Wheat irrigation management using multispectral crop coefficients: I. Crop evapotranspiration prediction

A method widely used for irrigation management determines crop evapotranspiration (ET _c) from reference evapotranspiration (ET _o) calculations and estimated crop coefficients. However, standard time-based crop coefficients may fail to represent the actual crop water use, for example, when deviations in weather or agronomic constraints appreciably change crop development patterns from typical conditions. In this study, the FAO-56 dual crop coefficient procedures were applied during experiments with wheat to calculate the estimated ET _c for irrigation scheduling. The objective of this research was to determine whether basal crop coefficients (K _cb) determined from a normalized difference vegetation index (NDVI treatment) improve the prediction of ET _c over a standard application with a locally developed time-based K _cb curve (FAO treatment). The experiments conducted for two seasons in central Arizona included subtreatments, equally replicated within the NDVI and FAO treatments, of three plant densities (typical, dense, and sparse) and two nitrogen levels (high and low) to provide a range of crop development and water use conditions. The effects of plant density and N level resulted in significant differences in measured seasonal ET _c. Large variations that occurred in the observed K _cb and ET _c trends between subtreatments were better correlated with the NDVI than the FAO treatment. The mean absolute percent difference for predicted ET _c was significantly smaller for NDVI than FAO during both seasons. The treatment difference was 5% for the first season, but 10% for the second season when an unexpected early decline in ET _c and K _cb was effectively predicted by the NDVI treatment but not by the FAO treatment. NDVI appears to be a robust approach for K _cb estimation of wheat, able to reliably predict actual ET _c for both typical and abnormal water use conditions.