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Comparison of Evapotranspiration from the National Water Model Retrospective Analysis with Remotely Sensed Estimates from OpenET
  • +9
  • Ayman Nassar,
  • David Tarboton,
  • Martha Anderson,
  • Yun Yang,
  • Gabriel B Senay,
  • Joshua B Fisher,
  • Adam J Purdy,
  • Furqan Baig,
  • Cenlin He,
  • David Gochis,
  • Forrest Melton,
  • John Volk
Ayman Nassar
Department of Civil and Environmental Engineering, Utah Water Research Laboratory, Utah State University

Corresponding Author:[email protected]

Author Profile
David Tarboton
Department of Civil and Environmental Engineering, Utah Water Research Laboratory, Utah State University
Martha Anderson
Department of Agriculture, Agricultural Research Service, Hydrology and Remote Sensing Laboratory, BARC-West
Yun Yang
Department of Forestry, Mississippi State University
Gabriel B Senay
United States Geological Survey Earth Resources Observation and Science Center/North Central Climate Adaptation Science Center
Joshua B Fisher
Joint Institute for Regional Earth System Science and Engineering, University of California Los Angeles
Adam J Purdy
Department of Applied Environmental Sciences, California State University Monterey Bay
Furqan Baig
Department of Geography & Geographic Information Science, University of Illinois at Urbana-Champaign
Cenlin He
National Center for Atmospheric Research
David Gochis
National Center for Atmospheric Research
Forrest Melton
NASA Ames Research Center Moffett Field
John Volk
Hydrologic Sciences, Desert Research Institute


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.
01 Dec 2023Submitted to ESS Open Archive
01 Dec 2023Published in ESS Open Archive