Drivers of evapotranspiration in Central Africa: investigating seasonality and change in interactions with soil moisture, and solar radiation

The present study uses 41 years of ERA5 and MERRA2 reanalysis data to assess Soil moisture-Evapotranspiration Interactions (SEIs) and the contribution of surface solar radiation (SSR) to evapotranspiration (ET) variability in Central Africa (CA). The study area is clustered using the k-means method, and the nature and strength of changes in ET induced by soil moisture (SM) and SSR are assessed. A comparative evaluation of the performance of MERRA2 and ERA5 in representing SEIs is also made. The results indicate that transitional areas (wet and dry areas) show a strong significant control of SM on ET, while very wet areas show a weak but almost significant sensitivity of ET to changes in both SM and SSR. MERRA2 shows an extreme sensitivity of ET to changes in SM, resulting in a failure to capture the competitive controls on ET exerted by surface versus root zone SM during wet and dry seasons, particularly in dry and very dry soil regimes. The two reanalysis datasets reveal a possible interaction between SM and SSR in very wet area, which could impact SEIs. A rapid drying process, accompanied by a hydrometeorological regime transition, may be currently underway in the CA region. The most significant and strongest transition seems to be occurring in very wet area. The seasonal meridional migration of the strong response of ET to changes in SM is associated with a switch in the control of ET between SM and SSR. The switch in control depends on soil water content, cloud cover and land cover. When considering the surface versus the root zone soil layer, a shared control of ET can be observed.