Ruixiu Sui

　　USDA-ARS Crop Production System Research Unit, Stoneville, MS 38776, USA

　　Precision irrigation (also called variable rate irrigation (VRI) or site-specific irrigation (SSI)) is a new irrigation method developed in precision agriculture. VRI technologies allow the producers to site-specifically apply irrigation water at variable rates within a field to adjust the temporal and spatial variability in soil and plant characteristics. Adoption of VRI has the potential to improve water use efficiency. VRI systems are commercially available. VRI technologies are normally implemented on self-propelled center-pivot and linear-move sprinkler irrigation systems. VRI practices require specialized hardware and software. The hardware requirements include GPS receiver to determine the spatial position of the irrigation system and intelligent electronic device to control individual sprinklers or groups of sprinklers to deliver the desired amount irrigation water on each specific location within the field. The software required includes the algorithms to calculate the water application rates and the computer programs to create VRI prescription maps. Two control methods can be used for VRI, speed control and duty-cycle control. The speed control method changes the travel speed of sprinkler irrigation system to vary water application depth. The speed control is able to change the application rate only in the travel direction of the irrigation system, not along the lateral pipeline, resulting in difficulty to develop VRI for randomly-shaped management zones to address the variability of soil and plant characteristics across the field. The duty-cycle control method changes the duty cycle of individual sprinklers or groups of sprinklers installed along the lateral pipeline. This method is capable of varying the irrigation rate both in the system’s travel direction and along the lateral pipeline, which offers more flexibility in development of VRI management zones. VRI practice requires a prescription map. A prescription map provides the information to the controller of VRI system for how much water to deliver at each specific management zone. The VRI prescription map should include spatial coordinates of each management zone and the irrigation water depth associated with each management zone within the field. Normally the prescription map can be created using the software with the VRI system. One or multiple inputs including soil properties, plant water stress, crop yield potential, field topography, and other relevant parameters can be used with geographical information system (GIS) software to delineate each management zone and determine the irrigation water application rate. These inputs can be obtained through various means including soil sampling, soil moisture monitoring, soil electric conductivity mapping, yield monitors, and remote sensor imagery. Development of algorithms and models using the various inputs for calculating the appropriate amount of water to apply is one of the important components in VRI technologies and a great challenge faced by VRI researchers.