a design program for the pipe connection between the city water main and the valve box.
• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Landscape Irrigation Systems. In Irrigation and Drainage Engineering. Springer International Publishing. doi:10.1007/978-3-319-05699-9_15
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  As with turf irrigation, the goal of landscape irrigation is aesthetic. Plants (trees, shrubs, groundcover, and flowers) can be irrigated to just survive or to thrive, to maintain plant biomass or to have vegetative growth. Research on landscape plant water requirements has been limited, and many systems are not adjusted to match seasonal changes or changes in plant canopy area. The largest water user in many cities is irrigation so improved irrigation management is critically important in water stressed regions. Although traditional landscape drip irrigation systems have proven to be unreliable, new systems are more reliable: multiport emitters mounted on PVC pipe, inline drip irrigation tubing, and bubbler irrigation systems. Another key to successful system performance is the proper design and installation of the control zone. The typical control zone has a ball valve, solenoid valve, filter, and pressure regulator installed in a valve box. This chapter focuses on the installation methods and components in landscape irrigation systems.
• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Low-Head Gravity Bubbler Irrigation System. In Irrigation and Drainage Engineering. Springer International Publishing. doi:10.1007/978-3-319-05699-9_22
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  Low-head gravity bubbler irrigation systems apply water to the soil as a small stream from a small diameter tubes known as delivery hoses. To avoid runoff, small basins around the plant are sometimes needed when the application rate from the delivery hoses exceed the infiltration rate of the soil. The system is particularly well-suited for irrigation of perennial tree crops such as orchards, olives, vines, and palm trees. Traditional irrigation systems, such as furrows, can be easily converted into bubbler systems. As the name implies the systems are based on gravity where the flowrate through the delivery tube is dependent on the elevation difference between the source and the outlet of the tube. For that reason, the systems can operate at a pressure head as low as 1 meter (3.3 ft) and do not require mechanical pumps. Because the emmision point of the delivery tubes are large compared to other microirrigation emitters, the system does not require an elaborate filtration system. Keller (1990, Modern irrigation in developing countries. In: Proceedings of the 14th international congress on irrigation and drainage. Rio de Janeiro, International Commission on Irrigation and Drainage, No 1-E, pp 113–138) ranked bubbler systems in the same low-risk category as surface irrigation systems, since both systems are based on gravity flow and they do not require mechanical pumps or filtration systems. The initial cost of installing bubbler systems is comparable to or lower than the cost of most other microirrigation systems that include pumps and filters. Operation and maintenance costs for bubbler systems are substantially less than for other systems due to lower energy requirements and no mechanical breakdowns.
• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Open Channel Flow. In Irrigation and Drainage Engineering. Springer International Publishing. doi:10.1007/978-3-319-05699-9_11
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  Open channels can range from small irrigation furrows to huge irrigation canals that are hundreds of kilometers long and supply billions of cubic meters per year for irrigation, industry, and domestic purposes. Agricultural canal categories include the irrigation district main, secondary and tertiary canals, laterals, on-farm irrigation ditches, and drainage channels. This chapter covers the structures and principles that are related to open channel delivery of water to agriculture: water diversion structures, conveyance efficiency, siphons, canal hydraulics (uniform flow, energy drop structures, and gradually varied flow), and flow measurement. The government has encouraged farmers to conserve water by lining irrigation ditches with concrete (Fig. 11.1); however, concrete channels can develop cracks and gaps that have excessive water loss. Economic analysis can determine whether lining a canal is worth the cost. Manning’s equation calculates the head loss along a canal based on slope, roughness, and channel geometry. Energy dissipation structures use supercritical flow and hydraulic jumps to dissipate energy. The Froude number determines the relationship between subcritical and supercritical flow. A finite difference solution calculates water depth changes along a canal with gradually varied flow.
• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Subsurface Drainage Design and Installation. In Irrigation and Drainage Engineering. Springer International Publishing. doi:10.1007/978-3-319-05699-9_30
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  Subsurface drainage systems remove excess groundwater below the ground surface. Perforated plastic drain tubes are placed between 1 and 2 m below the soil surface. The technique was originally called tile drainage because tile cylinders were laid end to end in a trench. Spacing and depth of drain tubes as well as lateral hydraulic conductivity of soil layers determine the rate of water removal from the field. Lateral hydraulic conductivity is generally measured with auger hole tests. Drainage can impact downstream water quality because it changes the timing of water and chemical leaching through the soil. Drainage planning and design require extensive analysis of hydrology, soil structure and texture, soil chemistry, crop rotations, field equipment, topography, waterways, and construction materials. The sizing of drainage pipes is based on the land slope and expected flow rate to pipes. Pipe layout and slope is a function of land slope and discharge location in the field. If the discharge waterway is higher than the drain outlet, then a sump and pump must be installed. Gravel envelopes or fabrics protect drain tubes from sedimentation.
• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Subsurface Drainage Modeling. In Irrigation and Drainage Engineering. Springer International Publishing. doi:10.1007/978-3-319-05699-9_31
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  The original subsurface drainage model is the Hooghoudt equation, which is a one-dimensional steady-state simplification of the two-dimensional transient flow to parallel drains. It calculates the midpoint water table elevation between drains. Bower and van Schilfegaarde modified the Hooghoudt equation for transient analysis. The Bureau of Reclamation also developed drainage equations for transient analysis of midpoint water table elevation. Kirkham developed a Laplace analytic solution for the two-dimensional subsurface drainage geometry. He also adapted this solution for transient analysis with the concept of fixed streamtubes along the path of water flow. The advantage of this approach is that water table height can be simulated as a function of distance from the drain rather than just the midpoint water table elevation. The Kirkham streamtube approach is used in the WINDS drainage model, which enables WINDS to model water, salinity, and nitrogen in the soil profile as a function of distance from the drain. The chapter also includes an example of the economic analysis of drain spacing and depth.
• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Turf Sprinkler Irrigation Systems. In Irrigation and Drainage Engineering. Springer International Publishing. doi:10.1007/978-3-319-05699-9_13
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  The green industry, which includes turf and landscape irrigation, has an economic impact in the United States of $150 billion/year. Effective turf irrigation systems keep turf areas healthy and attractive and use water efficiently, which are the primary goals of turf irrigation. This chapter begins with a review of sprinkler system layout and spacing. The Sprinkler Uniformity program allows the user to visualize and quantify the effect of spacing on uniformity. This is followed by an economic rationale for sprinkler spacing. The last section covers sprinkler pipe network design and scheduling calculations. There is a range of turf sprinkler types. Fixed spray sprinklers cover small areas ( 5 m radius) and have a lower application rate (
• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). WINDS Agricultural Simulations. In Irrigation and Drainage Engineering. Springer International Publishing. doi:10.1007/978-3-319-05699-9_29
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  This chapter demonstrates the use of WINDS for modeling crops, ET, and water content in agriculture. The chapter describes the WINDS algorithms for infiltration, evapotranspiration, crop stress, dry and wet zones in fields, and remote sensing. Infiltration can be specified as a fixed depth or calculated with the Green-Ampt infiltration model inside the WINDS model. Evapotranspiration is calculated with the dual component evapotranspiration model. Yield reduction is based on water stress and the sensitivity to water stress during the growing season. Fields with drip irrigation or alternative furrow irrigation can be modeled as partially wetted fields, which can provide a better estimate of available water capacity in some cases. Two case studies are presented. The WINDS model is used to schedule irrigations in a cotton remote sensing experiment in Maricopa, Arizona, and it is used to evaluate farming practices for a rainfed farm in a tropical region. Finally, this chapter reviews the stress algorithms for salinity and nitrogen in the WINDS model.
• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). WINDS Salinity and Nitrogen Algorithms. In Irrigation and Drainage Engineering. Springer International Publishing. doi:10.1007/978-3-319-05699-9_25
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  This chapter introduces the WINDS (Water-use, Irrigation, Nitrogen, Drainage, and Salinity) model, which simulates water, salts, and nitrate in agricultural fields. The WINDS model uses daily time steps and allows for up to 13 soil layers and multiple field positions in simulations. The focus of this chapter is the nitrogen and salinity models in WINDS. The input salinity parameters include initial concentration in each soil layer, date of manure application, manure salinity, crop salinity equation threshold and slope, and irrigation water salinity. Unlike the salinity model, the nitrate model includes reaction terms: mineralization, denitrification, and plant uptake. Because nitrate is the primary form of nitrogen used by plants, the mass balance for nitrogen focuses on the sources and sinks for nitrate. The rate of microbial activity (mineralization and denitrification) is strongly dependent on temperature; thus, this chapter presents algorithms for calculating annual and diurnal temperature variation in soils. The input nitrate parameters include initial concentration in layers (mg/kg), organic matter distribution with depth, fertilization application dates and depths, and irrigation water nitrate concentration. Reaction terms include mineralization rate constants, Michaelis-Menton uptake coefficients, seasonal nitrate requirement, optimal soil nitrate concentration, fraction of plant nitrogen uptake as nitrate, nitrogen dissolution rate, and nitrogen stress factors.
• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Waste Treatment in Wetlands and Agriculture. In Irrigation and Drainage Engineering. Springer International Publishing. doi:10.1007/978-3-319-05699-9_24
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  Municipal and animal waste can be treated in ponds, wetlands, and agricultural fields. The sizing of ponds, wetlands, and field application areas is based on waste characteristics and volume. Wetland and pond areas are calculated based on standard treatment times, such as 5 day hydraulic retention time, or based on decay rate equations. The decay rate constant is primarily a function of temperature. Application of animal waste to soils by solids spreading (vehicle), liquid spreading (vehicle) or through a sprinkler system requires a series of calculations: the nutrient content in the wastewater, nutrient needs of the crop, the degradation and volatilization of nutrients in storage, mineralization, denitrification, and plant uptake in soils. The rate of application by sprinklers is dependent on percent solids in the waste and soil texture. This chapter includes an NRCS example calculation of dairy waste application to cropped fields. The soil salinization hazard from animal waste is calculated from the manure salinity load. Finally, an example shows how to calculate the blended ratio of wastewater to fresh water when wastewater concentration of nitrogen exceeds the crop nitrogen requirements.
• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Wastewater Contaminants and Treatment. In Irrigation and Drainage Engineering. Springer, Cham. doi:10.1007/978-3-319-05699-9_23
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  With the utilization of irrigation systems for wastewater treatment and disposal, engineers should be aware of common contaminants, methods of on-farm wastewater treatment, and methods of risk evaluation. The primary concerns with wastewater treatment and reuse are prevention of disease and prevention of nutrient contamination and eutrophication of surface and subsurface water resources. Regulations and treatment processes have been set up as a barrier between contaminants and people. In addition, best management practices have been established for optimal and safe utilization of waste. Epidemiological studies and quantitative risk assessment methods can help to establish the risk to human health of wastewater treatment, containment, and reuse practices. Based on risk assessment, regulators establish rules for wastewater treatment and disposal. The last wall of defense against disease and death is the immune system.
• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Water and Energy Relationships in Soils. In Irrigation and Drainage Engineering. Springer International Publishing. doi:10.1007/978-3-319-05699-9_27
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  This chapter begins with the derivation of the conservation of energy equation. Water is driven through the soil by energy gradients while friction between water molecules in small pores dissipates energy and restricts flow. Hydraulic conductivity is the ratio of water velocity to energy dissipation, and Darcy’s Law calculates velocity of water as the product of hydraulic conductivity and the energy gradient. Soil energy diagrams include gravitational potential energy and matric potential and can help to determine energy gradients. The chapter shows that vertical movement through the soil profile, horizontal flow through subsurface wetlands, or flow in any direction through any media, is calculated with the same energy gradient principle. Hydraulic conductivity decreases in unsaturated soils. Equations that describe the relationships between unsaturated hydraulic conductivity, matric potential, and water content have been developed by Brooks-Corey and by van Genuchten.
• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Water and Salinity Stress. In Irrigation and Drainage Engineering. Springer, Cham. doi:10.1007/978-3-319-05699-9_4
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  Continually applying salt-laden irrigation water to soils can lead to soil salinization because plants leave most of the salts behind as they uptake water. Osmotic potential energy in saline soils is negative and resists the movement of water toward plant roots. According to the FAO, approximately 3 ha per minute are lost to soil salinization in the world, and 80 million ha have already been lost to soil salinization. Salinity management practices such as leaching water below the root zone are needed to prevent salt accumulation in the root zone. Leaching takes place when irrigation is increased beyond the evapotranspiration requirement. On the other hand, excess water in the soil restricts the movement of oxygen into the soil. Plants require oxygen for the roots as the conduct respiration at night. Insufficient water reduces crop evapotranspiration, and there is generally a linear relationship between percent depletion beyond the management allowed depletion and yield reduction. Crop yield reduction due to water or salt stresses can be quantified by plant stress coefficients, which reflect the plant sensitivity to stress. Water and salinity stress coefficients as well as methods to measure and control salinity are described in this chapter.
• Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Water and Solute Mass Balance Models. In Irrigation and Drainage Engineering. Springer, Cham. doi:10.1007/978-3-319-05699-9_26
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  This chapter describes the WINDS tipping bucket approach to water, salt, and nitrate transport in soils. The tipping bucket model is based on the law of conservation of mass (Fig. 26.1). The model runs quickly because rapid changes due to irrigation or storm events are simulated with conservation of mass, which allows for daily time steps. The algorithms march forward in time with Euler’s finite difference method. Because the model is fast, it can simulate daily changes in water, salinity, and nitrogen at hundreds of locations within fields over a growing season.
• Yitayew, M., Yitayew, M., Waller, P. M., Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Hydroponic Irrigation Systems. In Irrigation and Drainage Engineering. Springer, Cham. doi:10.1007/978-3-319-05699-9_21
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  Hydroponic irrigation systems in high-tech commercial greenhouses utilize drip irrigation with soilless growing media such as sand, rock wool or coco-coir. Hydroponic drip systems use online barbed drip emitters and deliver water through distribution tubing to a stake in the media. With their closely spaced barbed fittings, the friction loss calculation for hydroponic drip laterals must include the losses due to barbed fittings. The irrigation schedule is generally triggered by a light sensor placed above the greenhouse. The duration of watering cycle, a few minutes, is primarily due to the need to regularly add oxygen to the growing media. All nutrients, including micronutrients, must be supplied by fertigation. The calculations are generally made on a concentration basis, and proportional injectors add fertilizer solution to the irrigation water. A sequential calculation method for the five macronutrients and fertilizer amounts is presented. The chapter also includes a VBA/Excel program that solves for fertilizer amounts for both macronutrients and micronutrients.
• Yitayew, M., Yitayew, M., Waller, P. M., Yitayew, M., Yitayew, M., & Waller, P. M. (2016). Modeling Soil Moisture. In Irrigation and Drainage Engineering. Springer International Publishing. doi:10.1007/978-3-319-05699-9_28
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  The conservation of energy and conservation of mass equations are combined in Richards equation, which is a model of water content and soil water flux in response to energy gradients. The WINDS model is used to demonstrate the use of the Richards equation and the effect of different van Genuchten soil parameters. Special algorithms are required with a water table. Determining the rate of water table movement in response to water gain or loss requires calculation of specific yield. The WINDS model utilizes integrated forms of the Brooks-Corey and van Genuchten equations to calculate water volume in the soil profile above a water table and to calculate the specific yield of the water table. This approach requires that some soil layers in the model are in hydraulic equilibrium with the water table while flow between upper layers is calculated based on energy differences and hydraulic conductivity. Lastly, this chapter discusses the upward movement of water from the water table to the root zone and compares Anat’s equation to a discretized solution.
• Waller, P. M. (2015). Turf and Landscape Irrigation. In Irrigation and Drainage Engineering(pp 337-361). John Wiley & Sons, Ltd. doi:10.2134/AGRONMONOGR30.2ED.C10

Journals/Publications

• Wang, Z., Nie, T., Lu, D., Zhang, P., Li, J., Li, F., Zhang, Z., Chen, P., Jiang, L., Dai, C., & Waller, P. (2024). Effects of Different Irrigation Management and Nitrogen Rate on Sorghum (Sorghum bicolor L.) Growth, Yield and Soil Nitrogen Accumulation with Drip Irrigation. Agronomy, 14(1). doi:10.3390/agronomy14010215
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  Sorghum (Sorghum bicolor L.) has emerged as a pivotal global food crop. Consequently, it is imperative to explore sustainable and eco-friendly strategies to achieve sustainable sorghum production with a high yield. This study aimed to reveal the effects of irrigation management and nitrogen rates and their interactions on sorghum growth traits, yield and soil nitrate-N and ammonium-N accumulation to improve irrigation and nitrogen practices under drip irrigation. A 2-year (2021 and 2022) field experiment was conducted on drip-irrigated fertilized sorghum in Heilongjiang Province to investigate the effects of three lower levels of soil moisture (80% (HI), 70% (NI), and 60% (LI) of field capacity) with four nitrogen rates at 225, 150, 75 and 0 kg/ha (designated as HN, NN, LN and WN, respectively) on sorghum growth, yield and soil nitrogen accumulation. The results indicated that irrigation management and nitrogen rate interaction had a significant effect on sorghum growth (plant height, stem diameter, leaf area index (LAI), and SPAD value), yield, aboveground biomass and 0~60 cm soil nitrogen accumulation (p < 0.05). The NNHI treatment demonstrated the highest plant height (120.9 and 121.8 cm) and LAI (2.738 and 2.645) in 2021 and 2022, and there was a significant positive correlation between plant height, LAI, and yield (p < 0.01). However, the NNNI treatment exhibited the highest yield (7477.41 and 7362.27 kg/ha) in 2021 and 2022, sorghum yield increased and then decreased with an increase in irrigation management and nitrogen rate. In addition, soil nitrate-N and ammonium-N accumulation were significantly affected by the interaction of irrigation management and nitrogen rate (p < 0.05) while irrigation management had no significant effect on the accumulation of nitrate-N and ammonium-N. Soil nitrate-N and ammonium-N accumulation increased with the increasing nitrogen rate. Although yield differences between the NNNI and HNNI treatments were not significant, the NNNI treatment with a lower soil moisture limit of 70% field capacity and a nitrogen rate of 150 kg/ha accumulated 10.4% less nitrate-N in soil than the HNNI treatment, reduced risk of nitrate nitrogen leaching. The regression analysis indicated that the optimal irrigation management and nitrogen rate management practices of 71.93% of the soil moisture lower limit and 144.58 kg/ha of nitrogen rate was an optimal strategy for favorable sorghum growth, high-yielding and low soil nitrate-N accumulation of sorghum. This study provides a scientific reference for precise water and fertilizer management in sorghum.
• Elshikha, D., Wang, G., Waller, P., Hunsaker, D., Dierig, D., Thorp, K., Thompson, A., Katterman, M., Herritt, M., Bautista, E., Ray, D., & Wall, G. (2023). Guayule growth and yield responses to deficit irrigation strategies in the U.S. desert. Agricultural Water Management, 277. doi:10.1016/j.agwat.2022.108093
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  Deficit irrigation can maximize the water productivity (WP) of guayule and increase the percent rubber (%R) in shrubs compared to irrigation meeting full crop evapotranspiration (ETc). In this study, we hypothesize that certain deficit irrigation strategies that impose soil water deficits during specific periods of growth or throughout the growing season might produce higher %R and equivalent rubber yield (RY), thereby, increasing WP compared to full irrigation. Herein, growth and yield responses of direct-seeded guayule to different water deficit schemes were evaluated in an experiment on a silty clay loam soil, in a field in central Arizona using furrow irrigation. Two guayule cultivars (AZ2 and AZ6) were grown for 22.5 months (Apr. 2020-Mar. 2022) in a split-plot design, with six irrigation treatments in whole plots and cultivars in split-plots. After homogeneous irrigation for two months, irrigation treatments were begun. A control treatment was irrigated to meet full ETc. The other five treatments were irrigated with less water using various deficit irrigation strategies imposed during the two-year growing period. Measurements included plant height (h), cover fraction (fc), soil water contents, harvest of dry biomass (DB), RY, resin yield (ReY), %R, and percent resin (%Re). Total water applied (TWA) by irrigation and precipitation to treatments varied from 2780 to 1084 mm and DB varied from 20.5 to 9.1 Mg ha−1. The h and fc were significantly greater at higher irrigation levels, while they were also significantly greater in AZ6 than AZ2. The DB, RY, and ReY generally increased linearly with TWA. However, it was found that a treatment applying every other irrigation of the control resulted in statistically equivalent yields to the control, with 36% less irrigation. The %R generally decreased with TWA, while %Re did not change. However, DB, %R, and %Re were significantly greater for AZ2 than AZ6, as were RY, ReY, and WP. Among the deficit treatments evaluated, every other irrigation offers the best strategy to significantly increase guayule WP without causing a yield penalty.
• Katterman, M. E., Waller, P. M., Elshikha, D. E., Wall, G. W., Hunsaker, D. J., Loeffler, R. S., & Ogden, K. L. (2023). WINDS model simulation of guayule irrigation. Water Journal, 15(19), 15.
• Katterman, M. E., Waller, P. M., Elshikha, D. E., Wall, G. W., Hunsaker, D. J., Loeffler, R. S., Ogden, K. L., Elshikha, D. E., Waller, P. M., Hunsaker, D. J., Thorp, K. R., Wang, G., Dierig, D., Cruz, V. V., Attalah, S., Katterman, M. E., Williams, C., Ray, D. T., Norton, E. R., , Orr, E. R., et al. (2023).

  Water Use, Growth, and Yield of Ratooned Guayule under Subsurface Drip and Furrow Irrigation in the US Southwest Desert

  . Water Journal, 15(19), 15.
• Maqsood, H., Hunsaker, D. J., Waller, P. M., Thorp, K. R., French, A., Elshikha, D. E., & Loeffler, R. S. (2023). WINDS model demonstration with field data from a furrow-irrigated cotton experiment. Water Journal, 15(8), 15.
• Elshikha, D., Hunsaker, D., Waller, P., Thorp, K., Dierig, D., Wang, G., Cruz, V., Katterman, M., Bronson, K., Wall, G., & Thompson, A. (2022). Estimation of direct-seeded guayule cover, crop coefficient, and yield using UAS-based multispectral and RGB data. Agricultural Water Management, 265. doi:10.1016/j.agwat.2022.107540
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  Guayule (Parthenium argentatum, A. Gray), a perennial desert shrub, produces high-quality natural rubber and is targeted as a domestic natural rubber source in the U.S. While commercialization efforts for guayule are on-going, crop management requires plant growth monitoring, irrigation requirement assessment, and final yield estimation. Such assistance for guayule management could be provided with remote sensing (RS) data. In this study, field and RS data, collected via drones, from a 2-year guayule irrigation experiment conducted at Maricopa, Arizona were evaluated. In-season field measurements included fractional canopy cover (fc), basal (Kcb) and single (Kc) crop coefficients, and final yields of dry biomass (DB), rubber (RY), and resin (ReY). The objectives of this paper were to compare vegetations indices from MS data (NDVI) and RGB data (triangular greenness index, TGI); and derive linear prediction models for estimating fc, Kcb, Kc, and yield as functions of the MS and RGB indices. The NDVI and TGI showed similar seasonal trends and were correlated at a coefficient of determination (r2) of 0.52 and a root mean square error (RMSE) of 0.11. The prediction of measured fc as a linear function of NDVI (r2 = 0.90) was better than by TGI (r2 = 0.50). In contrast to TGI, the measured fc was highly correlated with estimated fc based on RGB image evaluation (r2 = 0.96). Linear models of Kcb and Kc, developed over the two years of guayule growth, had similar r2 values vs NDVI (r2 = 0.46 and 0.41, respectively) and vs TGI (r2 = 0.48 and 0.40, respectively). Final DB, RY, and ReY were predicted by both NDVI (r2 = 0.75, 0.53, and 0.70, respectively) and TGI (r2 = 0.72, 0.48, and 0.65, respectively). The RS-based models enable estimation of irrigation requirements and yields in guayule production fields in the U.S.
• Wang, G., Elshikha, D., Katterman, M., Sullivan, T., Dittmar, S., Cruz, V., Hunsaker, D., Waller, P., Ray, D., & Dierig, D. (2022). Irrigation effects on seasonal growth and rubber production of direct-seeded guayule. Industrial Crops and Products, 177. doi:10.1016/j.indcrop.2021.114442
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  Understanding guayule's response to environmental factors, such as location, soil type, drought stress, and seasonal growth variation is critical for irrigation management to maximize and estimate rubber and resin accumulation throughout the growing seasons. A study was conducted at two sites with different soil types (sandy loam soil at Maricopa, AZ and a clay soil at Eloy, AZ) to compare plant growth and rubber accumulation among different irrigation treatments during a two-year growing season. The above- and below-ground biomass, biomass growth, rubber/resin content, and rubber/resin accumulation were measured every other month from establishment to final harvest in well-watered treatments, which received 100% replacement of crop evapotranspiration (ETc) and irrigated with subsurface drip and furrow (denoted as D100 and F100, respectively), and compared to reduced irrigation treatments (D50 and F50), which received 50% replacement of ETc. Drip irrigation with high water input (D100) decreased root mass partition, but leaf, stem, and flower partitions were not significantly affected by irrigation treatment. Biomass yield was higher in the well-watered treatments as expected, while rubber and resin content were lower, indicating rubber and resin dilution by higher biomass. For all treatments, rubber and resin yield increased linearly over the two-year growing season. However, the rates of increase were different among the irrigation treatments. The D100 treatment had a higher rubber yield increase rate compared to F100 and D50 in sandy loam soil at Maricopa, while the D100 treatment had the lowest increase rate compared to the F100, F50, and D50 treatments in clay soil at Eloy. Top branches of guayule plants in the D100 treatment at Eloy lodged in the second year and likely contributed to lower rubber content and rubber yield in the treatment. The drip irrigation treatments D50 and D100 had higher water productivity for rubber yield at Maricopa. However, the D50 and F50 treatment had the highest water productivity for guayule rubber yield, while the D100 treatment had the lowest due to lower rubber content at Eloy. Root rubber content was 31–39% lower than stem at the two locations. This study indicates that rubber biosynthesis occurred in guayule year-round and that it is possible in clay soils to reduce irrigation without a significant loss in rubber yield.
• Elshikha, D. M., Waller, P. M., Hunsaker, D. J., Dierig, D., Wang, G., Cruz, V. V., Thorp, K. R., Katterman, M. E., Bronson, K. F., & Wall, G. W. (2021). Growth, water use, and crop coefficients of direct-seeded guayule with furrow and subsurface drip irrigation in Arizona. Industrial Crops and Products, 170. doi:https://doi.org/10.1016/j.indcrop.2021.113819
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  Direct-seeded guayule had the highest yield when irrigated with 75–100 % of crop ET.Sandy loam soil gave 20–30 % higher rubber/resin yield than clay with 25 % more water.Furrow irrigation is favored for direct-seeded guayule planted in clay soil.Subsurface drip is preferred for direct-seeded guayule planted in sandy loam soil.
• Gutierrez-Jaramillo, A., Davidowitz, G., Waller, P. M., & Pryor, B. M. (2021). Bioregenerative Food Production System: Using integrated food production systems to feed the future. International Conference on Environmental Systems ICES, 2021-169.
• Steichen, S., Gao, S., Waller, P. M., & Brown, J. K. (2020). Association between algal productivity and phycosphere composition in an outdoor Chlorella sorokiniana reactor based on multiple longitudinal analyses.. Microbial biotechnology, 13(5), 15.
• Wang, G., Elshikha, D. E., Katterman, M., Sullivan, T., Dittmar, S., Cruz, V. M., Hunsaker, D., Waller, P. M., Ray, D., & Dierig, D. (2021). Irrigation effects on seasonal growth and rubber production of direct-seeded guayule.. Industrial Crops and Products, 177. doi:https://doi.org/10.1016/j.indcrop.2021.114442
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  Reduced irrigation decreased guayule biomass yield, but increased rubber content.Rubber yield increased linearly, indicating rubber was biosynthesized year-round.Rubber content in guayule root was 31–39% lower than in stem.
• Brown, J. K., Waller, P., Steichen, S. A., & Gao, S. (2020). Association between algal productivity and phycosphere composition in an outdoor Chlorella sorokiniana reactor based on multiple longitudinal analyses. Microbial Biotechnology, 13(5), 1546-1561. doi:10.1111/1751-7915.13591
• Waller, P. M., Brown, J. K., Steichen, S., & Gao, S. (2020). Association between algal productivity and phycosphere composition in an outdoor Chlorella sorokiniana reactor based on multiple longitudinal analyses.. Microbial biotechnology, 13(5), 1546-1561. doi:10.1111/1751-7915.13591
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  Microalgae as a biofuel source are of great interest. Bacterial phycosphere inhabitants of algal cultures are hypothesized to contribute to productivity. In this study, the bacterial composition of the Chlorella sorokiniana phycosphere was determined over several production cycles in different growing seasons by 16S rRNA gene sequencing and identification. The diversity of the phycosphere increased with time during each individual reactor run, based on Faith’s phylogenetic diversity metric versus days post‐inoculation (R = 0.66, P < 0.001). During summer months, Vampirovibrio chlorellavorus, an obligate predatory bacterium, was prevalent. Bacterial sequences assigned to the Rhizobiales, Betaproteobacteriales and Chitinophagales were positively associated with algal biomass productivity. Applications of the general biocide, benzalkonium chloride, to a subset of experiments intended to abate V. chlorellavorus appeared to temporarily suppress phycosphere bacterial growth, however, there was no relationship between those bacterial taxa suppressed by benzalkonium chloride and their association with algal productivity, based on multinomial model correlations. Algal health was approximated using a model‐based metric, or the ‘Health Index’ that indicated a robust, positive relationship between C. sorokiniana fitness and presence of members belonging to the Burholderiaceae and Allorhizobium–Neorhizobium–Pararhizobium–Rhizobium clade. Bacterial community composition was linked to the efficiency of microalgal biomass production and algal health.
• Waller, P., Steichen, S., Gao, S., & Brown, J. (2020). Association between algal productivity and phycosphere composition in an outdoor Chlorella sorokiniana reactor based on multiple longitudinal analyses. Microbial Biotechnology, 13(5), 1546-1561. doi:10.21203/rs.2.21781/v1
• Attalah, S., Waller, P., Steichen, S., Brown, C. C., Mehdipour, Y., Ogden, K., & Brown, J. K. (2019). Cost minimization of deoxygenation for control of Vampirovibrio chlorellavorus in Chlorella sorokiniana cultures. ALGAL RESEARCH-BIOMASS BIOFUELS AND BIOPRODUCTS, 42.
• Attalah, S., Waller, P., Steichen, S., Gao, S., Brown, C. C., Ogden, K., & Brown, J. K. (2019). Application of deoxygenation-aeration cycling to control the predatory bacterium Vampirovibrio chlorellavorus in Chlorella sorokiniana cultures. ALGAL RESEARCH-BIOMASS BIOFUELS AND BIOPRODUCTS, 39.
• Brown, J. K., Ogden, K. L., Gao, S., Brown, C., Steichen, S., Waller, P. M., & Attalah, S. (2019). Application of deoxygenation-aeration cycling to control the predatory bacterium Vampirovibrio chlorellavorus in Chlorella sorokiniana cultures. Algal Research, 39. doi:https://doi.org/10.1016/j.algal.2019.101427
• Brown, J. K., Ogden, K. L., Gao, S., Brown, C., Steichen, S., Waller, P. M., & Attalah, S. (2019). Cost minimization of deoxygenation for control of Vampirovibrio chlorellavorus in Chlorella sorokiniana cultures. Algal Research, 42, 1061. doi:https://doi.org/10.1016/j.algal.2019.101615/
• Khawam, G., Waller, P., Gao, S., Edmundson, S., Huesemann, M., Attalah, S., & Ogden, K. L. (2019). Simulation of shading and algal growth in experimental raceways. ALGAL RESEARCH-BIOMASS BIOFUELS AND BIOPRODUCTS, 41.
• Khawam, G., Waller, P., Gao, S., Edmundson, S., Wigmosta, M. S., & Ogden, K. (2019). Model of temperature, evaporation, and productivity in elevated experimental algae raceways and comparison with commercial raceways. ALGAL RESEARCH-BIOMASS BIOFUELS AND BIOPRODUCTS, 39.
• Ogden, K. L., Brown, J. K., Gao, S., Steichen, S., Waller, P. M., & Attalah, S. (2019). Deoxygenation-aeration cycling-driven management of a predatory bacterium Vampirovibrio chlorellavorus in Chlorella sorokiniana algae culture.. Algal Research, 39. doi:https://doi.org/10.1016/j.algal.2019.101427
• Gao, S., Waller, P., Khawam, G., Attalah, S., Huesemann, M., & Ogden, K. (2018). Incorporation of salinity, nitrogen, and shading stress factors into the Huesemann Algae Biomass Growth model. ALGAL RESEARCH-BIOMASS BIOFUELS AND BIOPRODUCTS, 35, 462-470.
• Ogden, K. L., Huesemann, M., Attalah, S., Khawam, G., Waller, P. M., & Gao, S. (2018). Incorporation of salinity stress, nitrogen stress, and shading into the HABG algae growth model.. Algal Research, 35, 462-470. doi:https://doi.org/10.1016/j.algal.2018.09.021.
• 10 other co authors, ., Olivares, J., Waller, P. M., Ogden, K. L., & Lammers, P. (2017). Review of the cultivation program within the National Alliance for Advanced Biofuels and Bioproducts. Algal Research. Algal Research, 22, 166-186. doi:http://dx.doi.org/10.1016/j.algal.2016.11.021
• Huesemann, M. H., Wigmosta, M. S., Crowe, B. J., Waller, P. M., Chavis, A., Chavis, A. R., Hobbs, S. J., Edmundson, S. J., Chubukov, B., Tocco, V. J., & Coleman, A. M. (2017). Estimating the maximum achievable productivity in outdoor ponds: Microalgae biomass growth modeling and climate-simulated culturing. Microalgal Production, 113-137. doi:10.1201/B19464
• Waller, P. M. (2016). Review of the cultivation program within the National Alliance for Advanced Biofuels and Bioproducts. Algal Research, 1-21. doi:10.1016/j.algal.2016.11.021
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  The was a review of the large NAABB program, 6/26 figures were based on models that I contributed to or the ARID raceway.
• Waller, P. M., Huesemann, M., Crowe, B., Chavis, A., Hobbs, S., Scott, S., & Wigmosta, M. (2016). A validated model to predict microalgae growth in outdoor pond cultures subjected to fluctuating light intensities and water temperatures.. Algal Research, 13, 195-206.
• Attalah, S., Waller, P. M., Khawam, G., Ryan, R., & Huesemann, M. (2015). Energy Productivity of the High Velocity Algae Raceway Integrated Design (ARID-HV).. Applied Engineering in Agriculture, 31(3), 365-375.
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  The original Algae Raceway Integrated Design (ARID) raceway was an effective method to increase algae culture temperature in open raceways. However, the energy input was high and flow mixing was poor. Thus, the High Velocity Algae Raceway Integrated Design (ARID-HV) raceway was developed to reduce energy input requirements and improve flow mixing in a serpentine flow path. A prototype ARID-HV system was installed in Tucson, Arizona. Based on algae growth simulation and hydraulic analysis, an optimal ARID-HV raceway was designed, and the electrical energy input requirement (kWh ha-1 d-1) was calculated. An algae growth model was used to compare the productivity of ARID-HV and conventional raceways. The model uses a pond surface energy balance to calculate water temperature as a function of environmental parameters. Algae growth and biomass loss are calculated based on rate constants during day and night, respectively. A 10 year simulation of DOE strain 1412 (Chlorella sorokiniana) showed that the ARID-HV raceway had significantly higher production than a conventional raceway for all months of the year in Tucson, Arizona. It should be noted that this difference is species and climate specific and is not observed in other climates and with other algae species. The algae growth model results and electrical energy input evaluation were used to compare the energy productivity (algae production rate/energy input) of the ARID-HV and conventional raceways for Chlorella sorokiniana in Tucson, Arizona. The energy productivity of the ARID-HV raceway was significantly greater than the energy productivity of a conventional raceway for all months of the year.
• Huesemann, M., Crow, B., Waller, P. M., Chavis, A., Hobbs, S., Edmunson, S., & Wigmosta, M. (2016). A validated model to predict microalgae growth in outdoor pond cultures subjected to fluctuating light intensities and water temperatures.. Algal Research, 13, 195-206.
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  This paper is the foundational description of our work in algae modeling.
• Hunsaker, D., French, A., Waller, P. M., Bautista, E., Thorp, K., Bronson, K., & Andrade-Sanchez, P. (2015). Comparison of traditional and ET-based irrigation scheduling of surface-irrigated cotton in the arid southwestern USA. Agricultural Water Management, 159, 209-224. doi:doi:10.1016/j.agwat.2015.06.016
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  ET-based and traditional and irrigation scheduling methods were evaluated for cotton grown with surface irrigation. ET-based irrigation scheduling methods utilized remote sensing and other ancillary measurements, but these methods did not improve lint yields compared to traditional irrigation scheduling method that was not dependent on field data. Higher irrigation application of the traditional method increased measured ET but also increased lint yield while achieving higher irrigation water productivity than normally achieved in the region.
• Xu, B., Li, P., Waller, P. M., & Huesemann, M. (2015). Evaluation of flow mixing in an ARID-HV algal raceway using statistics of temporal and spatial distribution of fluid particles. Algal Research, 9, 27-39. doi:doi:10.1016/j.algal.2015.02.027
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  This paper analyzes and evaluates the flow mixing in an open channel algal raceway for biofuel production. The flow mixing governs the frequency of how algae cells are exposed to sunlight, due to the fluid movement between the surface and the bottom of the algal raceway, thereby affecting algal growth rate. In this work, we investigated the flow mixing performance in a table-sized model of the High Velocity Algae Raceway Integrated Design (ARID-HV). Various geometries of the raceway channels and dams were considered in both the CFD analysis and experimental flow visualization. In the CFD simulation, the pathlines of fluid particles were analyzed to obtain the distribution of the number of times that particles passed across a critical water depth, Dc, defined as a cycle count. In addition, the distribution of the time period fraction that the fluid particles stayed in the zones above and below Dc was recorded. Such information was used to evaluate the flow mixing in the raceway. The CFD evaluation of the flow mixing was validated using experimental flow visualization, which showed a good qualitative agreement with the numerical results. In conclusion, this CFD-based evaluation methodology is recommended for flow field optimization for open channel algal raceways, as well as for other engineering applications in which flow mixing is an important concern.
• Ben, X., Peiwen, L. i., Waller, P., Ben, X., Peiwen, L. i., & Waller, P. (2014). Study of the flow mixing in a novel ARID raceway for algae production. Renewable Energy, 62, 249-257.
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  Abstract: A novel flow field for algae raceways has been proposed, which is fundamentally different from traditional paddlewheel-driven raceways. To reduce freezing and heat loss in the raceway during cold time, the water is drained to a deep storage canal. The ground bed of the new raceway has a low slope so that water, lifted by propeller pump, can flow down in laterally-laid serpentine channels, relying on gravitational force. The flow rate of water is controlled so that it can overflow the lateral channel walls and mix with the main flow in the next lower channel, which thus creates a better mixing. In order to optimize the design parameters of the new flow field, methods including flow visualization, local point velocity measurement, and CFD analysis were employed to investigate the flow mixing features. Different combinations of channel geometries and water velocities were evaluated. An optimized flow field design and details of flow mixing are presented. The study offers an innovative design for large scale algae growth raceways which is of significance to the algae and biofuel industry. © 2013 Elsevier Ltd.
• Waller, P. M., Khawam, G., Attalah, S., & Ryan, R. (2014). ARID Raceway Temperature Model Evaluation. Transactions of the ASABE, 57(1), 1-8.
• Waller, P. M., Li, P., & Xu, B. (2014). Study of the flow mixing in a novel open-channel raceway for algae production.. Renewable Energy, 62, 249-257.
• Xu, B., Li, P., & Waller, P. M. (2014). Study of the flow mixing in a novel ARID raceway for algae production. Renewable Energy, 62, 249-257.
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  A novel flow field for algae raceways has been proposed, which is fundamentally different from traditional paddlewheel-driven raceways. To reduce freezing and heat loss in the raceway during cold time, the water is drained to a deep storage canal. The ground bed of the new raceway has a low slope so that water, lifted by propeller pump, can flow down in laterally-laid serpentine channels, relying on gravitational force. The flow rate of water is controlled so that it can overflow the lateral channel walls and mix with the main flow in the next lower channel, which thus creates a better mixing. In order to optimize the design parameters of the new flow field, methods including flow visualization, local point velocity measurement, and CFD analysis were employed to investigate the flow mixing features. Different combinations of channel geometries and water velocities were evaluated. An optimized flow field design and details of flow mixing are presented. The study offers an innovative design for large scale algae growth raceways which is of significance to the algae and biofuel industry
• Attalah, S., Waller, P., Khawam, G., & Ryan, R. (2012). Energy evaluation in the High Velocity Algae Raceway Integrated Design (ARID-HV). American Society of Agricultural and Biological Engineers Annual International Meeting 2012, 4, 2887-2901.
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  Abstract: The ARID raceway is an effective method to maintain temperature in the optimal growing range. However, the energy input is excessive. Thus, the ARID-HV raceway was developed in order to reduce energy input requirements. This was accomplished by improving pumping efficiency and using a serpentine flow pattern in which the water flows through channels instead of over barriers. A prototype ARID-HV raceway was installed in Tucson, Arizona in order to evaluate the flow and energy requirements of the raceway. Preliminary results show continued high energy usage, but this paper explores the possible energy reductions with the new ARID-HV design with efficient propeller pumps and properly engineered channel lengths. Channel lengths are evaluated with Manning's equation in order to determine the maximum channel length that could be implemented with the current pumping configuration.
• Crowe, B., Attalah, S., Agrawal, S., Waller, P., Ryan, R., Van, W. J., Chavis, A., Kyndt, J., Kacira, M., Ogden, K., & Huesemann, M. (2012). A comparison of Nannochloropsis salina growth performance in two outdoor pond designs: conventional raceways versus the ARID pond with superior temperature management. International Journal of Chemical Engineering.
• Hunsaker, D., French, A., Waller, P., Bautista, E., Royer, P., Thorp, K., Andrade-Sanchez, P., & Heun, J. (2012). Irrigation scheduling decision support for field-scale, surface irrigation using remote sensing and ground-based data. IAHS-AISH Publication, 352, 414-418.
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  Abstract: A spatial soil water balance modelling approach that utilized remote sensing and ground-based data was developed to guide surface irrigation scheduling of farm-size cotton borders in a field experiment in Arizona, USA. The objective was to evaluate spatial estimates of daily crop evapotranspiration (ETc) calculated for small, 4 × 8 m cells within 12 × 168 m cotton borders. Estimated ETc rates during the season compared favourably with ETc data based on soil water content measurements, though underestimating field-based ETc by an average of 0.25-0.46 mm/d. Results suggest that the spatial modelling approach could be a useful decision-making tool for improving irrigation scheduling of surface-irrigated fields. Copyright © 2012 IAHS Press.
• Kacira, M., Waller, P., Agrawal, S., Attalah, S., Crowe, B., Ryan, R., Van Wagenen, J., Chavis, A., Kyndt, J., Ogden, K. L., & Huesemann, M. (2012). A Comparison ofNannochloropsis salinaGrowth Performance in Two Outdoor Pond Designs: Conventional Raceways versus the ARID Pond with Superior Temperature Management. International Journal of Chemical Engineering, 2012, 1-9. doi:10.1155/2012/920608
• Khawam, G., Waller, P., Attalah, S., & Ryan, R. (2012). ARID raceway temperature management. American Society of Agricultural and Biological Engineers Annual International Meeting 2012, 2, 997-1011.
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  Abstract: One of the biggest causes of decreased algae production in open ponds is diurnal and seasonal temperature variation. The ARID (Algae Raceway Integrated Design) system maintains temperature in the optimal range by controlling the surface area of the system. A finite difference temperature model of the ARID raceway was developed in Visual Basic for Applications. The atmospheric boundary layer model uses hourly meteorological data from agricultural weather station networks. The latent heat of vaporization is calculated with the weather station reported values of evapotranspiration, which are calculated with the ASCE standardized Penman equation. The energy balance includes four terms: solar radiation, sensible heat flux, latent heat of vaporization, and long wave radiation. This research focused on calibrating the model for a one month experiment that was run during winter 2011. The results show a very good match between the simulated and the experimental temperature. The model was run in order to simulate the temperature and algae growth in ARID and conventional raceways in Tucson and Yuma, Arizona during the 12 months of 2011.
• Waller, P. M., WallerP, ., Ryan, R., Kacira, M., & Li, P. (2012). The Algae Raceway Integrated Design for optimal temperature management. Journal of Biomass and Bioenergy, 46, 702-709.
• Haberland, J. A., Colaizzi, P. D., Kostrzewski, M. A., Waller, P. M., Choi, C. Y., Eaton, F. E., Barnes, E. M., & Clarke, T. R. (2010). AgIIS, Agricultural Irrigation Imaging System. Applied Engineering in Agriculture, 26(2), 247-253.
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  Abstract: AgIIS (Agricultural Irrigation Imaging System, pronounced Ag Eyes), a ground-based remote sensing system, served as a research tool that generated data for research on remotely sensed canopy level water and nitrogen status indices. A rail was mounted on a 100-m long linear move irrigation machine, and a cart with a remote sensing unit ran back and forth on the rail. As the cart traveled along the rail and the linear move traveled through the field, the sensing unit collected one square meter area reflectance measurements every meter along the rail. Because the system was automated, the remotely sensed data was acquired with low labor cost compared to traditional handheld radiometers, and provided high temporal and spatial resolution. The system monitored a 0.5-ha research area with 16 research plots. The rail, made of steel tubing, was constructed of three parallel tubes in a triangular frame. The rail had almost no vertical deflection due to cart weight, and slip joints between sections were elastic enough to absorb the deformation of the linear move when loaded with water. The sensor package included four reflectance bands filtered to narrow wavelength intervals (10 nm) in the red (670 nm), green (555 nm), red-edge (720 nm), and near infrared (NIR) (790 nm) portions of the spectrum, and an infrared thermometer. The crop spectral signals were post-processed in order to construct georeferenced field maps of vegetation, nutrient, and water status indices. Analysis of the data showed that the rail and cart provided a platform for collection of consistent and reliable remote sensing data, and it served as a valuable tool for refinement of water and nitrogen status indices. The AgIIS design effectively and reliably collected remote sensing data from a constant elevation, at near nadir orientation, and at 1-m intervals. © 2010 American Society of Agricultural and Biological Engineers.
• Hunsaker, D. J., French, A. N., Bautista, E. M., Thorp, K. R., Waller, P. M., Royer, P. D., Andrade-Sanchez, P., & Heun, J. (2010). Spatial estimation of crop evapotranspiration, soil properties, and infiltrated water for scheduling cotton surface irrigations. ASABE - 5th National Decennial Irrigation Conference 2010, Held in Conjunction with Irrigation Show 2010, 2, 748-760.
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  Abstract: Estimates of spatially distributed crop evapotranspiration (ETc) over large fields could be particularly valuable for aiding irrigation management decisions in arid regions where surface irrigation systems are predominant. The objectives are to evaluate an irrigation scheduling approach that combines remote sensing inputs with field data to provide fine-scale, spatial monitoring of crop water use and soil water status within surface-irrigated fields. Remote sensing observations of vegetation index were used to spatially estimate basal crop coefficients within 4-m x 8-m zones within borders of a 4.9-ha cotton field. These data were used to compute ETc within zones using FAO-56 procedures. Spatial inputs of soil properties were estimated from a ground-based apparent soil electrical conductivity survey. Spatial distribution of infiltrated water along the furrow was estimated using hydraulic field measurements and irrigation simulation software. An existing daily time-step, soil water balance computer program was modified to incorporate the spatial information and provide simultaneous monitoring of crop and soil conditions in zones. Irrigation scheduling using the spatial monitoring approach compared favorably in yield to traditional cotton irrigation scheduling used in the area, but reduced water use by 7 to 9%, whereas it attained as much as 19% higher yield compared to scheduling based on assuming a uniform crop coefficient for all zones. Managing water for large surface-irrigated fields aided by decision support tools and approaches that allow spatial monitoring of crop water use and soil conditions could improve precision and timing of irrigation water scheduling.
• El-Shikha, D., Hunsaker, D. J., French, A., Waller, P., & Clarke, T. (2009). Sensitivity of canopy chlorophyll concentration index (CCCI) for water stress. American Society of Agricultural and Biological Engineers Annual International Meeting 2009, 3, 1427-1452.
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  Abstract: A variety of remote sensing indices have been used to infer crop nitrogen (N) status for field-scale nutrient management. However, such indices may indicate incorrect N status if there is a decrease in crop canopy density influenced by other growth retardation factors, such as water stress. The Canopy Chlorophyll Content Index (CCCI) is a two-dimensional remote sensing index that has been proposed for inferring cotton N status. The CCCI uses reflectances in the near-infrared (NIR) and red spectral regions to account for seasonal changes in canopy density, while reflectances in the NIR and far-red regions are used to detect relative changes in canopy chlorophyll, a surrogate for N content. The primary objective of this study was to evaluate the CCCI for detecting the N status for cotton, broccoli and wheat during the growing season without being affected by water stress. Remote sensing data were collected during cotton (1998 and 1999), broccoli (2001), and wheat (2004 and 2005) experiments. Experiments included treatments of optimal and low levels of N and water. They were carried out at The University of Arizona's Maricopa Agricultural Center (MAC) located approximately 40 km south of Phoenix, AZ, USA. The primary results indicated that the CCCI is significantly correlated with the measured parameters of nitrogen status, including petiole NO3-N, SPAD chlorophyll, and leaf total nitrogen. The CCCI was found to be highly sensitive to nitrogen, but mostly insensitive to water stress, especially at full cover. The CCCI can be used as a successful management tool for differentiating between the effects of nitrogen and water stress in wheat. However, CCCI was not very reliable with wheat or broccoli at times of severe water stress.
• Waller, P., Szidarovszky, F., Shikha, D. E., & Roanhorse, A. (2009). SELECTING THE BEST REMOTE SENSING PLATFORM FOR AGRICULTURAL ASSESSMENT USING MULTI-OBJECTIVE ANALYSIS. Misr Journal of Agricultural Engineering, 26(1), 11-43. doi:10.21608/mjae.2009.109860
• El-Shikha, D. M., Barnes, E. M., Clarke, T. R., Hunsaker, D. J., Haberland, J. A., Pinter Jr., P. J., Waller, P. M., & Thompson, T. L. (2008). Remote sensing of cotton nitrogen status using the Canopy Chlorophyll Content Index (CCCI). Transactions of the ASABE, 51(1), 73-82.
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  Abstract: Various remote sensing indices have been used to infer crop nitrogen (N) status for field-scale nutrient management. However, such indices may indicate erroneous N status if there is a decrease in crop canopy density influenced by other factors, such as water stress. The Canopy Chlorophyll Content Index (CCCI) is a two-dimensional remote sensing index that has been proposed for inferring cotton N status. The CCCI uses reflectances in the near-infrared (NIR) and red spectral regions to account for seasonal changes in canopy density, while reflectances in the NIR and far-red regions are used to detect relative changes in canopy chlorophyll, a surrogate for N content. The primary objective of this study was to evaluate the CCCI and several other remote sensing indices for detecting the N status for cotton during the growing season. A secondary objective was to evaluate the ability of the indices to appropriately detect N in the presence of variable water status. Remote sensing data were collected during the 1998 (day of year [DOY] 114 to 310) and 1999 (DOY 106 to 316) cotton seasons in Arizona, in which treatments of optimal and low levels of N and water were imposed. In the 1998 season, water treatments were not imposed until late in the season (DOY 261), well after full cover. Following an early season N application in 1998 for the optimal (DOY 154) but not the low N treatment, the CCCI detected significant differences in crop N status between the N treatments starting on DOY 173, when canopy cover was about 30%. A common vegetation index, the ratio of NIR to red (RVI), also detected significant separation between N treatments, but RVI detection occurred 16 days after the CCCI response. After an equal amount of N was applied to both optimal and low N treatments on DOY 190 in 1998, the CCCI indicated comparable N status for the N treatments on DOY 198, a trend not detected by RVI. In the 1999 season, both N and water treatments were imposed early and frequently during the season. The N status was poorly described by both the CCCI and RVI under partial canopy conditions when water status differed among treatments. However, once full canopy was obtained in 1999, the CCCI provided reliable N status information regardless of water status. At full cotton cover, the CCCI was significantly correlated with measured parameters of N status, including petiole NO 3-N (r = 0.74), SPAD chlorophyll (r = 0.65), and total leaf N contents (r = 0.86). For well-watered cotton, the CCCI shows promise as a useful indicator of cotton N status after the canopy reaches about 30% cover. However, further study is needed to develop the CCCI as a robust N detection tool independent of water stress.
• Lopez, J. C., Waller, P., Giacomelli, G., & Tuller, M. (2008). Physical characterization of greenhouse substrates for automated irrigation management. Acta Horticulturae, 797, 333-338.
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  Abstract: Over the last decade, the greenhouse industry experienced a significant increase in production capacity in response to enhanced demand of high-quality crops. To optimize yield and quality of greenhouse crops, substrates with optimal balance of aeration and water holding properties are essential. A wide variety of root zone media such as perlite, rockwool, coco coir, foamed glass or mixtures of these substrates have been successfully used in greenhouse agriculture. There is mounting empirical evidence that dual porosity (i.e., aggregated) media that contain small intra-aggregate pores for water storage and larger inter-aggregate pores for aeration create an improved rhizosphere environment for many crops. To investigate the suitability of these substrates and various mixtures thereof for cultivation of tomatoes, we conducted a comprehensive measurement campaign to characterize water retention properties. The measured substrate water retention curves exhibit various unimodal (e.g., coco coir and rockwool) and bimodal shapes (e.g., foamed class and perlite) with differing air entry potentials, providing valuable information for irrigation scheduling to balance water storage and aeration for optimum growth conditions. Furthermore, the obtained curves can be used to parameterize numerical simulation models for optimization of irrigation strategies and other controllable environmental variables.
• El-Shikha, D., Waller, P., Hunsaker, D., Clarke, T., & Barnes, E. (2007). Ground-based remote sensing for assessing water and nitrogen status of broccoli. Agricultural Water Management, 92(3), 183-193.
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  Abstract: Remote sensing (RS) can facilitate the management of water and nutrients in irrigated cropping systems. Our objective for this study was to evaluate the ability of several RS indices to discriminate between limited water and limited nitrogen induced stress for broccoli. The Agricultural Irrigation Imaging System (AgIIS) was used over a 1-ha broccoli field in central Arizona to measure green (550 nm), red (670 nm), far red (720 nm), and near infrared (NIR-790 nm) reflectances, and thermal infrared radiation. Measurements were taken at a 1 m × 1 m resolution, every several days during the season. The following indices were calculated: ratio vegetation index (RVI), normalized difference vegetation index (NDVI), normalized difference based on NIR and green reflectance (NDNG), canopy chlorophyll concentration index (CCCI), and the water deficit index (WDI). The experimental design was a two-factor, nitrogen × water, Latin square with four treatments (optimal and low water and optimal and low nitrogen) and four replicates. In addition to RS measurements, the following in-situ measurements were taken: SPAD chlorophyll (closely related to nitrogen status), plant petiole nitrate-nitrogen concentrations, soil water content, and plant height, width, and leaf area index (LAI). Fresh marketable broccoli yield was harvested from plots 130 days after planting. Seasonal water application (irrigation plus rainfall) was 14% greater for optimal than low water treatments, whereas total nitrogen application was 35% greater for optimal than low N treatments. Although both nitrogen and water treatments affected broccoli growth and yield, nitrogen effects were much more pronounced. Compared to the optimal water and nitrogen treatment, broccoli yield was 20% lower for low water but optimal nitrogen, whereas yield was 42% lower for optimal water but low nitrogen. The RVI, NDVI, and NDNG indices detected treatment induced growth retardation but were unable to distinguish between the water and nitrogen effects. The CCCI, which was developed as an index to infer differences in nitrogen status, was found to be highly sensitive to nitrogen, but insensitive to water stress. The WDI provided appropriate information on treatment water status regardless of canopy cover conditions and effectively detected differences in water status following several irrigation events when water was withheld from low but not optimal water treatments. Using a RS ground-based monitoring system to simultaneously measure vegetation, nitrogen, and water stress indices at high spatial and temporal resolution could provide a successful management tool for differentiating between the effects of nitrogen and water stress in broccoli. © 2007 Elsevier B.V. All rights reserved.
• Evans, R. G., & Waller, P. M. (2007). 8. Application of chemical materials. Developments in Agricultural Engineering, 13, 285-327.
• Colaizzi, P. D., Barnes, E. M., Clarke, T. R., Choi, C. Y., & Waller, P. M. (2003). Estimating soil moisture under low frequency surface irrigation using crop water stress index. Journal of Irrigation and Drainage Engineering, 129(1), 27-35.
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  Abstract: The present study investigated the relationship between the crop water stress index (CWSI) and soil moisture for surface irrigated cotton (Gossypium hirsutum, Delta Pine 90b) at Maricopa, Arizona during the 1998 season. The CWSI was linked to soil moisture through the water stress coefficient Ks that accounts for reduced crop evapotranspiration when there is a shortage of soil water. A stress recovery coefficient Krec was introduced to account for reduced crop evapotranspiration as the crop recovered from water stress after irrigation events. A soil water stress index (SWSI) was derived in terms of Ks and Krec. The SWSI compared reasonably well to the CWSI, but atmospheric stability correction for the CWSI did not improve comparisons. When the CWSI was substituted into the SWSI formulation, it gave good prediction of soil moisture depletion (fDEP; when to irrigate) and depth of root zone depletion (Dr; how much to irrigate). Disagreement was greatest for fDEP
• Colaizzi, P. D., Barnes, E. M., Clarke, T. R., Choi, C. Y., Waller, P. M., Haberland, J., & Kostrzewski, M. (2003). Water stress detection under high frequency sprinkler irrigation with water deficit index. Journal of Irrigation and Drainage Engineering, 129(1), 36-43.
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  Abstract: A remote sensing package called the agricultural irrigation imaging system (AgIIS) aboard a linear move irrigation system was developed to simultaneously monitor water status, nitrogen status, and canopy density at one-meter spatial resolution. The present study investigated the relationship between water status detected by AgIIS and soil moisture for the 1999 cotton (Gossypium hirsutum, Delta Pine 90b) season in Maricopa, Ariz. Water status was quantified by the water deficit index (WDI), an expansion of the crop water stress index where the influence of soil temperature is accounted for through a linear mixing model of soil and vegetation temperature. The WDI was best correlated to soil moisture through the FAO 56 water stress coefficient Ks model; stability correction of aerodynamic resistance did not improve correlation. The AgIIS did provide field images of the WDI that might aid irrigation scheduling and increase water use efficiency.
• Kostrzewski, M., Waller, P., Guertin, P., Haberland, J., Colaizzi, P., Barnes, E., Thompson, T., Clarke, T., Riley, E., & Choi, C. (2003). Ground-based remote sensing of water and nitrogen stress. Transactions of the American Society of Agricultural Engineers, 46(1), 29-38.
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  Abstract: A ground-based remote sensing system (Agricultural Irrigation Imaging System, or AgIIS) was attached to a linear-move irrigation system. The system was used to develop images of a 1-ha field at 1 X 1 m resolution to address issues of spatial scale and to test the ability of a ground-based remote sensing system to separate water and nitrogen stress using the coefficient of variation (CV) for water and nitrogen stress indices. A 2 X 2 Latin square water and nitrogen experiment with four replicates was conducted on cotton for this purpose. Treatments included optimal and low nitrogen with optimal and low water. ANOVA was not an adequate method to assess the statistical variation between treatments due to the large number of data points. In general, the coefficient of variation of water and nitrogen stress indices increased with water and nitrogen stress. In fact, the coefficient of variation of stress indices was a more reliable measurement of water and nitrogen status than the mean value of the indices. Differences in coefficient of variation of stress indices between treatments were detectable at 3 m grid resolution and finer for water stress and at 7 m grid resolution and finer for nitrogen stress.
• Wedwick, S., Lakhani, B., Stone, J., Waller, P., & Artiola, J. (2001). Development and sensitivity analysis of the GLEAMS-IR model. Transactions of the American Society of Agricultural Engineers, 44(5), 1095-1104.
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  Abstract: In response to the need for improved agricultural best management practices in irrigated lands, the GLEAMS (Groundwater Loading Effects of Agricultural Management Systems) model was modified to include a component that models furrow, basin, and border irrigation practices. The irrigation component of the new model, GLEAMS-IR, was validated with results from SRFR, a full hydrodynamic irrigation model. Sensitivity of nutrient and irrigation output parameters to model input parameters was assessed with a single-variable method and a stochastic method, Monte Carlo. Means and distributions of soil parameters for a surface-irrigated cotton field in Marana, Arizona, were used for sensitivity analysis. For both single-variable and Monte Carlo sensitivity analyses, output parameters were most sensitive to infiltrated depth after 120 min, permanent wilting point, and field capacity. A study of sludge application on the same field (1x and 3x sludge application, nitrogen fertilizer application, and control with no nitrogen) was used to evaluate the GLEAMS-IR model results for different management scenarios and to compare GLEAMS-IR results with nutrient concentrations at different locations along the furrow. The variability of measured nitrate concentrations along the furrow indicated the need to account for variable infiltration along the furrow in GLEAMS-IR.
• Wedwick, S., Waller, P., Stone, J., Lakhani, B., & Artiola, J. (2001). DEVELOPMENT AND SENSITIVITY ANALYSIS OF THE GLEAMS–IR MODEL. Transactions of the ASABE, 44(5), 1095-1104. doi:10.13031/2013.6437
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  In response to the need for improved agricultural best management practices in irrigated lands, the GLEAMS (Groundwater Loading Effects of Agricultural Management Systems) model was modified to include a component that models furrow, basin, and border irrigation practices. The irrigation component of the new model, GLEAMS–IR, was validated with results from SRFR, a full hydrodynamic irrigation model. Sensitivity of nutrient and irrigation output parameters to model input parameters was assessed with a single–variable method and a stochastic method, Monte Carlo. Means and distributions of soil parameters for a surface–irrigated cotton field in Marana, Arizona, were used for sensitivity analysis. For both single–variable and Monte Carlo sensitivity analyses, output parameters were most sensitive to infiltrated depth after 120 min, permanent wilting point, and field capacity. A study of sludge application on the same field (1x and 3x sludge application, nitrogen fertilizer application, and control with no nitrogen) was used to evaluate the GLEAMS–IR model results for different management scenarios and to compare GLEAMS–IR results with nutrient concentrations at different locations along the furrow. The variability of measured nitrate concentrations along the furrow indicated the need to account for variable infiltration along the furrow in GLEAMS–IR.
• Barnes, E. M., Clarke, T. R., Richards, S. E., Colaizzi, P. D., Haberland, J., Kostrzewski, M., Waller, P., Waller, P. M., Choi, C. Y., Choi, C. Y., Riley, E., Thompson, T. L., Thompson, T. L., Lascano, R. J., Li, H., Li, H., Moran, M., Robert, P. C., Rust, R. H., & Larson, W. E. (2000). Coincident detection of crop water stress, nitrogen status and canopy density using ground-based multispectral data.. Proceedings of the Fifth International Conference on Precision Agriculture.
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  Remotely sensed data has been identified as an important tool for precision crop management (PCM). The data has been used to assist in the identification of management zones, map crop nutrient status, and detect pest infestations. However, in many of the examples cited, the correlation between a multispectral signature and the variation of interest was limited to single factor experiments (i.e., only one factor was primarily responsible for the variability in crop condition). A water by nitrogen experiment was conducted during the 1999 cotton season near Phoenix, Arizona, where one objective was to test the ability of remotely sensed data to distinguish between water and nitrogen stress. Multispectral (visible, near infrared and thermal) data were collected using a prototype sensor mounted on a linear move irrigation system. Neutron probe data were used to quantify crop water status, and petiole samples were used to
• Colaizzi, P. D., Choi, C. Y., Waller, P. M., Barnes, E. M., & Clarke, T. R. (2000). Determining irrigation management zones in precision agriculture using the water deficit index at high spatial resolutions. 2000 ASAE Annual Intenational Meeting, Technical Papers: Engineering Solutions for a New Century, 1, 2635-2656.
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  Abstract: A method using the Water Deficit Index (WDI) as a guide to determining irrigation management zones and reducing soil samples needed to characterize plant available water is proposed. The WDI is an expansion of the Crop Water Stress Index (CWSI) where soil background under partial canopy cover is accounted for. WDI maps did not appear similar to a map of plant available water; thus, it had limitations in defining irrigation management zones. A WDI map generated four days after an irrigation did provide a suitable guide for soil sampling locations. Point locations where soil plant available water was known could be selectively reduced by 75% using the WDI map with only an average error of 0.08 cm, and error was within 10% for 98% of the field area.
• Suarez-Rey, E., Choi, C., Waller, P., & Kopec, D. (2000). Comparison of subsurface drip irrigation and sprinkler irrigation for Bermuda grass turf in Arizona. Transactions of the American Society of Agricultural Engineers, 43(3).
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  A subsurface drip irrigation (SDI) system is potentially efficient because it provides water directly to the root zone, minimizing evaporative loss, especially in arid lands. In this study, subsurface drip irrigation was compared to standard overhead sprinkler irrigation of Bermuda grass turf using reclaimed water. Research focused on the response of Bermuda grass to two irrigation treatments, subsurface drip irrigation and overhead sprinkler irrigation. Soil moisture content was calculated via time domain reflectometry (TDR) and neutron probe data. The remotely sensed crop water stress index (CWSI) could not be used to schedule irrigation. When the average soil moisture of all eight plots was depleted to 50% of readily available water, they were irrigated until the soil moisture content reached field capacity. No significant differences were observed between the two irrigation systems in total irrigation depth, relative root weight, dry clipping mass per unit area, or visual quality. The electrical conductivity (EC) of a soil water extract measured at the beginning and end of the season indicated salt accumulation near the ground surface in the subsurface plots, but there was not sufficient accumulation to affect the appearance of turf. Visual inspection of emitters after one irrigation season showed signs of root intrusion because of water stress in certain plots with high surface sand content. A subsurface drip irrigation (SDI) system is potentially efficient because it provides water directly to the root zone, minimizing evaporative loss, especially in arid lands. In this study, subsurface drip irrigation was compared to standard overhead sprinkler irrigation of Bermuda grass turf using reclaimed water. Research focused on the response of Bermuda grass to two irrigation treatments, subsurface drip irrigation and overhead sprinkler irrigation. Soil moisture content was calculated via time domain reflectometry (TDR) and neutron probe data. The remotely sensed crop water stress index (CWSI) count not be used to schedule irrigation. When the average soil moisture of all eight plots was depleted to 50% of readily available water, they were irrigated until the soil moisture content reached field capacity. No significant differences were observed between the two irrigation systems in total irrigation depth, relative root weight, dry clipping mass per unit area, or visual quality. The electrical conductivity (EC) of a soil water extract measured at the beginning and end of the season indicated salt accumulation near the ground surface in the subsurface plots, but there was not sufficient accumulation to affect the appearance of turf. Visual inspection of emitters after one irrigation season showed signs of root intrusion because of water stress in certain plots with high surface sand content.
• Yuan, Z., Choi, C. Y., Waller, P. M., & Colaizzi, P. (2000). Effects of liquid temperature and viscosity on Venturi injectors. Transactions of the American Society of Agricultural Engineers, 43(6), 1441-1447.
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  Abstract: The effect of chemical temperature change on the injection flow rate of a Venturi injector was evaluated. The percent change in flow rate corresponding with changes in temperature should be quantified because Venturi injectors are connected to chemical tanks at various temperatures due to radiative and convective heat transfer. Water, CAN17 (calcium ammonium nitrate), UAN32 (urea ammonium nitrate), soybean oil, and Orchex® were injected from a thermal reservoir into a PVC pipeline with a Venturi injector. Both CAN17 and UAN32 are soluble in water, while soybean oil and Orchex oil are insoluble. The injection flow rate for the four chemicals and water was measured over a range of pressure differentials between the upstream and downstream side of the Venturi, and over a range of chemical temperatures. The viscosity of water was less than 1.5 mPa·s. The viscosity of the other four chemicals ranged from 3.1 mPa·s to 121 mPa·s. The injection flow for water, with low viscosity, did not change significantly with temperature. However, the injection rate for the four chemicals was correlated with temperature and viscosity. If the chemical tank temperature variation is 20°C during the day, then the injection flow rate variation would be in the range of 50% for soybean oil, 30% for Orchex®, 10% for UAN32, and 5% for CAN17. Insoluble chemicals had much higher injection rates than soluble chemicals at the same viscosity. Because the injection rate for Venturi injectors is temperature dependent, and flow increases as chemical temperature increases, the increased cost of chemicals, environmental contamination, and crop loss might be greater than capital and maintenance savings.
• Marouelli, W. A., & Waller, P. M. (1999). Foil drop generator for foliar chemigation: Field evaluation. Transactions of the American Society of Agricultural Engineers, 42(6), 1599-1607.
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  Abstract: Chemigation field experiments were conducted with a new drop generator for oil-based foliar chemicals. The drop generator was adjusted to generate two drop-size distributions with d(max) of 980 μm and 98 μm. Soybean oil concentration in sprinkler sprays was measured along the 379-m-long center pivot irrigation pipeline; uniformity coefficients along the center pivot pipeline were 73% and 98%, respectively. An experiment with a conventional straight tube injector was also conducted, and calculated d(max) was 1850 μm; oil discharge uniformity coefficient along the pipeline was 61%. The calculated drop-size distributions for the three experiments were input into a dispersed phase pipeline transport model; the model calculates oil discharge concentration versus distance along the pipeline. The root mean square deviation error between experimental and simulated concentration curves were 40%, 28%, and 3% for the straight tube, 980 μm, and 98 μm experiments, respectively. The models were used to develop graphs for discharge concentration uniformity as a function of d(max) and relative density of oil and water in the pipeline. The model simulations and experiments showed that the drop generator and two-phase transport model provide the ability to control oil discharge concentration uniformity along a center pivot pipeline.
• Marouelli, W. A., & Waller, P. M. (1999). Oil drop generator for foliar chemigation: Theory and laboratory evaluation. Transactions of the American Society of Agricultural Engineers, 42(5), 1289-1301.
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  Abstract: Precise control of oil drop size optimizes oil retention on plants and oil discharge uniformity along the pipeline for foliar chemigation. However, control of oil drop size with existing chemigation injection systems is difficult. A new system was developed that injects oil drops of known size distribution into a center pivot irrigation pipeline. The system removes water from the irrigation pipeline, increases water pressure with a pump, injects oil into the water stream, increases dispersion velocity in small diameter tubes in order to break up oil drops, and finally injects the water-oil dispersion back into the irrigation pipeline. In order to calculate the maximum drop size (d(max)) of viscous oil drops in the drop generator, a correction term with effective viscosity, effective density, and dispersed phase volume fraction was added to the Hinze (1955) equation for d(max) in turbulent two-phase pipe flow. The new equation calculates d(max) as a function of water flow rate, oil and water viscosity, oil volume fraction, and other measurable parameters. The average relative error and root mean square deviation between d(max) and literature data was 3 and 17%, respectively. The tubing in the drop generator is coiled in order to reduce the length of the system; thus, the coiled tubing friction factor must be used in the d(max) equation. Two equations (the Ito and Srinivasen equations)for friction factor in helical coiled tubing were evaluated in laboratory experiments with the drop generator. The Ito equation performed best; average root mean square deviation between calculated friction factor and experimental data was 2%. Three equations for the effective viscosity of oil-water dispersions (the Einstein, Taylor, and Richardson equations) were evaluated with literature and experimental data. The Richardson equation performed best; average root mean square deviation between calculated viscosity and experimental and literature data was 7%.
• Waller, P., & Marouelli, W. A. (1999). OIL DROP GENERATOR FOR FOLIAR CHEMIGATION: THEORY AND LABORATORY EVALUATION. Transactions of the ASABE, 42(5), 1289-1301. doi:10.13031/2013.13293
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  Precise control of oil drop size optimizes oil retention on plants and oil discharge uniformity along the pipeline for foliar chemigation. However, control of oil drop size with existing chemigation injection systems is difficult. A new system was developed that injects oil drops of known size distribution into a center pivot irrigation pipeline. The system removes water from the irrigation pipeline, increases water pressure with a pump, injects oil into the water stream, increases dispersion velocity in small diameter tubes in order to break up oil drops, and finally injects the water-oil dispersion back into the irrigation pipeline. In order to calculate the maximum drop size (dmax) of viscous oil drops in the drop generator, a correction term with effective viscosity, effective density, and dispersed phase volume fraction was added to the Hinze (1955) equation for dmax in turbulent two-phase pipe flow. The new equation calculates dmax as a function of water flow rate, oil and water viscosity, oil volume fraction, and other measurable parameters. The average relative error and root mean square deviation between dmax and literature data was 3 and 17%, respectively. The tubing in the drop generator is coiled in order to reduce the length of the system; thus, the coiled tubing friction factor must be used in the dmax equation. Two equations (the Ito and Srinivasen equations) for friction factor in helical coiled tubing were evaluated in laboratory experiments with the drop generator. The Ito equation performed best; average root mean square deviation between calculated friction factor and experimental data was 2%. Three equations for the effective viscosity of oil-water dispersions (the Einstein, Taylor, and Richardson equations) were evaluated with literature and experimental data. The Richardson equation performed best; average root mean square deviation between calculated viscosity and experimental and literature data was 7%.
• Yuan, Z., Waller, P. M., & Choi, C. Y. (1998). Effects of organic acids on salt precipitation in drip emitters and soil. Transactions of the American Society of Agricultural Engineers, 41(6), 1689-1696.
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  Abstract: Three organic acid compounds were evaluated for preventing precipitation of salts and/or removing salts in drip irrigation systems and soils. Three experiments were conducted to measure drip emitter clogging, ponded infiltration and soil salinity change. All acid compounds included maleic acid, a form of dicarboxylic acid. The first organic acid was composed of polymaleic acid, maleic acid, surfactant blend, and inert ingredients. The second was an anionic polymer with maleic acid as the organic acid. The third included a soap and was a 1:1 stoichiometric equivalent of an organic carboxylic acid and an amine base. The first and third organic acid significantly reduced drip emitter clogging compared to a water-only treatment. The third organic acid was significantly better than the first for reducing clogging. The third organic acid and water-only treatments significantly reduced soil sodicity below the drip irrigation laterals during the study. Ponded infiltration tests with organic acid in water were also conducted. All three organic acid treatments produced significantly lower infiltration rates than the water-only treatment. This may have occurred because salt precipitates in the soil were removed and pores were clogged.
• Choi, C. Y., & Waller, P. M. (1997). Momentum transport mechanism for water flow over porous media. Journal of Environmental Engineering, 123(8), 792-799.
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  Abstract: The momentum transport phenomena at the interface of the porous medium and fluid have been numerically investigated. The single domain approach is used with matching boundary conditions; that is, the Brinkman-Forchheimer-extended Darcy equation is used for the present study. Five typical porous media found in natural and engineered systems are selected in order to cover a wide range of the Darcy number (6.25 × 10-4 ≤ Da ≤ 5.90 × 10-11). In addition, six different Reynolds numbers (10 ≤ R ≤ 1,000) are tested for each case. When Da > 10-7, the results showed the importance of viscous shear in the channel fluid. The viscous shear propagates across the interface into the porous medium and forms a transition region of disturbed flow in the porous medium. The depth of penetration is only dependent on the Darcy number of the porous medium rather than the Reynolds number and the shape of velocity profile. In the vicinity of the interface, it is clear that Darcy's law is inappropriate to describe flow in a permeable wall fracture or flow over porous media. ©ASCE.
• Choi, C. Y., Waller, P. M., & Dennehy, T. M. (1997). Insect control with carbon dioxide foam. Transactions of the American Society of Agricultural Engineers, 40(5), 1475-1480.
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  Abstract: The purpose of this study was to investigate a safer pest control method for both urban and agricultural applications. In this method, carbon dioxide gas and air were applied within a non-toxic aqueous foam. The CO2 foam was designed to spread over crops or substrate and to persist until insects suffocate in the anoxic foam after a sufficient length of exposure. The surfactants that were mixed with water to create the foam are harmless to the environment, as well aS inexpensive. The foam would be removed by a spray of water or would simply disintegrate. Laboratory toxicity studies on common urban and agricultural pests were conducted. For German cockroaches in particular, mortality for CO2 foam was significantly higher than mortality for air foam. The results indicated that CO2 foam could effectively control some urban and agricultural pests without the coincident use of pesticides.
• Colaizzi, P. D., Jordan, K. A., & Waller, P. M. (1997). Overwatering controller for landscape irrigation systems. Paper - American Society of Agricultural Engineers, 2.
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  Abstract: Water conservation in metropolitan areas of Arizona is critical if limited water resources are to meet current and future demands. Timer controlled landscape irrigation systems contribute to a large portion of municipal water use, and there is a great potential for water savings if deficit irrigation is practiced. The overwatering controller used in this research senses soil water and prevents irrigation if sufficient water is in the soil. The controller was evaluated at 50 sites in Tucson, Arizona. An average of 35% water savings was observed and 30% of irrigation cycles were skipped.
• Hla, A. K., Tipton, J. L., & Waller, P. M. (1997). Sap flow gauge measurement of transpiration for acacia and mesquite trees. Paper - American Society of Agricultural Engineers, 2.
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  Abstract: Transpiration was measured with sap flow gauges on two landscape plant species (Acacia and Mesquite). The trees were subjected to three watering treatments: 1, 4, and 8 eight drip emitters. All treatments received equal amounts of water based on seasonal transpiration rates. The seasonal sap flow patterns on an hourly and daily basis were plotted. Transpiration measurements indicated that the threshold temperature limits were not experienced. Transpiration for the acacia and the mesquite as a function of canopy area were 0.4 and 0.65 of the reference ET, respectively, during summer.
• Marouelli, W. A., & Waller, P. M. (1997). Field evaluation of new foliar chemigation system. Paper - American Society of Agricultural Engineers, 2.
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  Abstract: Center pivot field experiments for oil-based pesticide application were conducted using a new turbulent dispersion chemigation injector and a straight tube injector for oil-pesticide application through irrigation pipelines. For the straight tube injector system, oil uniformity coefficient along the lateral was 61%. The turbulent dispersion injector was set to deliver 2 oil-pesticide dispersions to the irrigation pipeline with dmax of 980 μm and 98 μm, and uniformity coefficients along the center pivot pipeline were 73% and 98%, respectively. Injector drop-size models were used to calculate an oil drop size distribution for the straight tube and turbulent injector trials. The drop-size distributions and other parameters were input into a pipeline transport model for nonsoluble chemicals in irrigation pipelines, and the model calculated oil concentration vs. distance curves for each trial. The root mean square deviation error between experimental and simulated concentration curves were 40%, 28%, and 3% for the straight tube, 980 μm, and 98 μm experiments, respectively.
• Marouelli, W. A., & Waller, P. M. (1997). New chemigation droplet generator for foliar pesticide application. Paper - American Society of Agricultural Engineers, 2.
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  Abstract: A new droplet generation system for chemigation of foliar pesticides removes water from the irrigation pipeline, increases water pressure with a pump, injects oil-pesticide into the water stream, increases dispersion velocity in small tubes with high velocity in order to break up oil-pesticide droplets, and finally injects the water-oil dispersion back into the irrigation pipeline. Droplet breakup research was reviewed, and a model was developed to predict oil-pesticide maximum droplet size. Two equations were required: one for droplets in the inertial subrange scale, and one for droplets at the large eddy scale. The model was evaluated against 5 data sets in the literature and compared to 3 existing models. For the new model, maximum relative error and average root mean square deviation for maximum droplet size were 40- and 17-%, respectively.
• Martin, E., Hla, A., Waller, P., & Slack, D. (1997). Heat unit-based crop coefficient for grapefruit trees. Applied Engineering in Agriculture, 13(4).
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  The onset and rate of sap moving up the branches of grapefruit (Citrus paradisi Macfadyen) trees were monitored hourly using portable sap flow sensors at Waddell, Arizona. Hourly reference evapotranspiration (ETo) estimates were calculated using data from a nearby weather station. Crop water use was estimated from soil moisture measurements using a neutron probe. These data were used to first delineate the upper and lower temperature threshold values for the determination of heat units. A heat unit-based crop coefficient was then derived from a correlation of the crop coefficient with heat units over a crop year. The heat unit-based crop coefficient was found to be similar to crop coefficients derived by other researchers.
• Wedwick, S. J., & Waller, P. M. (1997). Modified GLEAMS assessment of sludge application on irrigated desert agriculture. Paper - American Society of Agricultural Engineers, 2.
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  Abstract: In response to the need for improved agricultural management practices, the GLEAMS model was modified to include a component that models irrigation practices. This component models furrow, basin, and border irrigation. Other irrigation practices, such as sprinkler or drip, can be modeled if the performance parameters are given. After completing the modifications, a partial validation was performed using soil nitrate data from a municipal sludge experiment performed on a cotton farm in Marana, Arizona. The predicted results from the model (GLEAMS-IR) were within the EPA's recommended factor of 3.0 for each of the sludge treatments evaluated. Sensitivity analyses were performed using a standard, or deterministic, method and a stochastic, Monte Carlo method. The results of the standard sensitivity analysis were distinguished as no sensitivity, low sensitivity, moderate sensitivity, high sensitivity, and very high sensitivity. The Monte Carlo results were ranked by significance (F value.).
• Waller, P. M., & Hills, D. J. (1995). Chemigation pipeline transport model for nonsoluble pesticide. I. Theory. Transactions of the American Society of Agricultural Engineers, 38(6), 1699-1709.
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  Abstract: A mathematical model of dispersed phase transport of a nonsoluble pesticide in an irrigation pipeline is derived. Eulerian and Lagrangian approaches are used at high and low turbulence intensities, respectively. An equation is derived by equating pesticide surface tension and viscous forces to irrigation water drag force at the injection nozzle in order to estimate mean dispersed-phase chemical drop size within the irrigation pipeline.
• Waller, P. M., Hills, D. J., & Giles, D. K. (1995). Chemigation pipeline transport model for nonsoluble pesticide. II. Numerical and field validation. Transactions of the American Society of Agricultural Engineers, 38(6), 1711-1718.
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  Abstract: A computer model describing the injection and transport of a nonsoluble pesticide in a linear move irrigation lateral is developed and validated. Inputs to the model are both intensive and extensive properties of irrigation water and pesticide, and the model outputs chemical concentration in water for each sprinkler along the linear move irrigation lateral. Numerical studies are performed that compare Eulerian and Lagrangian methods and provide the transition between the two methods. The transport model and a model for evaluating drop diameter are used to calculate sprinkler discharge concentration as a function of distance along the pipeline, and the model is compared with field data for a single port injector. The model is also evaluated for multiorifice and multiport injection systems. A technique for optimizing placement of injectors for the multiport system is described and evaluated.
• Waller, P. M., & Wallender, W. W. (1993). Changes in cracking, water content, and bulk density of salinized swelling clay field soils. Soil Science, 156(6), 414-423.
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  Abstract: Shrinkage and cracking characteristics at two sites under border strip irrigation were examined. At one site, irrigation water salinity was varied. Image analysis of soil surface photographs was used to quantify surface ped geometry. There was no spatial dependence for water content and bulk density at the scale of several centimeters or between peds. Larger, more polygonal peds were observed in higher salinity treatments, wheel traffic rows, and after repeated irrigations. Surface shrinkage increased with salinity. Dry bulk density was higher on the high salinity treatment both on the surface and with depth. Perimeters of the 9000 ppm treatment peds were less than perimeters for other treatments. Surface percent crack area followed percent change in bulk density. -from Authors
• Waller, P. M., & Wallender, W. W. (1991). Infiltration in surface irrigated swelling soils. Irrigation and Drainage Systems, 5(3), 249-266.
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  Abstract: Infiltration characteristics for border strip irrigation at two sites with swelling clay soils were examined. Volume infiltrated was calculated from flow onto the field monitored with flow meters; depth of water in the soil estimated from soil samples taken before and after irrigation; and the advance profile which was used to calculate the volume infiltrated with time. Volume infiltrated was compared with volume of cracks before irrigation. Linear advance and observed crack closing supported the hypothesis that infiltration approached zero after about 10 min. Volume of cracks was less than 20% of the volume infiltrated. Wetting front was 3-10 times greater than depth of observed surface cracks. There was no significant correlation between intake opportunity time and depth of infiltration, but elevation irregularities were related to infiltration. © 1991 Kluwer Academic Publishers.
• Hills, D. J., & Waller, P. M. (1989). Lateral move chemigation of Lorsban-4E on field corn. Applied Engineering in Agriculture, 5(4), 534-538.
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  Abstract: A 150 m (500 ft) lateral move machine was equipped to apply Lorsban-4E (chlorpyrifos) at a rate of 2.2 L/ha (0.24 gal/acre) while transversing a corn field at a speed of 2 m/min (6.6 ft/min). Four formulations, utilizing additions of either water, soybean oil, light grade petroleum oil or a nonionic surfactant, were evaluated. Leaf and soil samples obtained throughout the field were analyzed in a GLC for Lorsban concentration. Of the four formulations, soybean oil was the most effective in securing Lorsban-4E to the foilage and minimizing the amount washed down to the soil surface. Supreme oil and the nonionic surfactant were the next most effective in sticking Lorsban to the foliage, whereas water was least effective. Foliage samples taken from transects across the field indicated that application was fairly uniform over the first three quarters of the lateral's length but decreased 20-40% in the last quarter. Time samples indicated a rapid decline in Lorsban concentration over the first 24 h following application and a gradual decline thereafter to near zero concentration after the seventh day.
• Hills, D. J., Nawar, F. M., & Waller, P. M. (1989). Effects of chemical clogging on drip-tape irrigation uniformity. Transactions of the American Society of Agricultural Engineers, 32(4), 1202-1206.
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  Abstract: The following four management schemes were evaluated for lessening the chemical clogging effects of high calcium content water in drip-tape: 1) above ground day-time water application, 2) above ground night-time water application, 3) subsurface placement of drip-tape, and 4) lowering the pH of the irrigation water. Calcium, magnesium and bicarbonate ions were injected into the water source to evaluate water qualities with electrical conductivities of 0.59, 1.12, and 2.02 dS/m. Irrigation duration for each management scheme was four hours daily over the 100-day investigation. Volumetric flow rate and emission uniformity were monitored. Partial and full clogging due to chemical precipitation occurred in all management schemes for the water with the highest salt content. By the end of the study, daily flow values in the laterals had decreased between 20 and 40% for this water. Corresponding flow reductions for the lowest salt content water varied between 3 and 15%. Of the management modes evaluated, reduction of water pH from 7.6 to 6.8, by sulfuric acid injection, provided the least clogging for all three water qualities.

Proceedings Publications

• Attalah, S., Elsadek, E., Waller, P., Hunsaker, D., Thorp, K., Bautista, E., Williams, C., Wall, G., Orr, E., & Elshikha, D. (2024, Summer). Evaluating the Performance of OpenET Models for Alfalfa in Arizona. In American society of Agricultural and Biosystems Engineering International Meeting.
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  This study was conducted to evaluate six satellite-based ET models (ALEXI/DisALEXI, eeMETRIC, geeSEBAL, PT-JPL, SIMS, and SSEBop) and their Ensemble, derived from the OpenET platform, in estimating the actual evapotranspiration (ET) of alfalfa. Then, identify the best-performing OpenET model for alfalfa irrigation management under arid climate conditions in Arizona, USA. Five statistical metrics, the index of agreement (Dindex), Nash-Sutcliffe efficiency coefficient (NSE), mean bias simulation error (MBE), prediction/simulation error (Pe), and coefficient of determination (R2), were used to evaluate the seven alternative estimates in comparison with measured ET (ETmea) at a field scale with four replicates during the 2023 alfalfa growing season in Buckeye, Arizona. Overall, OpenET models and their Ensemble were linearly correlated to average ETmea with R2 > 0.71. Our findings showed that ALEXI/DisALEXI, geeSEBAL, and PT-JPL had a general tendency to underestimate actual ET with acceptable to poor prediction errors (Pe ≤ -35.18 for PT-JPL). In contrast, eeMETRIC, SIMS, and SSEBop overestimated ETmea with acceptable to poor prediction errors (1.86 ≤ Pe ≤ 28.39). Our results highlighted the limitations of using the PT model in arid to semi-arid areas, even after the PT-JPL aridity correction. The Ensemble approach, which combined all OpenET models, showed a high degree of agreement (0.93 ≤ Dindex ≤ 0.96) with the ETmea of alfalfa during the growing period in 2023. R2 ranged from 0.77 to 0.86, with positive NSE values between 0.67 and 0.82. Moreover, the Ensemble approach had significantly lower prediction errors (-6.92 ≤ Pe ≤ 4.04) when compared with six OpenET models, making it the best to simulate alfalfa’s actual evapotranspiration over the study area. This will contribute to providing farmers and decision-makers with the best satellite-based approach for efficient irrigation management and water use in arid regions.
• Elshikha, D., Attalah, S., Elsadek, E., Waller, P., Thorp, K., Sanyal, D., Bautista, E., Norton, R., Hunsaker, D., Williams, C., Wall, G., Barnes, E., & Orr, E. (2024). The Impact of Gravity Drip and Flood Irrigation on Development, Water Productivity, and Fiber Yield of Cotton in Semi-Arid Conditions of Arizona. In 2024 ASABE Annual International Meeting.
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  The present study was conducted to examine the effects of two irrigation methods with different application rates on cotton (ST 4595B3XF variety) growth, irrigation water productivity, and fiber yield in Arizona. Five treatments, namely gravity drip (GD) with 100%, 80%, and 60% of crop evapotranspiration (ETc) and flood (F) with 100% and 80% of ETc (GD 100%, GD 80%, GD 60%, F100%, and F80%, respectively), were arranged in a randomized complete block design (RCBD) with three replicates during 2023. Deficit irrigation (DI) reduced cotton height and canopy cover with both GD and F irrigations. Compared with the F system, the enhanced vegetative growth resulted in a notable increase in total irrigation requirements, particularly evident in the GD 100% (1337.50 mm) and GD 80% (1106.60 mm). The highest fiber yield values of 1621 kg ha−1 and 1465 kg ha−1 were recorded under an irrigation rate of 100% ETc for both GD and F irrigation. However, decreasing irrigation rates to 80% and 60% ETc negatively affected fiber yield under the two irrigation systems. Improvement of irrigation water productivity (WPI), accompanied by saving irrigation water and a high fiber yield, could be obtained by shifting irrigation from the GD 100% (I = 1337.50 mm, WPI = 0.121 kg m−3) and F100% rates (I = 1162.00 mm, WPI = 0.125 kg m−3) toward the GD 80% rate (I =1106.60 mm, WPI = 0.130 kg m−3). The lowest micronaire (MIC) values were recorded under F 80 % and F 100% (4.36 and 4.97, respectively). The 100% treatment of F and GD showed higher fiber strength (STR) (29.51 and 28.29 HVI g tex−1, respectively). However, the upper half mean length (UHML), uniformity index (UI), and short fiber content (SFC) values showed a downward trend under both GD and F treatments, reflecting their correlation with the total applied water. Elongation at failure (ELO) was consistent among irrigation treatments. This study provides significant guidance for adopting DI strategies in cotton under semi-arid conditions.
• Elshikha, D. E., Attalah, S., Waller, P. M., Hunsaker, D. J., Thorp, K. R., Williams, C., Katterman, M. E., Sanyal, D., Wang, G., Dierig, D., Ray, D. T., Ray, D. T., Dierig, D., Wang, G., Sanyal, D., Katterman, M. E., Williams, C., Thorp, K. R., Hunsaker, D. J., , Waller, P. M., et al. (2023, Spring).

  Guayule germination and growth under subsurface gravity drip and furrow irrigation in Arizona

  . In 2023 ASABE Annual International Meeting, Paper number 2300034, 19.
• El-Shikha, D., Waller, P. M., & coauthors, 8. o. (2019, Summer). Direct seeded guayule grown in Arizona under furrow and subsurface drip irrigation. In 2019 ASABE Annual Meeting.
• Waller, P. M., Xu, B., & Li, P. (2013, Summer). Optimization of the Flow Field of a Novel ARID Raceway (ARID-HV) for Algal Production. In ASME 2013 7th International Conference on Energy Sustainability collocated with the ASME 2013 Heat Transfer Summer Conference and the ASME 2013 11th International Conference on Fuel Cell Science, Engineering and Technology, V001T13A001, 10.
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  This paper addresses issues of flow field optimization for a water raceway which is used to grow algae for biofuels. An open channel raceway is the typical facility to grow algae in medium to large scales. The algae growth rate in a raceway is affected by conditions of temperature, nutrients, and sunlight intensity etc. These conditions are essentially associated with the fluid mixing in the flow field. Good flow mixing at low consumption of pumping power for the water flow is desirable for an economic algal growth facility. A novel design of an open channel raceway for medium- and large-scale algae growth field has been proposed by the authors previously, which is called High Velocity Algae Raceway Integrated Design (ARID-HV). Optimization analysis using CFD and experimental visualization has been applied to a table-sized ARID-HV test model with various geometries of dams and their spacing in the system. CFD results and flow visualization allow us to understand the flow mixing in the entire raceway. Data is also processed to show the statistics of the locations of ‘fluid particles’ at different height and time period during one flow path. Different flow field designs were thus compared quantitatively based on this statistics according to the understanding that the “tumbling times” of fluid particles at bottom/top of the water is tightly related to the growth rate of algae.
• Waller, P., Khawam, G., & Huesemann, M. H. (2013). Climate Effects on Algae Production in the ARID-HV Raceway.. In 2013 Kansas City, Missouri, July 21 - July 24, 2013.
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  Abstract: The ARID (Aquaculture Raceway Integrated Design) system controls the diurnal and seasonal temperature fluctuation by controlling the surface area of the system. A finite difference model of the ARID raceway water temperature was developed in Visual Basic for Application. The input of this model is hourly meteorological data from an agricultural weather station networks. The ARID temperature model was integrated with a growth model for DOE species 1412. This research focused on optimizing the operation schedule and the geometry of the ARID system. The integrated AIRD model with the DOE species 1412 growth model was run in order to simulate the algae growth in the ARID and the conventional systems in different climates.
• Waller, P., Li, P., & Xu, B. (2013). Optimization of the Flow Field of a Novel ARID Raceway (ARID-HV) for Algal Production. In Energy Sustainability.
• Attalah, S., Waller, P., Ryan, R., & Khawam, G. (2012). ARID Raceway Temperature Management. In 2012 Dallas, Texas, July 29 - August 1, 2012.
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  One of the biggest causes of decreased algae production in open ponds is diurnal and seasonal temperature variation. The ARID (Algae Raceway Integrated Design) system maintains temperature in the optimal range by controlling the surface area of the system. A finite difference temperature model of the ARID raceway was developed in Visual Basic for Applications. The atmospheric boundary layer model uses hourly meteorological data from agricultural weather station networks. The latent heat of vaporization is calculated with the weather station reported values of evapotranspiration, which are calculated with the ASCE standardized Penman equation. The energy balance includes four terms: solar radiation, sensible heat flux, latent heat of vaporization, and long wave radiation. This research focused on calibrating the model for a one month experiment that was run during winter 2011. The results show a very good match between the simulated and the experimental temperature. The model was run in order to simulate the temperature and algae growth in ARID and conventional raceways in Tucson and Yuma, Arizona during the 12 months of 2011.
• Waller, P. M., Hunsaker, D., French, A., Bautista, E., Thorp, K., WallerP, ., RoyerP, ., Hunsaker, D., French, A., Bautista, E., Thorp, K., Waller, P., & RoyerP, . (2010, Fall). Spatial Estimation of Crop Evapotranspiration, Soil Properties, and Infiltrated Water for Scheduling Cotton Surface Irrigations. In ASABE Conference Presentation.

Presentations

• El-Shikha, D., Waller, P. M., & coauthors, 7. o. (2019, September). Growing direct-seeded guayule with furrow and subsurface drip irrigation in Arizona. Association for the Advancement of Industrial Crops Annual Meeting. Tucson, Arizona: Association for the Advancement of Industrial Crops.
• Hoare, D., Katterman, M., & Waller, P. M. (2019, September). Development of a remote sensing crop condition sensing system utilizing Internet of Things. Association for the Advancement of Industrial Crops Annual Meeting. Tucson, Arizona: Association for the Advancement of Industrial Crops.
• Maqsood, H., Angadi, S., Waller, P. M., & collaborators, 4. o. (2019, September). Evaluating crop water status for guar using WINDS model. Association for Advancement of Industrial Crops Annual Meeting. Tucson, Arizona: Association for the Advancement of Industrial Crops.
• Waller, P. M., Maqsood, H., & coauthors, 6. o. (2019, September). Assessment of Irrigation Requirement for guayule using WINDS model. Association for the Advancement of Industrial Crops Annual Meeting. Tucson, Arizona: Association for the Advancement of Industrial Crops.
• Johnson, M., Richardson, J., Ryan, R., & Waller, P. M. (2013, Summer). A Comparison of Two Cultivation Systems: Open Pond and Algae Raceway Integrated Design (ARID). Algae Biomass Summit. Orlando, Florida: ABO.
• Khawam, G. M., Waller, P., & Huesemann, M. (2013, Summer). Climate Effects on Algae production in the ARID-HV raceway. ASABE 2013 International meeting. Kansas City, Missouri: ASABE.
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  The ARID (Aquaculture Raceway Integrated Design) system controls the diurnal and seasonal temperature fluctuation by controlling the surface area of the system. A finite difference model of the ARID raceway water temperature was developed in Visual Basic for Application. The input of this model is hourly meteorological data from an agricultural weather station networks. The ARID temperature model was integrated with a growth model for DOE species 1412. This research focused on optimizing the operation schedule and the geometry of the ARID system. The integrated AIRD model with the DOE species 1412 growth model was run in order to simulate the algae growth in the ARID and the conventional systems in different climates.

Poster Presentations

• Dierig, D., Evancho, B. E., Ray, D. T., Waller, P. M., Mccloskey, W. B., Teegerstrom, T., Teegerstrom, T., Mccloskey, W. B., Waller, P. M., Ray, D. T., Dierig, D., & Evancho, B. E. (2020, October). Guayule Production in the Arid Southwest. Arizona Cooperative Extension 2020 Virtual Conference. Virtual: Arizona Cooperative Extension.
• Waller, P. M., Murat, K., Ryan, R., Choi, C., Li, P., Ogden, K., & Kyndt, J. (2012, November). Computational Fluid Dynamics Modeling of ARID Raceway. NAABB conference. Phoenix, AZ.
• Waller, P. M., Murat, K., Ryan, R., Choi, C., Li, P., Ogden, K., & Kyndt, J. (2012, November). New High Velocity ARID (HV-ARID) Raceway. NAABB conference. Phoenix, AZ.
• Waller, P. M., Murat, K., Ryan, R., Choi, C., Li, P., Ogden, K., & Kyndt, J. (2011, November). Computational Fluid Dynamics Modeling of ARID Raceway. NAABB conference. Phoenix, AZ.
• Waller, P. M., Murat, K., Ryan, R., Choi, C., Li, P., Ogden, K., & Kyndt, J. (2011, November). New High Velocity ARID (HV-ARID) Raceway. NAABB conference. Phoenix, AZ.
• Waller, P. M., Pepper, I., Quanrud, D., Gerba, C., Newman, D., & Saez, E. (2011, August). Fate of endocrine disruptors following Long-Term Application of Class B Biosolids and Risks to Public Health. 3rd International Conference on Occurrence, Fate, Effects, and Analysis of Emerging Contamnants in the Environment. Copenhagen, Denmark.