Spatial assessment of surface-atmosphere fluxes in the Mediterranean region: synergy between satellite estimates and local observations

Evapotranspiration (ET) is a major component of both the hydrological cycle and the surface energy balance. Furthermore, ET is strongly linked to the primary production of natural and cultivated vegetation covers. Therefore, obtaining spatialized estimates of ET is of paramount importance in Mediterranean areas, submitted to hot and dry summers, all the more so as climate change is expected to worsen the water deficit in this region. In the framework of the preparation of the TRISHNA satellite mission, the objective of this study was (1) to produce maps of ET over a small Mediterranean region from high resolution satellites, and (2) compare these satellite estimates with local measurements of ET from flux towers. The studied area was located in the Hérault river area, south of France. During the period 2013 - 2019, 63 clear sky Landsat 7 and 8 images were collected and processed, having spatial resolutions of 60 and 100 meters, respectively. Maps of ET were generated using the EVASPA processing tool (Gallego-Elvira, 2013), with various methods for estimating the soil heat flux and the evaporative fraction. These satellite estimates were compared to those measured by long term flux towers installed on three biomes representative of Mediterranean landscapes: Puechabon (Quercus ilex forest on a rocky soil), Larzac (grassland on a limestone plateau) and Roujan (vineyards in the plain). In the estimation of ET from satellite images, intermediate variables (albedo, net radiation, soil heat flux, surface temperature) were first compared to those measured locally, when available. Finally, instantaneous and daily satellite estimates of ET were compared to the local measurements. Depending on sites and EVAPSA methods, the RMSE of instantaneous estimates of ET ranged between 39 and 202 W.m-2. The RMSE of daily estimates of ET ranged between 0.96 and 1.82 mm.day-1. Future work will be conducted to analyze the effect of air temperature variations, induced by altitude variations, on ET satellite estimates. This will allow to extend this study to larger regions of southern France.