Preparing for the SWOT mission in Alaska

NASA SPoRT is an Early Adopter for the Surface Water Ocean Topography (SWOT) mission, which will launch in 2021 to provide the first global inventory of terrestrial surface water. SWOT features a radar interferometer that provides 120-km swath measurements of water surface elevation (WSE) for rivers wider than 50 m, reservoir height, and inundation. SPoRT is investigating potential SWOT applications for Alaska, since SWOT orbit characteristics will provide measurements with much higher spatial coverage than is currently available with in situ stream gauges (Figure 1).

Figure 1. SWOT observable rivers (legend) in southcentral Alaska, number of SWOT obs. per 21 day repeat cycle (background; grayscale colorbar), and current United States Geological Survey (USGS) stream gauge sites (green dots).

SPoRT has developed a Weather Research and Forecasting Hydrological extension package (WRF-Hydro; Gochis et al. 2018) configuration in Alaska that mimics the NOAA National Water Model (NWM) conterminous United States (CONUS) configuration. The WRF-Hydro domain contains the upper Tanana River basin (upstream of Nenana) in central Alaska, as well as the Susitna River basin in southcentral Alaska (Figure 1). Figure 2 shows WRF-Hydro streamflow output for May-June 2015 and provides insight into streamflow responses to snowmelt. The animation shows two strong signals in streamflow related to snowmelt due to the seasonal rise in temperature and stemming from diurnal heating. Rapid snowmelt regularly contributes to flooding in Alaska, and monitoring streamflow in tributaries using modeling systems such as WRF-Hydro provides essential information for improving forecasts and enhancing situational awareness.

Figure 2. May-June 2015 WRF-Hydro streamflow for the upper Tanana River basin.

Using proxy SWOT data generated from an Observing System Simulation Experiment (OSSE), virtual stream gauges are used to quantify SWOT impacts on hydrological modeling in Alaska prior to mission launch (Figure 3). SPoRT has demonstrated the utility of SWOT in improving WRF-Hydro streamflow prediction through data assimilation (Elmer et al. 2018) and in calibrating hydrological models such as WRF-Hydro in ungauged (lacking in situ stream gauges) river basins (Elmer et al. 2019). Ongoing and future work seeks to integrate these new capabilities into the future NWM Alaska domain to enable immediate use of SWOT observations within operational systems immediately after launch.

Figure 3. Proxy SWOT virtual gauge WSE along the Chena River in Alaska derived from WRF-Hydro using an OSSE.

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