Computer control system for spatially varied water and chemical application studies with continuous-move irrigation systems

doi:10.1016/S0168-1699(99)00063-0

Computers and Electronics in Agriculture

Volume 24, Issue 3, December 1999, Pages 177-194

https://doi.org/10.1016/S0168-1699(99)00063-0 Get rights and content

Abstract

A microcomputer driven open-loop digital control system for implementing spatially variable water and chemical application from center pivot and linear-move irrigation systems is described. The control system is comprised of readily available commercial components and provides an effective means of implementing variable rate water and chemical application in crop production studies for evaluating potential benefits. The control system was implemented on a three-span 100 m linear-move irrigation system and on a 39-ha commercial center pivot irrigation system. Water and chemical application were monitored to document control system performance. The results show that spatially varied water and chemical application was achieved with the same accuracy as that of conventional uniform application. This paper provides details on the control system hardware and software and documents field performance.

Introduction

Increasing public awareness of environmental degradation and demand for clean air and water is putting pressure on agriculture to better manage soil and water resources. Irrigated agriculture, especially, is viewed as an industry that contributes significantly to excessive ground and surface water degradation through inefficient water management.

Current commercially available center pivot and linear-move irrigation systems are capable of applying relatively uniform controlled amounts of water and injected chemicals (i.e. chemigation) for efficient crop production. The systems are normally managed to apply a uniform water and chemical amount along the system lateral. Water application depth is determined by the system sprinkler parameters (i.e. pressure, nozzle size, spacing) and system travel speed. Chemical application through the irrigation system is controlled by the chemical injection rate and system travel speed. Commercial systems providing programmable control of travel speed allow one-dimensional control of water and chemical application.

Historically, efforts to improve linear-move and center pivot irrigation system design have primarily concentrated on reducing energy use by reducing operating pressure and increasing water application uniformity. Technological advances on both fronts are now to the point that the upper practical limits are being approached with very little return from incremental improvements. The next major advancement in center pivot irrigation technology will likely come from site-specific application of water and chemicals addressing the spatial variability inherent in irrigated agriculture landscapes.

Spatial variability in available soil water often develops during the irrigation season under conventional uniform irrigation. This can create problems in irrigation scheduling for optimum crop yield and quality, particularly for shallow-rooted, water-sensitive crops such as potatoes. Spatial variability in available soil water under a properly designed and managed conventional irrigation system is caused by spatial variability in evapotranspiration and water application efficiency. Evapotranspiration is largely dependent upon micrometerological conditions and crop growth, both of which vary spatially and temporally. Water application efficiency is influenced by many factors that vary spatially and temporally, but soil infiltration rate is likely the most important.

Considerable research and development is needed to realize the potential benefits of site-specific irrigation and chemigation and insure a net economic return to the producer. Cost effective and reliable equipment and control systems need to be developed and tested. Techniques for efficient and effective real-time system management need to be developed, field tested, and validated. Methodologies for predicting the potential environmental and economic benefit for a particular site are needed to facilitate adoption and implementation of the technology where appropriate. To begin addressing these needs, a control system was developed to implement variable rate irrigation and chemigation studies. The control system is constructed from commercially available components. The objective of the work reported here was to develop and test a computer control system for variable rate water and chemical application with a linear-move and center pivot irrigation system. The computer control system hardware and software are described and data demonstrating system performance are presented.

Section snippets

System description

The continuous-move irrigation systems used are representative of those commonly found in irrigated production systems of the Pacific Northwest and Intermountain West, USA. Typical sprinkler packages consist of medium pressure (138 kPa) fixed or rotating plate type sprinklers equipped with pressure regulators to minimize flow variation along the system lateral. Sprinklers were spaced 3 m apart on the linear-move system. The sprinkler spacing on the center pivot system changed from approximately

Control system testing

The control system has operated at various levels of development since 1990 on the linear-move irrigation system. During this time, two chemical applications were monitored in detail to document system performance.

In 1992, system chemigation performance was evaluated by injecting a KBr solution and measuring the amount of Br⁻ applied to each cell of an arbitrary chemigation map. A double sprinkler arrangement with the nozzles sized for 1/3 and 2/3 of the original sprinkler flow rate was used

Results and discussion

The results for the 1992 Br⁻ application with the linear-move system are summarized in Table 1. The mean relative water applications were 69, 35, and 0%, very near the target applications of 67, 33, and 0%, respectively. The target Br⁻ concentration in the applied water was 20 mg/l. The field average measured Br⁻ concentration was 19.03 mg/l with a standard deviation of 2.51 mg/l.

The important measure of performance is the actual Br⁻ mass delivered to each cell. The relative Br⁻ mass

Conclusions

A computer control system for implementing variable rate water and chemical application with continuous-move irrigation systems was constructed from commercially available components and readily implemented. A multiple sprinkler approach was used to achieve two-dimensional step-wise variable water application. This approach has proven to be highly reliable and facilitates variable rate chemical application as total system flow rate is easily estimated and remains steady. Chemical application

Acknowledgements

This material is based upon work supported by the Cooperative State Research Service, U.S. Department of Agriculture, under Agreement No. 93-34214-8861. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the U.S. Department of Agriculture.

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^☆: Approved as Paper No. 97303 and Paper No. 97305 of the Idaho Agricultural Experiment Station.

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Computer control system for spatially varied water and chemical application studies with continuous-move irrigation systems☆