The Impact of Calibration Strategies on Future Evapotranspiration Projections: a SWAT-T Comparison of Three Hydrological Modeling Approaches in West Africa

Actual evapotranspiration (AET) is a key component of the water cycle and a crucial source of uncertainty in hydrological modeling, particularly for sub-humid and AET-dominant regions such as West Africa. In this region, climate change is projected to be substantial, which will catalyze hydrological changes. In the climate-hydrological modeling chain for impact assessment, multiple sources of uncertainty are embedded. Hence, the present study investigates how different calibration strategies influence simulated future hydrological projections in West Africa. Given the key role of AET in West Africa, the study particularly evaluates how calibration shapes simulated future AET dynamics. In addition, we test whether a specific plant growth modeling, attributed as leaf area index (LAI), can be used as a proxy to predict AET. The Bétérou Catchment in Benin is selected as a demonstration case along hydrological modeling with the eco-hydrological SWAT-T model. To investigate calibration impacts, we apply three strategies, which range from simple (discharge (Q) only) to more comprehensive (Q and LAI; Q, LAI, and AET) approaches. We apply the Robust Parameter Estimation algorithm in each calibration strategy to address parameter equifinality. We use the standardized future climate data from ISIMIP3b (CMIP6) with five GCMs and three emission scenarios and evaluate changes for the near (2031–2050) and far (2070–2099) future periods. The calibration results demonstrate that the combined “Q + LAI” improves representation of the AET seasonal pattern compared with “Q only”. From “Q only” to “Q + LAI”, the minimum (median) performance increases from EKGE,AET=0.59 (EKGE,AET=0.84) to EKGE,AET=0.81 (EKGE,AET=0.86), respectively. The findings moreover show that projected changes in annual AET depend on the calibration strategy, where all strategies indicate increasing future AET. More precisely, the “Q + LAI” (886 to 918 mm yr−1) and “Q + LAI + AET” (904 to 932 mm yr−1) approaches show similar climate sensitivities and project higher future annual AET rates than the “Q only” approach (846 to 889 mm yr−1). For discharge, contrasting predictions of future changes depending on single GCMs are simulated, mostly indicating decreases across all calibration strategies. The present study provides insights into how calibration choices affect AET simulations for hydrological projections in West Africa. While this study demonstrates that detailed LAI modeling can improve the simulation of seasonal AET dynamics, further research is required to evaluate whether LAI can be used as a reliable proxy for AET in hydrological watershed models.