The added value of considering evapotranspiration fluxes in the calibration of a semi-distributed hydrological model

Trust in hydrological model projections is often challenged by the model’s capacity to simulate streamflows under non-stationary conditions imposed by climate change. While many studies focus on uncertainties arising from climate scenarios or downscaling methods of climate projections, uncertainties linked to the hydrological model structure and calibration strategies remain comparatively overlooked. In this work, we investigate how model calibration choices influence the performance and robustness of a semi-distributed conceptual hydrological model under climate change. Beyond the traditional calibration based solely on discharge data, we introduce an additional constraint on actual evapotranspiration (ET) data to better represent the catchment’s water balance under historic climate, and enhance model transferability to simulate flows under future climate conditions. Our results show that the calibration strategy markedly affects future hydrological projections. The ET-constrained calibration strategy produces more spatially coherent fluxes and a more stable model response in non-stationary conditions. By better framing internal fluxes, this approach enhances the trustworthiness of hydrological projections under future climate scenarios, which is essential for water resources availability assessments and adaptive management.