A WRF-Hydro-based retrospective simulation of water resources for US integrated water availability assessment

Abstract. A systematic and periodic evaluation of water supply across the United States is critical for gaining comprehensive insights into the present state of the nation's water resources and strategically planning for the future. The U.S. Geological Survey (USGS) Integrated Water Availability Assessments (IWAAs) is a national initiative designed to characterize past, present, and future water availability at selected basins in the United States. The Weather Research and Forecasting model hydrological modeling extension package (WRF-Hydro) is one of the selected hydrologic models used to generate an estimate of national hydrological fluxes and storage across the conterminous United States (CONUS). The WRF-Hydro application is being forced using the state-of-the-art CONUS404 dataset, a regional hydroclimate dataset over the CONUS, and evaluated over water years 2010–2021. Calibration leads to substantial improvements in simulated streamflow across most of the CONUS. Following parameter regionalization, streamflow performance is reasonable at USGS gages, particularly in the eastern and western regions. However, certain challenges arise in the central US, Arizona, and south Florida, where the model exhibits poor performance. The observed shortcomings in these regions can be attributed to a combination of deficiencies within the framework of the model code, its configuration and atmospheric forcing errors, with a specific emphasis on temporal accuracy issues.

Throughout the CONUS, WRF-Hydro IWAAs based simulations of snow water equivalent closely align with the Snow Data Assimilation System (SNODAS) during the snow accumulation season but show low biases during the snow ablation season. WRF–Hydro IWAAs based actual evapotranspiration (ET) simulations generally exhibit close agreement with Global Land Evaporation Amsterdam Model (GLEAM) ET estimates when comparing cumulative distribution functions across CONUS. Despite this overall agreement, simulated WRF-Hydro IWAAs ET is higher in parts of the central US and lower in parts of the northeast, southeast, and northwest regions of the US, and in urban areas when compared to GLEAM. There is a strong agreement between WRF-Hydro IWAAs based simulations and GLEAM surface soil moisture (top 10 cm) values, with the WRF-Hydro IWAAs model simulating some higher estimates particularly over the eastern US. Similarly, simulated WRF-Hydro IWAAs root-zone soil moisture is underestimated in the southeast US while there are positive biases observed in the western US, relative to the GLEAM simulations. These comparisons to independent datasets indicate the WRF-Hydro application developed for the USGS IWAAs is producing reasonable simulations in many locations across CONUS but is over- or underestimating model variables in some regions.