Abstract

The U.S. National Water Model (NWM) is a hydrologic modeling framework that uses the Weather Research and Forecasting Hydrological modeling system (WRF-Hydro) to simulate land surface hydrology and energy fluxes at 1-km spatial resolution. Understanding the performance of the operational NWM in simulating evapotranspiration (ET) is necessary to identify problems and biases in streamflow forecasts that may result from poor partitioning of runoff and ET. In this study, we compared NWM ET fluxes against OpenET, a satellite-driven dataset that provides interpretive or diagnostic information on actual ET at 30-m spatial resolution. Monthly ET simulations from the NWM version 2.1 (NWM V2.1) retrospective analysis over the Bear River Basin (BRB), U.S. were compared against OpenET products from 2017 to 2020 for different months and seasons. Comparisons showed that there was general agreement between the ET assessments at the 1-km scale, but with notable discrepancies for some landcover types, such as irrigated agriculture and riparian areas. The NWM showed less spatial variability and tended to predict lower ET fluxes compared to OpenET, particularly from June to August. In comparison with water balance estimates of ET derived from precipitation and USGS streamflow observations in four sub-watersheds within the BRB, OpenET modeled ET was biased high in two watersheds dominated by evergreen forest. The results from this study provide useful information for both NWM and OpenET developers, demonstrating the power of comparing predictive and interpretive modeling systems. This study serves as a prototype for broader assessment of both NWM and OpenET via intercomparison. Plain Language Summary This study compared the retrospective U.S. National Water Model (NWM) version 2.1 evapotranspiration (ET) fluxes with OpenET, a satellite-driven data product offering actual ET information at 30-m resolution from 2017 to 2020, aggregated to match the 1 km NWM grid. Results indicated that the NWM tends to underpredict ET fluxes when compared against the different OpenET component models used in this study. OpenET showed a high bias in comparison with water balance assessments of ET in two natural sub-watersheds characterized by evergreen forest. Significant spatial discrepancies were observed in NWM results for certain landcover types, including irrigated agricultural lands, riparian areas, and in one watershed that appears to be mis-calibrated. Key Points: • Compared with OpenET, the U.S. National Water Model (NWM) tends to underpredict evapotranspiration (ET) fluxes in all seasons. • OpenET overpredicts ET in comparison to water balance estimates from observed streamflow and precipitation in two forested sub-watersheds. • Spatial discrepancies between NWM ET and OpenET were observed in irrigated lands, riparian areas, and one mis-calibrated watershed.