Novel AirSWOT Measurements of River Height and Slope, Tanana River, AK
Altenau, E.H., Pavelsky, T., Moller, D., Lion, C., Pitcher, L.H., Allen, G.H., Bates, P.D., Calmant, S., Durand, M.T., and Smith, L.C. (13-Dec-16)
The Surface Water and Ocean Topography (SWOT) satellite mission aims to increase our knowledge of the global water cycle by providing 2D measurements of terrestrial and ocean water surface elevations (WSE). For the first time, SWOT will provide high accuracy heights and slopes of rivers 100 m wide and greater for the majority of the globe. With a planned launch date in spring of 2021, it is imperative to better understand the novel measurements SWOT will be recording. Here, we present new measurements of river height and slope from AirSWOT, the SWOT mission's airborne analog. To assess AirSWOT's ability to measure river slopes and heights, we used a survey-grade GPS mounted on a boat to measure WSE continuously along the centerline of an anabranching river in Alaska on June 9th, 2015. Additionally, we installed a network of 20 pressure transducers throughout the river to record temporal variations in WSE. We used a water mask created from color infrared imagery, along with incidence angle, sigma naught, and error raster files to filter the AirSWOT heights. Next, we calculated AirSWOT slope errors using a moving window of 100 m to divide the observational profile into 499 10 km reach segments. Differences between field-measured and AirSWOT-derived slopes range between -3.4 to 3.8 cm/km with a mean slope error of -0.48 cm/km over 10 km reaches. Standard deviation of the slope errors is ±1.6 cm/km with 71% of slope errors falling below the SWOT mission error requirement of 1.7 cm/km for a 10 km reach. RMSE for the vertical heights is 21 cm/km2, but falls to 10 cm/km2 when subtracting out the bias caused by a difference in heights associated with different flight directions. Further work is needed to develop more sophisticated filtering and layover identification methods. However, early results indicate AirSWOT's ability to record high accuracy river height and slope measurements and it's potential for use as a calibration and validation instrument for SWOT.