Utility of Copernicus-Based Inputs for Actual Evapotranspiration Modeling in Support of Sustainable Water Use in Agriculture

Quantifying spatial and temporal patterns of the actual evapotranspiration (ET) using Earth observation data can significantly contribute to the accurate and transparent monitoring of sustainable development goals (SDGs) target 6.4, which focuses on the increase of the water-use efficiency and sustainable freshwater withdrawals. Irrigated agriculture is by far the largest consumer of freshwater worldwide, and ET can serve as a direct proxy of crop water use. Various ongoing initiatives encourage the use of remote sensing data for the monitoring of SDG 6.4, including the WaPOR portal run by the Food and Agriculture Organization of the United Nations. However, none of these initiatives use Copernicus satellite and modeled data to the fullest extent. Copernicus provides operational high-quality data freely and openly, contains all the inputs required for ET modeling, and has long-term continuity and evolution plans, thus allowing for the establishment of baseline for SDG 6.4 and continuous monitoring in mid- and long term. In this study, we evaluate the utility of Copernicus data for this task with WaPOR products serving as a comparison benchmark. Thus, the modeled ET has to be able to accurately capture the field-scale activity at 10-day timesteps while also scaling to national coverage and providing consistent estimates at different spatial resolutions, ranging from tens to hundreds of meters. Results indicate that Copernicus-based ET can reach a correlation of 0.9, mean bias of 0.3 mm/day, and root-mean-square error of less than 1 mm/day when compared against the field lysimeter and eddy covariance measurements, and with proper approach, can achieve a better spatial-scale consistency than WaPOR data. This sets a path toward the Copernicus-based ET product and its use within the SDG monitoring and reporting.