Integrating reservoirs and lakes in the CoSWAT global hydrological model

Global water models are essential tools for assessing water resource challenges in the context of climate change, land use changes, and human activities. The CoSWAT Global Water model is a global application of the Soil and Water Assessment Tool (SWAT+). It is a high-resolution tool designed to simulate water systems using a basin-oriented structure. The CoSWAT model currently lacks a realistic representation of reservoirs and lakes, limiting its ability to adequately represent basins where these water bodies play a significant hydrological role. The scarcity and limited accessibility of global reservoir operation or lake outflow data make it challenging to represent these water bodies with the current tools that SWAT+ supports, particularly at a global scale. In this study, we address this limitation by combining commonly used reservoir and lake modelling schemes from other global water models and the capabilities of SWAT+, and by implementing a topological approach to estimate irrigation water demands for reservoirs. We also introduce an improved workflow to integrate lakes and reservoirs into the river network, setting up the CoSWAT model at 0.01° resolution on selected regions worldwide, where validation of reservoir and lake storage, inflow, outflow, and the impact on streamflow performance was assessed. The results show that the model captures monthly storage dynamics with moderate performance. Overall, 34 % of evaluated reservoirs achieved a positive KGE, a performance comparable to other state-of-the-art global hydrological models, with CoSWAT surpassed by only two of five models in the comparison. Moreover, results demonstrate an improvement in performance in streamflow representation on 135 out of 192 (70 %) evaluated stations following the integration of lakes and reservoirs. Despite these improvements, persistent streamflow biases that consequently affect reservoir storage point to the need for hydrological calibration, which remains a priority for future developments. Overall, these methodological advancements represent a substantial improvement for the CoSWAT global model, enabling more robust and realistic assessments of inland water systems at the global scale.