Enhancements to the WRF-Hydro Hydrologic Model Structure for Semi-Arid Environments

The NOAA National Water Model (NWM), operated by the NOAA National Weather Service (NWS) Office of Water Prediction (NWS/OWP), is an operational system, which provides hydrologic guidance throughout the Contiguous United States (CONUS). Based on the WRF-Hydro model architecture developed by the National Center for Atmospheric Research (NCAR), the NWM as it is currently configured does not explicitly resolve infiltration from ephemeral channels into the underlying unsaturated soil layers. Channel infiltration is an important source of groundwater recharge and can be a significant component of the water budget in the region. As a result of this limitation, the NWM has difficulty simulating low water conditions in many basins in the southwest CONUS, despite multiple iterations of calibration. Previous work by Lahmers et al. (2018, manuscript submitted to J. Hydromet.) demonstrated the value added of a conceptual channel infiltration function, based on that of the KINEROS2 semi-distributed hydrologic model, for streamflow forecasts from NWM v1.1 in four basins in Arizona.

NWM v1.2, with an added channel infiltration function, is calibrated in 50 basins around the western CONUS. As in previous NWM development work, model calibration utilizes 500 iterations of the Dynamically Dimensioned Search algorithm, optimizing the model using the Kling-Gupta Efficiency (KGE) statistic for hourly streamflow. The model is forced with regridded NLDAS-2 atmospheric variables and NCEP Stage-IV precipitation. Our results and analysis consider 1) performance of the calibrated NWM with channel infiltration around the western CONUS and limits of the calibration configuration, 2) interchangeability of calibrated parameters to basins within the same eco-region, and 3) fidelity of other model states (soil moisture and evapotranspiration) compared to observations. Results suggest that the calibrated model with channel infiltration is able to realistically reproduce the hydrologic responses in both the calibration and validation periods in basins around the southwest CONUS; however, some basins are subject to forcing precipitation errors. This testing of the NWM v1.2 calibration methods and evaluation of model states may be used to inform further development of WRF-Hydro for both operational and experimental purposes.