Integrating reservoirs and lakes in the CoSWAT global hydrological model

Abstract. 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+. Moreover, to model irrigation reservoirs, we implemented an approach that combines global datasets with a topological method to estimate irrigation demand for each reservoir. With these new implementations, the CoSWAT model was restructured for selected regions worldwide, where validation of reservoir or lake storage, inflow, and outflow was performed, and the impact of these implementations on streamflow performance was assessed. The results show that the model captures storage dynamics with reasonable performance, comparable to other state-of-the-art global models, and demonstrate a general improvement (70 % of evaluated stations) in streamflow representation following the integration of these water bodies. The new 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.