Comparison of estimated and observed evapotranspiration from farmland using inverse analysis and FLUXNET2015

Evapotranspiration (ET) is a critical concern for water management and hydrological cycle; thus, studies of ET have been performed to aid irrigation and water resource planning. Moreover, global warming-related studies are critical, as sensible heat contributes to warming, while the latent heat flux contributes to cooling. Recently, FLUXNET2015, a large energy flux dataset comprising climatic elements, was updated with a corrected heat balance relationship. In this study, we aim to applicability of the inverse analysis (IA) for estimating farmland ET. Practically, we evaluated the estimated ET (LEest) consistency using IA, which compared common climate data with observed data (LEobs) from US-Ne1 (irrigated), US-Ne2 (irrigated), and US-Ne3 (non-irrigated) land in FLUXNET2015. For an hourly time step, net radiation (Rn) and heat flux into the ground (G) were reasonably allocated into sensible (H) and latent (LE) heat fluxes, and LEobs was reasonably reproduced by LEest. For daily and monthly time steps, LEobs was reproduced well by LEest, with similar accuracies. For a yearly time step, LEobs was reproduced by LEest with an R2 of 0.933. Reasonability of the IA method also confirmed ET in crop growing season by comparing LEobs and LEest. A cooling effect under the canopy was observed on irrigated farmland in eight of the 22 analyzed years, whereas non-irrigated farmland did not exhibit a cooling effect. The maximum cooling effect was 4.26 °C of the monthly average. The results confirm that IA can be applied to non-irrigated and irrigated farmland if a cooling effect is not observed. IA can therefore be used to improve farmland water utilization because of accurate LEest and determining the capacities of irrigation facilities. The findings can be used to evaluate cooling effects on farmland, as well as reasonable allocations of Rn into H and LE, which promote the advancement of global warming issues.