Abstract. The use of conceptual hydrological models for projections of future freshwater resources is challenged by non-stationary climate conditions, as these conditions may affect whether models calibrated under historical climates are suitable for future scenarios. This study aims to (i) develop a framework for the parameterization of a conceptual hydrological model under non-stationary climate conditions and (ii) bias-correct, downscale and evaluate the performance of an 18-member ensemble of Regional Climate Models (RCMs) for simulating future streamflow. The framework was applied to generate streamflow projections for 38 mountain watersheds in the eastern Mediterranean island of Cyprus over the next decades (2030–2060) with the GR4J model. Six Nash-Sutcliffe Efficiency (NSE)- and Kling-Gupta Efficiency (KGE)-based functions and a composite scaled score were used for model calibration and validation across multiple 5-year calibration and 5-year validation periods (1980–2015). Climate non-stationarity was represented by differences in total precipitation between calibration and validation periods (ΔP= Pval–Pcal) using the differential split-sample test approach. The best-performing parameterization during drier periods (ΔP≤-5 %) was obtained with an NSE objective function applied to square-root transformed streamflow, achieving NSE of 0.48, KGE of 0.54 and total streamflow bias of 9 % during validation. This optimized model was selected for future streamflow simulations. Nine RCMs were excluded from the impact assessment because they underestimated the fraction of wet period precipitation (60 %–73 % instead of 82 %), resulting in streamflow biases up to 40 % in the 1980–2010 reference period. The median of future projections for 2030–2060 shows a 6 % reduction in precipitation and a 17 % reduction in streamflow. In the worst case, reductions could reach 16 % and 39 %, respectively. Notably, during the driest years, streamflow reductions could reach 70 % relative to historical dry years. Our findings suggest that terrestrial water resources in Cyprus may decrease significantly in the coming decades.
Sofokleous et al. (Wed,) studied this question.