Automating Variable Rate Irrigation Management Prescription for Center Pivots from Field Data Maps

Abstract

Abstract. Variable rate irrigation (VRI) enables center pivot systems to match irrigation application to non-uniform field needs. This technology has potential to improve application and water-use efficiency while reducing environmental impacts from excess runoff and poor water quality. Proper management of VRI systems depends on correctly matching the pivot application to specific field temporal and areal conditions. However, translating field maps to VRI prescriptions can be difficult and time consuming. The objective of this study was to automate development of VRI pivot control programs based on GIS maps of field conditions. Specifically, a computer program written in Visual Basic was developed to match pivot system geometry and VRI capability (i.e. resolution of machine control for azimuth and sprinkler cycling) with GIS grid-based field data in a simple X, Y, Z format text file. It requires the user to input specific pivot system information including sprinkler wetted diameter and spacing, system flow rate, lateral length, sprinkler zone locations, and design application depth when set at 100 percent rotation speed. Examples of field data maps useful to develop prescriptions were made based on variations in soil texture. The program can be used on VRI systems with individual sprinkler control, defined sprinkler zones, or system rotation speed changes. Results for a specific field indicated that all three methods of VRI management resulted in reduced potential runoff, and that individual sprinkler and zone control provided the best reduction. VRI with speed control was improved when the inner portion of the pivot was not included in developing prescription maps. Irrigating the field as a standard pivot with no VRI and maximum application depth for the soil with highest sand content resulted in over 15% of the applied water being lost to potential runoff.