Comparison of three crop water stress index models with sap flow measurements in maize

Both empirical and theoretical models have been widely used to calculate a crop water stress index (CWSI) - a metric often used to describe crop water status. The purpose of this study was to determine the accuracy, limitation, and uncertainty of an empirical (CWSI-E) and two theoretical models compared with sap flow measurement in maize. One theoretical model used a calculated aerodynamic resistance (CWSI-T1), and the other theoretical model used seasonal average aerodynamic resistance (CWSI-T2). Considering the uncertainty of crop coefficient and sap flow measurement, CWSI-T2 and CWSI-E models gave reasonable overall estimates of water stress. The average root mean square deviation at each growth stage from each model ranged from 0.16 to 0.33. CWSI-T2 and the CWSI-E provided relatively accurate prediction of crop stress, both between growth stages and irrigation events. However, CWSI-T1 did not accurately predict water stress between growth stages or between irrigation events. By including climate factors, crop water stress estimated by CWSI-T2 showed less variation and uncertainty than CWSI-E. The uncertainty of both CWSI-T2 and CWSI-E decreased with increasing vapor pressure deficit (VPD), and CWSI-E show larger crop water stress prediction uncertainty. The intercept of non-water stress baseline was the main source of the uncertainty for CWSI-E and CWSI-T2. Considering both uncertainty and stability, we recommend CWSI-T2 model (i.e., seasonal average aerodynamic resistance) for maize water stress assessment.