Retrieving daily evapotranspiration from the combination of geostationary and polar-orbit satellite data

Near-real time estimates of evapotranspiration (ET) are generated every 30 minutes over the whole field of view of the Meteosat Second Generation (MSG) satellite. It is a product that has been designed and is operated in the framework of the Land Surface Analysis Satellite Application Facility (LSA-SAF). Its temporal resolution and continental coverage is unprecedented. Nevertheless, the spatial resolution of the LSA-SAF ET might be too coarse for ecosystem monitoring; certainly in fragmented landscapes. This study evaluated the feasibility of generating ET estimates at moderate spatial resolution (similar to 1 km) by incorporating vegetation data and using the spatial grid from a polar-orbit satellite platform in the estimation process. We used SPOT-Vegetation data and followed two methodological paths. The first method was based on including SPOT-Vgt data in the forcing of the LSA-SAF ET algorithm. The second method consisted in deriving an empirical relationship between daily ET and major ET covariates (radiation, vegetation, surface temperature and soil moisture). The estimates were contrasted against measurements at flux towers in different biomes across Europe. The resulting ET estimates exhibited general good correlation with the validation data and allowed the detection of periods of the growing season where improvement is needed. The performance of the first method was better in most of the cases.