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Nitrogen management in surface irrigated VEGETABLE PRODUCTION systems
Dawit Zerihun, Charles A. Sanchez and K. Bali
Inefficient N-fertigation operations contribute to excessive water and nitrogen loss from irrigated lands as well as to nitrate contamination of water resources in the Southwestern U.S. The use of technically sound irrigation systems design and management approaches can bring about substantial improvement in water use efficiencies. However, the traditional system design and management approaches are inadequate in the context of integrated resource management operations - such as N-fertigation, a practice common in the desert Southwestern U.S. This project, therefore, seeks to develop new design and management approaches and guidelines, for N-fertigation operations in furrow irrigated vegetable production systems, through experimental and modelling studies.
The specific objectives of the project are: (1) To identify and define a self-contained set of performance indices that can adequately characterize the effectiveness of N-fertigation management systems. (3) To develop a numerical one-dimensional dispersion model that is capable of simulating the surface transport of conservative chemicals and NO3- and to couple the transport model with an existing surface irrigation hydraulics model. (2) To conduct field experiments to develop a database that will be used in the calibration as well as verification of the coupled surface hydraulics and mass transport models. (4) To perform N-fertigation management scenario analysis and to develop performance curves for improved management of N-fertigation operations. The principal contribution of this project will be the development of best management practices for the N-fertigation operations in furrow irrigated vegetable production systems of the low desert Southwestern U.S.
In the framework of the proposed project, a self-contained set of system performance indices will be defined. A numerical mass transport model will be developed, verified, and coupled with an existing surface irrigation simulation model. Field experiments will be performed to develop a database for model calibration and validation. The coupled mass transport and surface hydraulics model will be used to develop improved management guidelines for the N-fertigation operations in the Southwestern U.S.
Key words: N-fertigation, Irrigation, design and management
IntroductionIrrigated desert soils are commonly used for the production of high value horticultural crops. The low desert region of Arizona and California and environs represent more than 300,000 ha of irrigated cropland producing multiple crops each year. During the winter months, production from this area accounts for about 95% of the leafy vegetables produced in the United States. In the Southwestern U.S., irrigation is not only by far the largest consumer of water but also a very in efficient user. In 1990, irrigation of agricultural crops accounted for 81% (4.5 billon m3) of the over all water consumption in the state of Arizona (Arizona Department of Water Resources, 1991). Each year, over 3.4 billion m3 of water is withdrawn from the Colorado River to irrigate agronomic and vegetable crops in the Imperial Valley of Southern California (Imperial Irrigation District, 1999). This amounts to 95% of the water consumed in the area.
While scarcity of water in the arid Southwestern United States is a major impetus for improving water use efficiency in agriculture, inefficient irrigation practices are also of paramount significance in water quality related issues. Nitrate contamination of surface and ground water resources in the desert Southwestern United States is associated with inefficient irrigations in areas used for vegetable production (Adriano et al., 1977; Letey et al., 1978; Stark et al., 1983). Excessively high drainage water has been identified as an important factor affecting NO3--N loss through leaching below the root zone and tail-water runoff (Letey, et al., 1977; Sanchez et al., 1994). Consequently, agricultural interests in the region are being challenged to evolve into a more efficient and environmentally benign technology.
It has been reported by researchers in Arizona and California that pressurized irrigation systems can be used to manage water and N efficiently in irrigated vegetable crop production settings (Feigin et al. 1982; Hartz 1993; Pier and Dorege, 1995a; 1995b; Thompson and Dorege 1995a; 1995b; 1996a; 1996b). Nevertheless, practical considerations such as initial cost, complications associated with agronomic practices, and salinity management makes it likely that furrow irrigation will, for the foreseeable future, remain the principal irrigation method in vegetable production systems of the low desert regions of Arizona and California. Besides, recent research in the United States and Egypt has demonstrated that there is a potential for substantial improvement in furrow irrigation performance through the application of technically sound systems design and management practices (Clemmens et al., 1999, Strelkoff et al., 1999; Fangmeier et al., 1999). The use of surface hydraulics simulation-optimization models in systems design and management can help improve performance substantially (Singh, 1990; Alemi and Goldhammer, 1990; Zerihun et al., 1999a, 1999b). Such an approach is currently being used to enhance irrigation performance in the low desert Southwestern United States. However, the approach disregards the effects of irrigation design and management decisions on ancillary operations, such as N-fertigation.
While chemigation, including fertigation, is commonly practiced in conjunction with pressurized irrigation systems, its use in surface irrigation systems is expanding (Jaynes et al., 1992). Threadgill (1995) reported that about 4.6 million ha of land are chemigated at least once in the United States. In furrow irrigated vegetable production systems of the low desert regions of Arizona and California, nitrogen fertilizers are commonly applied via surface fertigation. In such systems, the spatial distribution and fate of N are closely intertwined with irrigation design and management decisions. Irrigation uniformity, efficiency, and target depth of application have a direct bearing on the efficient and uniform application of nitrogen. Mode of application of nitrogen (pulse, continuous) in an irrigation event, the timing of the fertigation event during an irrigation operation, the duration of the fertigaton and irrigation event, the concentration of N fertilizer applied, and the seasonal fertigation schedule all have an influence on the efficiency and adequacy of N over the season. Traditionally, on-farm water application methods are designed and operated with the express objective of maximizing irrigation performance without regard to other affiliated operations, such as fertigation. This project seeks to develop guidelines for the optimal management of irrigation water and water applied N in furrow irrigated vegetable production systems of the low desert Southwestern U.S.
IALC areas of research and development being addressed
The results of the proposed research project will enable growers in the Southwestern U.S. to better manage the combined water and N-fertilizer application operations in furrow irrigated vegetable production systems. Efficient N-fertigation operation will reduce soil, water, and nutrient loss from irrigated lands, thereby contributing to soil and water resource conservation and the enhancement of water quality.
The specific objectives of the project are outlined as follows: (1) To identify and define a self-contained set of performance indices that can adequately characterize the effectiveness of N-fertigation management systems. (2) To develop a numerical one-dimensional dispersion model that is capable of simulating the surface transport of conservative chemicals and NO3- and to couple the transport model with an existing surface irrigation hydraulics model. (3) To conduct field experiments to develop a database that will be used in the calibration as well as verification of the coupled surface hydraulics and mass transport models. (4) To conduct N-fertigation management scenario analysis and to develop performance curves for improved management of N-fertigation operations.
Literature reviewIn many engineered systems, including surface irrigation systems, mathematical models are the primary research as well as design, management, and analytical tools. During the last couple of decades surface irrigation hydraulic modeling has been an area of intensive research. Depending on the form of the momentum/energy equation used, surface irrigation models can broadly be classified into three major groups: the hydrodynamic, zero-inertia, and kinematic-wave models, all of which are based on the numerical solution of the continuity and a variant of the momentum/energy conservation equation (Bassett and Ftzsimons, 1976; Sakkas and Strelkofff, 1974; Strelkoff and Katopodes, 1977; Katopodes and Strelkoff, 1977; Elliott et al., 1982; Walker and Humphereys, 1983; Bautista and Wallender, 1992; Walker, 1993; Strelkoff et al., 1997). A fourth class of surface irrigation model is the volume-balance model, which is based on the analytical or numerical solution of the spatially and temporally lumped form of the continuity equation, while the dynamic equation is supplanted by gross assumptions (Lewis and Milne, 1938; Davis, 1961; Hall, 1956; Philip and Farrel, 1964; Christiansen et al., 1966; Walker and Skogerboe, 1987). Although the literature in surface irrigation is voluminous, currently only two surface irrigation models, SRFR (Strelkoff et al., 1997) and SIRMOD (Walker, 1993), are commonly used by researchers in real-life applications. These models have been extensively validated, have well developed user-interface, and have capabilities to analyze the effects of various management scenarios. In addition, SIRMOD and SRFR have capabilities to simulate processes in any of the three primary surface irrigation systems at three levels of complexity and accuracy (the hydrodynamic, the zeroinertia, and the kinematic-wave models) in the framework of a single integrated model.
Numerical solutions of the one-dimensional dispersion (turbulent diffusion and differential convection) equation have been used successfully to model the transport of conservative chemicals in watercourses and canals (Brebion et al., 1971, Krenkel and Novotney, 1980; Cunge et al., 1980). In the context of surface transport modeling of NO3- fertilizers in irrigation furrows, NO3- can be treated as a conservative chemical. In light of the fact that the reduction of NO3- to N2 gas through denitrification or the assimilation NO3- by microorganisms requires considerably longer time than the hydraulic residence time of NO3- in the irrigation stream, such an assumption is not unrealistic. Moreover, provided N-fertigation systems are designed such that the fertilizer solution is well mixed with the irrigation stream at the upstream end of irrigation furrows, an approach similar to that used for water courses and canals can be used to model the transport of NO3- in the surface stream. In fact, furrows are miniature channels in which changes on the transport and hydrodynamic variables in the transverse and vertical direction are insignificant in comparison with the longitudinal direction, hence the one-dimensional approximation is highly pertinent. Nevertheless, modeling transport processes in surface irrigation setting is in its infancy. Strelkoff et al. (1997) have developed a quasi-steady erosion, sediment transport, and deposition model coupled with a surface irrigation simulation model, SURFR. Playan and Faci (1997) reported an experimental and a (simplified) modeling study of the fertigation process in border irrigation. Playan and Faci reported that the performance of their simplified model, which treats the irrigation border as a "plug flow reactor", did not perform well in predicting NO3- transport processes. The work of Playan and Faci (1997) and the tracer studies conducted on irrigation basins by Jayens et al. (1992) and on irrigation furrows by Izadi et al. (1996) are indications of the growing research interest, among engineers, in the efficient management of surface irrigated N-fertigation systems.
Real-life data is required in order to calibrate and verify the surface hydraulic and transport models. There are a variety of methods to estimate the hydraulic parameters, mainly: infiltration, roughness, and furrow geometry parameters (Strelkoff et al., 1999; Elliott and Walker, 1982; Katopodes et al., 1990; Walker and Busman, 1992; Bautista and Wallender, 1993). Currently, the approach proposed by Strelkoff et al. (1999) is most suitable for real-life applications. Strelkoff et al. have developed an operational parameter estimation model called EVALUE, which is simple and has already been used in a real-life irrigation setting in Egypt. The principal transport parameter of the one-dimensional dispersion model is the longitudinal dispersion coefficient. In most practical applications, the longitudinal dispersion coefficient is determined empirically based on cross-sectional average concentrations measured at regular temporal and spatial intervals (Elder, 1959; Brebion, 1971).
The ultimate use of mathematical models in engineering applications is to help analyze, design, and manage systems for "optimal" performance. Evidently, quantitative performance indices are required to evaluate alternate design and management scenarios. While the definition, related assumptions, as well as methods of quantification of surface irrigation performance indices have been the subject of various past studies (Zerihun et al., 1997, Burt et al., 1997), evaluation of system performance from the perspective of system effectiveness vis-à-vis water and nitrogen application is a relatively new area of research. Existing surface irrigation performance indices are inadequate to characterize the performance of N-fertigation management systems. Consequently, there is a need to identify and define a new set of performance indices for the integrated water and fertilizer application operations. In this endeavor, the methodology used by Zerihun et al. (1997) and Burt et al. (1997) to identify and describe a self-contained set of irrigation performance indices can be used to advantage.
The proposed research project aims at the development of best management practices for the integrated furrow irrigation/nitrogen management system. A complete set of performance indicators for the integrated resource management system will be identified. A numerical model that solves the one-dimensional dispersion equation will be developed and verified and will be coupled with an existing surface irrigation simulation model. Field experiment will be performed to develop a database for use in the calibration and validation of the coupled surface hydraulics and chemical transport models. The coupled nitrogen transport and surface irrigation simulation model will be used to develop improved management guidelines for N-fertigation operations in the Southwestern U.S.
Identification, description and quantification of a self-contained set of performance indices: A couple of indices are required to fully characterize the performance of a given N-fertigation event. In this study, a self-contained set of performance indices for the integrated resource management system will be identified and defined and methods to quantify them will be developed. An approach similar to that used by Zerihun et al. (1997) will be used.
Mathematical modeling of the combined resource management system: Mathematical models will be used to develop best management practices for the integrated irrigation-nitrogen-fertilizer management system for the Southwestern United States. A surface irrigation hydraulics model that simulates the complete cycle of a surface irrigation event forms the hydrodynamic basis for the chemical transport model. SRFR (Strelkoff et al., 1997), a surface irrigation simulation model developed at the US Water Conservation Laboratory, in Phoenix, will be used in this study.
A one-dimensional dispersion (turbulent diffusion and differential convection) model will be used to describe the mass transport of chemicals in the surface irrigation stream. The governing partial differential equation will be solved using a finite difference scheme (Holly and Preissmann, 1977; Cunge et al, 1980). The mass transport model will be coupled with the surface irrigation simulation model. For each time line, mean cross-sectional velocity and flow depth (discharge and flow cross-sectional area) as well as infiltration calculated for each computational node using SRFR forms the input matrix for the chemical transport model. The resulting model will have the capability to predict N losses in surface runoff and the potential for N leaching from the root zone (the latter will be estimated as a function of deep percolation and mean cross-sectional concentration).
Field experiment and the determination of surface hydraulic and transport parameters: Field experiments will be performed at the University of Arizona Yuma Agricultural Center (YAC) in Yuma and at the University of California Desert Research and Extension Center (DREC) near Holtville. The primary objective of the experimental study is to determine model parameters. Among the furrow irrigation system hydraulic parameters: hydraulic resistance, infiltration, and furrow geometry parameters are the most difficult to quantify. In this study, the method of Strelkoff et al. (1999) is to be used in the determination of all these parameters. To estimate the transport parameter, particularly the dispersion coefficient, advance and recession as well as concentrations of the nitrogen fertilizer and the bromide tracer will be measured at regular spatial and temporal intervals during each irrigation event. The longitudinal dispersion coefficient will be estimated empirically using the method proposed by Elder (1959) and Brebion (1971). Concurrently, tracer studies, using a Br- conservative tracer, will be performed to estimate the potential for nitrogen leaching by drainage water. In addition, a database will be developed for the verification of the coupled surface irrigation and chemical transport model.
Model validation: Water advance and recession data will be used to evaluate the predictive quality of the surface irrigation hydraulics model. The transport model will be validated by comparing the field observed and calculated advance and recession of the chemical plume, the temporal and spatial evolution of the concentration of the chemical in the surface stream, and the potential for leaching of nitrate. Based on results of model validation, further changes could be made to the models.
N-fertigation management scenario analysis and development of performance curves for N-fertigation systems management: Once the coupled surface hydraulics and chemical transport models are validated, they will be used to study the effects of different management scenarios. The coupled model will be used to study the effects of (1) mode of application (pulse, continuous, intermittent), (2) timing of application in an irrigation event, (3) concentration of fertilizer solution, (4) inlet flow rate, and (5) cutoff length or cutoff time. In addition, the mathematical model will be used to evaluate the effects of variations in furrow length, infiltration, roughness parameters, and bed slope on uniformity, adequacy, and efficiency of nitrogen fertilization. The models will further be used to develop performance curves via simulation experiments for use in the optimal management of fertigation systems.
Field experiments will be conducted in the fall/winter seasons of the year 2000 and 2001 at each of the experimental farms of the University of Arizona Yuma Agricultural Center (YAC) in Yuma and at the University of California Desert Research and Extension Center (DREC) near Holtville. The experiment in Yuma will focus on the N-fertigation of diked-end furrows, while the experiment in Holtville will be performed on freely-draining furrows.
During each irrigation event, a three level complete factorial experiment, with respect to inlet flow rate and fertilizer concentration, will be performed. In addition, a two level complete factorial experiment will be conducted, with respect to inlet flow rate and Br- concentration, for the Br- tracer study. As can be seen from Table 1, during each irrigation event thirteen equally spaced furrows will be used in the N-fertigation and tracer study. In order to take into account the effect of time-to-the-initiation of fertigation on fertigation system performance, the timing of the fertigation event will be varied throughout the irrigation seasons.
Table 1. Experimental design
Qmax = maximum nonerosive flow rate, AP = application time-irrigation (observe that NO3- concentration is controlled by duration of fertigation), X = 10 g m-2 of Br-.
Description of responsibility of each collaborator
Dr. Dawit Zerihun and Dr. Charles Sanchez will be responsible for the experiments to be conducted at the University of Arizona Yuma Agricultural Center. Dr. Khalid Bali will be in charge of the experimental work to be performed at the University of California Desert Research and Extension Center. Dr. Zerihun and Dr. Sanchez will also be responsible for data processing and database construction, model building and verification, N-fertigation management scenario analysis, as well as the development of the N-fertigation system management curves.
Schedule of work
The project will be undertaken in three phases spanning over three calendar years. The first phase (between May 1999 September 1999) of the project will deal with the detailed planning of the various elements of the project and procurement of supplies and necessary equipment. In addition, the fundamental components of the research, such as model building and performance indices identification and description, will be initiated during this period. The second phase will be launched in September 2000 and ends in April 2001. The field experiments in all the collaborating institutions are to be conducted
during the fall/winter seasons of 2000/2001 and 2001/2002. Concurrently, data processing and analysis as well as model development activities will be performed during this period at the Yuma Agricultural Centre. The last twelve months of the project life (October, 2001-October 2002) will be devoted to model calibration and validation, management scenario analysis, performance curve development, and writing and submission of report. The first and second interim reports will be submitted on January 2001 and January 2002, respectively. A detailed outline of the project schedule is shown in Table 2.
Description of investigators institutional support
The University of Arizona Yuma Agricultural Research Center is located in the lower Colorado River Valley, Southwestern Arizona. The Agricultural center has excellent surface and pressurized irrigation system research facilities. An extensive network of concrete lined field supply canals is used to convey and distribute Colorado River water to irrigated fields. The field supply canals are equipped with the necessary water measuring and control structures. The Yuma Agricultural Center also has a well-equipped chemical analysis laboratory. In addition, the experimental farm is equipped with the necessary farm machinery required to produce winter vegetables (Disk plows, chisels, harrows, planters, cultivators, laser leveling equipment, etc.). The farm has a staff of knowledgeable technicians, farm attendants, and a farm management. It also has a clerical staff to support resident research scientists.
Dr. Sanchez is Resident Director and Soil and Water Research Scientist at this research facility. Dr. Zerihun is an Irrigation and Environmental Engineer currently employed at the center and works jointly with Dr. Sanchez on irrigation and nonpoint source pollution control projects. Dr. Sanchez has been active in Soil and Water Research as related to vegetable production for the past 12 years. Dr. Zerihun has been active in surface irrigation modeling, design, and management as well as nonpoint source pollution control research over the last eight years. Dr. Sanchez and Dr. Zerihun have a team of educated and experienced research technicians capable of providing the necessary technical support in field and laboratory works.
List of current funded research and development
Item 1 Salaries
Item 2. Fringe benefits for staff at 22.4%.
Item 3. Supplies include:
Subcontract with Dr. Khaled Bali at the University of California
Travel among experimental sites for site instrumentation,
Curriculum vitae and List of relevant publications
Ph.D. 1995 Agricultural (Irrigation) Engineering, Catholic University of Leuven, Belgium
M.S. 1990 Irrigation Engineering, Catholic University of Leuven, Belgium
B.S. 1984 Agricultural Engineering, Addis Ababa University, Ethiopia
Currently working on part-time basis on an M.S. in Environmental
Present Research Associate in Irrigation and Environmental Engineering,
Yuma Agricultural Center, University of Arizona.
Postdoctoral fellow in Irrigation Engineering, Catholic University of
Graduate Assistant and Assistant Lecturer, Jimma College of
Current employment responsibilities
Currently working as a Postdoctoral Research Associate in irrigation and environmental engineering at the University of Arizona, Yuma Agricultural Center. My primary responsibility is the development of a decision support system for the optimal design and management of irrigation systems and minimization of irrigation induced nonpoint source pollution.
AWARDS AND HONORS
MEMBERSHIP OF PROFESSIONAL COMMITTEES
Member of the American Society of Civil Engineers Task Committee on Soil and Crop Hydraulic Properties
SELECTED REFEREED PUBLICATIONS