Variability and Trends of Actual Evapotranspiration over West Africa: The Role of Environmental Drivers.

This paper evaluates the atmospheric controls contributing to actual evapotranspiration (ET) variability and trends across the ecological zones in the data-scarce region of West Africa. Spatial and temporal ET dynamics in the region were analysed using three selected prognostic- and machine learning-based global ET over 1983 to 2012. In addition, the responses of annual actual ET to changes in annual rainfall, atmospheric evaporative demand (AED) and incident solar radiation were evaluated using joint time-frequency wavelet decomposition method and pixel-wise partial correlation analysis. Based on the Mann-Kendall statistics, results from the Global Land Evaporation Amsterdam Model (GLEAM) data revealed high variability (> 70 mm yr(-1)) and significant rising trends (> 5 mm yr(-1)) in annual ET in the Western Sahel. In contrast, results from CSIRO Penman-Monteith Leuning (PML) ET data showed declining trends by more than 4 mm yr(-1) in some parts of the Savannah zone during the study period. Results also showed some similarities in the coupling between annual ET, rainfall and AED, particularly along the Sahelian belt time-frequency domain. This implies that these atmospheric variables largely explain the signals in ET variations across the zones. However, the atmospheric variables have different modulating powers on the ET variations in the region. Additionally, we find that other non-climatic factors may have also influenced the variations in ET during the study period. This could serve as a pointer to focusing on other non-climatic drivers, potentially affecting ET trend and variability in the study area. Therefore, the study highlights the importance of providing essential and robust information on ET variability and trend as affected by different atmospheric drivers and its potential impacts on water resources, particularly in the Sahelian zone where agricultural biomes are often threatened by water stress and drought.