Sensitivity analysis of modeled soil water dynamics within variable rate irrigation zones for winter wheat

Abstract

Understanding within-field and season variability of soil water supply and crop water stress is critical for successful variable-rate irrigation (VRI) management. The objectives of this study were to (1) assess within-field variability of field capacity (FC), wilting point (WP), total available water (TAW), and initial soil volumetric water content (VWCinit); (2) validate a water balance model based on the American Society of Civil Engineers (ASCE) standardized Penman-Monteith estimation of reference evapotranspiration (ET0) using measured within-field VWC; and (3) evaluate the model sensitivity to FC, WP, VWCinit, and crop coefficients. A 22-ha field of winter wheat (Triticum aestivum L.) near Grace, Idaho, United Sates of America was delineated into zones and managed with VRI. Spatial variability of soil water characteristics among 102 sites was measured in the field: FC (355–488 mm), WP (103–153 mm), TAW (230–361 mm), and VWCinit (325–464 mm) in a 1.2 m soil profile. Model validation against measured VWC resulted in root mean square error (RMSE) values of 23.2–61.3 mm for different sampling dates. These RMSE values showed the ability to model within-field spatially variable soil water dynamics and crop water stress. The model showed high sensitivity of predicted ET and soil water depletion output values to FC and VWCinit inputs, but lower sensitivity to WP. The sensitivity analysis also suggested spatially variable crop coefficients improve prediction of spatially variable ET rates. The model validation highlights opportunities for a spatially variable modelling approach, while the model’s sensitivity to soil and crop parameters highlights practical limitations to scheduling VRI.