Understanding the spatial and temporal dynamics of soil water and crop water stress within a field is critical for effective Variable Rate Irrigation (VRI) management. Proper VRI can result in improved protection of the crop from early onset of crop water stress while minimizing runoff and drainage losses. The objectives of this study are (1) to examine zone delineation for informing irrigation recommendations from volumetric water content (VWC) and field capacity (FC) to grow similar or greater wheat yields with less water, (2) evaluate the ability to model soil and crop water dynamics within a season and within a field of irrigated winter wheat, and evaluate the sensitivity of crop water stress, evapotranspiration and soil water depletion outputs within a water balance model with Penman-Monteith evapotranspiration (ET) in response to adjusted soil properties, spring volumetric water content (VWC), and crop coefficient model input values. Five irrigation zones were delineated from two years of historical yield and evapotranspiration (ET) data. Soil sensors were placed at multiple depths within each zone to give real time data of the VWC values within each soil profile. Soil samples were taken within a 22 ha field of winter wheat (Triticum aestivum ‘UI Magic’) near Grace, Idaho, USA multiple times during a growing season to describe the spatial variation of VWC throughout the field, and to assist in modeling soil water dynamics and crop water stress through energy balance and water balance equations. Spatial variation of VWC was observed throughout the field, and on a smaller scale within each zone, suggesting the benefit of breaking portions of the field into zones for irrigation management purposes. Irrigation events were triggered when soil sensors detected low values of VWC, with each zone receiving unique rates intended to refill to zone specific FC. Cumulative irrigation rates varied among zones and the VRI approach saved water when compared to an estimated uniform Grower Standard Practice (GSP) irrigation approach. This method of zone management with soil sampling and sensors approximately represented the VWC within each zone and proved beneficial with effective reduction of irrigation rates in every zone compared to an estimated GSP. As such, there was a delay in the premature onset of crop water stress throughout some areas of the field. Variability in soil properties and spring soil moisture were key in giving accurate values to the model in order to make proper VRI management decisions. When assessing the model sensitivity, changing the inputs such as FC, wilting point (WP), total available water (TAW), spring VWC and crop coefficient (Kc) by -4 to +4 standard deviations away from their spatially average values, impacted the outputs of the model, with Kc having a large impact all three of the outputs. Further work is needed to improve the accuracy of representing VWC throughout a field, thus improving VRI management, and there is potential benefit in using a variable crop coefficient could to more accurate VRI management decisions from a soil water depletion model.



College and Department

Life Sciences; Plant and Wildlife Sciences



Date Submitted


Document Type





variable rate irrigation, winter wheat, zone delineation, field capacity, volumetric water content, soil sensors, evapotranspiration, soil spatial variability, crop coefficient, sensitivity analysis, soil water depletion model