Assessment of three remote sensing methods for estimating actual evapotranspiration in a Mediterranean region

A semi-empirical method based on ancillary and remotely sensed data, NDVI-Cws, is currently applied over the Tuscany region to yield daily actual evapotranspiration (ETa) estimates at moderate spatial resolution (250 m) for a 20-year period (2005–2024). The outcome of this exercise is then statistically analysed in comparison with two similar ETa products provided by the MODIS and LSA SAF systems at 500 m and 5 km resolutions, respectively. The analysis relies on the triple collocation strategy and comprises the following two steps: i) examination and inter-comparison of the spatial and temporal ETa variations which occur in 12 areas of the region representative of different climatic conditions and biome types; ii) repetition of these operations at pixel level. The experimental results indicate the existence of clear spatial and temporal ETa variations over most of the region which are differently represented by the three ETa products. The MODIS ETa estimates are significantly higher and lower than those of the other two products for forest and non-forest areas, respectively; the ETa trends estimated by MODIS are poorly concordant with those of the other products, particularly for forests. Consequently, the MODIS ETa estimates reflect only marginally the ETa increases which are evidenced by the other two methods over most of the region. Out of these methods, NDVI-Cws allows a more spatially detailed prediction of the local ETa variability depending on the NDVI dataset used. The implications and consequences of these findings are finally discussed, together with the main future research prospects.