Two source energy balance maize evapotranspiration estimates using close-canopy mobile infrared sensors and upscaling methods under variable water stress conditions

Mobile infrared thermometers (IRTs) mounted on moving platforms provide one-time-of-day radiometric measurements (T_r), which can be used to calculate instantaneous actual evapotranspiration (ET_a) using the two-source energy balance (TSEB) model. However, irrigation scheduling decisions utilize daily ET_a estimates, hence the need for time scaling. This study evaluated different upscaling methods to calculate daily maize ET_a using one-time-of day T_r under varying water stress conditions. Mobile IRTs were mounted on a high clearance mobile sensing platform and collected T_r in remote locations under full, deficit and rainfed conditions. Seven scaling methods via two pathways were employed to obtain daily ET_a. First pathway was scaling one-time-of-day T_r (SC) whereas the second pathway involved use of six upscaling methods of instantaneous ET_a including: original and modified evaporative factor ((EF)_o, (EF)_m) as well as crop coefficient ((K_c)_o, (K_c)_m), direct canopy resistance (Direct- r_c), and solar radiation ratio (R_n/R_s); and all were compared to a neutron-based soil water balance (SWB) determined ET_a. From the results, SC outperformed other methods in comparison to SWB ET_a across all the selected treatments with smaller discrepancies and lower RMSE (0.9–1.7 mm d⁻¹ vs. 0.7–4.3 mm d⁻¹ for other methods). Furthermore, methods including SC, (EF)_o, (EF)_m, and R_n/R_s had their daily average ET_a values in close agreement to SWB ET_a with mean ET_a differences ranging between 0.2 and 1.6 mm d⁻¹. Overall, SC method performed better in fully irrigated maize (r² = 0.52, RMSE = 0.9 mm d⁻¹) than in deficit irrigated maize ( r² = 0.48, RMSE = 1.4 mm d⁻¹) but worst in rainfed maize (r² = 0.16, RMSE = 1.7 mm d⁻¹). This implies that SC is more suited for irrigated rather than rainfed settings. Importantly, the choice of any method depends on data requirements, irrigation water management strategy, and ET_a estimation accuracy.