Crop evapotranspiration calculation using infrared thermometers aboard center pivots

Irrigation scheduling using remotely sensed surface temperature can result in equal or greater crop yield and crop water use efficiency compared with irrigation scheduling using in-situ soil water profile measurements. Crop evapotranspiration (ETc) is useful for irrigation scheduling, and can be calculated using surface temperature. Recent advances in wireless infrared thermometers (IRTs) have made surface temperature measurement a viable alternative to in-situ soil water profile measurements, and wireless IRTs are practical for deployment aboard moving irrigation systems, such as center pivots. However, ETc calculation has not been tested using IRTs aboard center pivots in conjunction with recent advances in a two-source energy balance (TSEB) model. We compared daily ETc calculated by a TSEB model to daily ETc estimated by a simple soil water balance (SSWB), where the SSWB used volumetric soil water measured by a field calibrated neutron probe to the 2.4-m depth. Crops included two seasons each of corn (Zea mays L.), cotton (Gossypium hirsutum L.), and grain sorghum (Sorghum bicolor L.) at Bushland, Texas, USA. Discrepancies of TSEB vs. SSWB daily ETc were similar for each crop and season, and had root mean squared error from 1.5 to 1.8 mm per day, mean absolute error from 1.1 to 1.5 mm per day, and mean bias error from −0.51 to 0.63 mm per day. A sensitivity analysis was conducted for daily evaporation (E), daily transpiration (T), and ETc calculated by the TSEB model. These were most sensitive to radiometric surface temperature, air temperature, the reference temperature used in time scaling (i.e., to convert instantaneous to daily E, T, and ETc), and incoming solar irradiance. Because over half of the irrigated area in the USA is now by center pivot, ETc calculated using IRTs aboard center pivots will be useful to maintain or increase crop water productivity.