Implementation of a new lake scheme in the Weather Research Forecasting Model Hydrological modeling system (WRF-Hydro)

Lakes and reservoirs (herein lakes) are key elements of geophysical systems, playing an important role in ecosystem function, natural resource management, water supply, and flood control yet they are often neglected or oversimplified in modeling and research. For example, in large-scale fully-distributed hydrological models such as the National Water Model (NWM) lake routing is limited to lumped water budget estimations in terms of net lake inflows and outflows. However, the representation of important physical processes such as thermal structure and surface fluxes (i.e., precipitation and evaporation) have been mostly neglected. To address these challenges, we adopted the Weather Research Forecasting Model Lake modeling scheme (WRF-Lake) to improve the representation of lake physical processes in the WRF Hydrological modeling system (WRF-Hydro). The upgraded WRF-Hydro model is capable of simulating lake temperature and ice formation in the water column, and accounts for surface evaporation and precipitation in the lake water balance estimation. This study focuses on the initial model development steps and validation of the new lake module in WRF-Hydro. The model was tested for the April-June period in 2020 for Standley Lake in Colorado, US, where direct measurements of lake surface temperature and evaporation are available. Preliminary results showed that over the validation period, the surface temperature and evaporation were estimated with a normalized root mean squared error of 0.48 and 0.63. Development and implementation of this new capability helps close a significant gap in closing hydrologic system water and energy budgets and should help lead to improved understanding of basin scale water budgets who have large internal water bodies.