Evaluation of Retrospective National Water Model Soil Moisture and Streamflow for Drought-Monitoring Applications

The National Oceanic and Atmospheric Administration (NOAA)'s National Water Model (NWM) provides analyses and predictions of hydrologic variables relevant to drought monitoring and forecasts at fine time and space scales (hourly, 0.25-1 km). We present results exploring the potential for NWM soil moisture and streamflow analyses to inform operational drought monitoring. Both agricultural and hydrologic drought monitoring rely either explicitly or implicitly on an accurate representation of anomalous soil moisture values. Much of our analysis focuses on comparisons of soil moisture anomalies in the NWM to those from in-situ observations. To establish benchmarks for NWM soil moisture skill, we also include other gridded data sets currently used to inform the US Drought Monitor, specifically those from the North American Land Data Assimilation System phase 2 (NLDAS-2) land surface models. We then compare NWM streamflow low flows with similar to 500 stream gauges from the United States Geological Survey (USGS) Hydro-Climatic Data Network of undisturbed basins. The NWM soil moisture simulation's skill parallels that from NLDAS-2. The accuracy of drought condition identification from NWM streamflow exceeds that based on soil moisture as determined by Critical Success Index scores for extreme dry percentiles. Different meteorological forcings are used in the operational NWM cycles than those used in this retrospective analysis. This forcing disconnect, together with concerns about current-generation land surface model soil moisture-transport schemes, inhibit its current operational use for drought monitoring. The National Oceanic and Atmospheric Administration's National Water Model offers output relevant for drought monitoring. This paper evaluates the National Water Model soil moisture and streamflow with drought applications in mind, and compares those evaluations to other modeling tools currently used to inform drought monitoring. We find the model's ability to estimate anomalous low flows exceeds its ability to estimate anomalously dry soils, and discuss its current potential to inform operational drought monitoring. Retrospective NOAA National Water Model soil moisture and streamflow were evaluated for drought-monitoring applications The National Water Model was comparable in skill to land model guidance currently used to inform the US drought monitor The National Water Model's operational forcing strategy currently limits its application for real-time drought monitoring