Evaluating a reservoir parametrisation in a vector-based global routing model for Earth System Model coupling

Humans have fundamentally altered global river flow by constructing reservoirs and building water-diversion schemes for irrigation. Reservoir operation and the regulation of river flow is important for estimating global water fluxes and water availability. Reservoirs and dam management are however generally not represented in Earth System Models. Recently, efforts are made to incorporate human water management in Land Surface Models by improving the irrigation representation and including high-resolution river networks. Here, we present the integration of a reservoir routine in the vector-based river routing model mizuRoute, to be coupled with the Community Terrestrial Systems Model (CTSM). We use the Hydrologic Derivatives for Modeling and Applications (HDMA) vector-based river network, which is intersected with lake and reservoir polygons from the HydroLAKES and GRanD databases to model both natural lakes and reservoirs. We implement reservoir management based on the parametrization of Hanasaki et al. (2006) and develop an irrigation topology to determine the irrigation water demand for every individual reservoir based on gridded water demands modeled by CTSM. We then evaluate our reservoir implementation both in a local setup, driven by observed inflow for 26 reservoirs, and in a global-scale setup, driven by gridded runoff from CTSM and using the Hydrologic Derivatives for Modeling and Applications (HDMA) river network. The local simulations show that accounting for reservoirs improves the skill compared to resolving reservoirs with a natural lake parametrization and not accounting for lakes/reservoirs. In the global-scale simulation, the reservoir management and natural lake parametrizations show however similar results, which could be attributed to biases in modeled reservoir inflow. These biases originate from biases in runoff simulated by CTSM and/or unresolved reservoirs on the river network. This study overall underlines the need to further develop and test water management parametrizations for improving the representation of anthropogenic interference with the terrestrial water cycle in Earth system models. References: Hanasaki, N., Kanae, S., & Oki, T. (2006). A reservoir operation scheme for global river routing models. Journal of Hydrology, 327(1–2), 22–41 Mizukami, N., Clark, M. P., Gharari, S., Kluzek, E., Pan, M., Lin, P., Beck, H. E., & Yamazaki, D. (2021). A Vector-Based River Routing Model for Earth System Models: Parallelization and Global Applications. Journal of Advances in Modeling Earth Systems, 13(6).

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