Exploring the physical consistency of evapotranspiration estimates over Madagascar using remote sensing.

Evapotranspiration (ET) plays a major role in climate processes by facilitating water redistribution between continental surfaces and the atmosphere. Accurately quantifying ET remains a challenge due to the scarcity of direct ET measurements, particularly in some regions with poor climate monitoring like Madagascar. Moreover the island has a very different climate, from semi-arid in the southwest to humid in the east. Estimating ET from remote sensing and climate reanalysis appears as a relevant way to provide spatially distributed data at a regional scale. Several operational or pre-operational usually, they providing quite different results. How to choose the most relevant product for a study area is a key question for any hydrological study.

Our study focused on evaluating five popular evapotranspiration products over Madagascar: GLDAS-NOAH, ERA5, ERA5-LAND, WAPOR, and GLEAM. The data covers years from 2009 to 2021. The analysis aims to provide a comprehensive understanding of their utility and accuracy in estimating ET over the different climatic zone.

Our initial findings involved a comprehensive assessment of various datasets, focusing on their differences and evaluating their validity in maintaining water and energy balance. This comprehensive analysis encompassed (i) analyzing jointly evapotranspiration estimates, potential evapotranspiration, and precipitation used by each ET dataset and (ii) validating ET estimates on the few catchments where streamflow data are available. The results indicate significant differences in ET estimates, as well as in each climate zone in Madagascar (in average 550 mm/year in semi-arid area and 1050 mm/year in humid area). The observed differences warrant a deeper exploration of the factors contributing to these differences and a careful assessment of the strengths and limitations of each datasets.