Application of an energy balance method for estimating evapotranspiration in cropping systems

Accurate quantification of evapotranspiration (ET, consumptive water use) from planting through harvest is critical for managing limited water resources for crop irrigation. Our objective was to develop and apply a land-crop surface residual energy balance (EB) method for quantifying ET and to estimate ET of corn (Zea mays L.) for the first time in the climate of the lower Mississippi Delta region. Actual ET (ETe) was estimated as the residual term of the energy balance equation from measurements of net solar irradiance (Rn) and computed sensible heat (H) and ground heat (Go) fluxes. The H flux was computed from measurements of the air and crop canopy temperature differential and modeling the aerodynamic resistance (ra) to heat and water transport in the turbulent atmospheric boundary layer above the canopy. The Go flux was estimated by measuring heat flux at 8 cm depth and accounting for heat storage in the soil layer above it. The developed EB procedure was tested using simultaneous measurements of EB data and lysimetric ET in a cotton (Gossypium hirsutum L.) field at Bushland, Texas, USA in 2008. The lysimeter measured ET compared well with the computed ETe under cotton (RMSE of daily ET = 1.2 mm, and seasonal ET within 1% error). Further, we quantified irrigated corn ET using EB in a silt loam soil at Stoneville, Mississippi, USA in 2016. The computed seasonal values of ETe were greater than shortgrass reference ET (ETo) by 27 mm and less than alfalfa reference crop ET (ETr) by 80 mm. The instrumentation used in the EB method can be moved, and the estimated ET was comparable with lysimeter measured ET. As such, this method provides a cost-effective, viable alternative for quantifying ET, which should be broadly tested in other locations and cropping systems.