Assessing crop evapotranspiration by combining ERA5-Land meteorological reanalysis data and visible and near-infrared satellite imagery

Measuring irrigation water volumes at national and regional scales is one of the priorities that the Italian legislation has identified among the requirements to support environmental policies and land monitoring. Crop evapotranspiration estimates under standard conditions represent a key component for the indirect evaluation of irrigation water volumes at regional scale. In this study, we present a way to assess crop evapotranspiration by combining visible and near-infrared (VIS-NIR) satellite crop imagery and meteorological reanalysis data, in the hypothesis that reanalysis products represent a valid proxy of past weather data when ground-based meteorological observations are missing. The study was conducted in Campania region (Southern Italy) where VIS-NIR high-resolution multispectral satellite crop images have been validated with ground LAI measurements performed in two maize fields during the irrigation seasons of years 2014 and 2015. For the same seasons, full sets of weather data were recorded by 18 automatic weather stations distributed across the region. The results show that the use of reanalysis data as proxy of past weather data for crop evapotranspiration estimates introduces acceptable errors (with overall RMSE of about 0.65 mm day-1) in the assessment of the crop evapotranspiration and clearly support the idea that for regions with limited past weather data archives or served by sparse and irregular monitoring networks, reanalysis data can be successfully exploited as a source of gridded weather data in similar agricultural and hydrological applications.