AirSWOT InSAR Mapping of Surface Water Elevations and Hydraulic Gradients Across the Yukon Flats Basin, Alaska

Abstract

AirSWOT, an experimental airborne Ka-band interferometric synthetic aperture radar, was developed for hydrologic research and validation of the forthcoming Surface Water and Ocean Topography (SWOT) satellite mission (to be launched in 2021). AirSWOT and SWOT aim to improve understanding of surface water processes by mapping water surface elevation (WSE) and water surface slope (WSS) in rivers, lakes, and wetlands. However, the utility of AirSWOT for these purposes remains largely unexamined. We present the first investigation of AirSWOT WSE and WSS surveys over complex, low-relief, wetland-river hydrologic environments, including (1) a field-validated assessment of AirSWOT WSE and WSS precisions for lakes and rivers in the Yukon Flats Basin, an Arctic-Boreal wetland complex in eastern interior Alaska; (2) improved scientific understanding of surface water flow gradients and the influence of subsurface permafrost; and (3) recommendations for improving AirSWOT precisions in future scientific and SWOT validation campaigns. AirSWOT quantifies WSE with an RMSE of 8 and 15 cm in 1 and 0.0625 km 2 river reaches, respectively, and 21 cm in lakes. This indicates good utility for studying hydrologic flux, WSS, geomorphic processes, and coupled surface/subsurface hydrology in permafrost environments. This also suggests that AirSWOT supplies sufficient precision for validating SWOT WSE and WSS over rivers, but not lakes. However, improvements in sensor calibration and flight experiment design may improve precisions in future deployments as may modifications to data processing. We conclude that AirSWOT is a useful tool for bridging the gap between field observations and forthcoming global SWOT satellite products.This study deployed AirSWOT and a field team to eastern interior Alaska in June 2015 for testing over the Yukon Flats Basin (YFB), a protected wetland area within the Yukon Flats National Wildlife Refuge, which straddles the Arctic circle (Figure 1a). The YFB has complex, low-relief topography and is underlain by PITCHER ET AL.

Estimating River Discharge With Swath Altimetry: A Proof of Concept Using AirSWOT Observations

“…These elevation and slope errors compared with in situ data are slightly higher than those previously reported (Altenau et al, 2017;Pitcher et al, 2018), and exceed the SWOT science requirements for 100-m rivers (10-cm elevation/1.7-cm/km slope RMSE), but the Willamette is a narrower river than the rivers investigated in previous studies and we are therefore evaluating elevation uncertainty over a smaller water surface area for 10-km reaches. At the pixel level, AirSWOT elevation RMSEs compared with in situ data are 99.1 cm, dropping to 80.4 cm when the incidence angle and land/water buffer filters were implemented.…”

“…We evaluate slope as a rise-over-run approach using only the nodes at the start and end of the reach; slope RMSE compared with in situ data for four reaches across 6 days of data was 3.2 cm/km. These elevation and slope errors compared with in situ data are slightly higher than those previously reported (Altenau et al, 2017;Pitcher et al, 2018), and exceed the SWOT science requirements for 100-m rivers (10-cm elevation/1.7-cm/km slope RMSE), but the Willamette is a narrower river than the rivers investigated in previous studies and we are therefore evaluating elevation uncertainty over a smaller water surface area for 10-km reaches.…”

“…As in Altenau et al (2017) and Pitcher et al (2018), the data used in this study were from the outer swath of the Kaband AirSWOT instrument, which provides data at incidence angles between nadir and 25°off nadir, leading to a~3-km swath width for the flight altitudes used over the Willamette (8,600 m above mean sea level). As in Altenau et al (2017) and Pitcher et al (2018), the data used in this study were from the outer swath of the Kaband AirSWOT instrument, which provides data at incidence angles between nadir and 25°off nadir, leading to a~3-km swath width for the flight altitudes used over the Willamette (8,600 m above mean sea level).…”

“…AirSWOT overflights occurred on 16, 17, 24, 25, 30, and 31 March 2015 and were timed to coincide with an atmospheric river event, which was expected to provide dynamic hydraulic conditions during an otherwise dry period for the region. As in Altenau et al (2017) and Pitcher et al (2018), the data used in this study were from the outer swath of the Kaband AirSWOT instrument, which provides data at incidence angles between nadir and 25°off nadir, leading to a~3-km swath width for the flight altitudes used over the Willamette (8,600 m above mean sea level). Compared to SWOT, which operates at incidence angles ranging from 1°to 5°, AirSWOT's broader range of incidence angles leads to larger variabilities in backscatter, interferometric coherence, and phase ambiguity across the swath and does not allow for land-water delineation using radar return data.…”

Arctic‐Boreal Lake Dynamics Revealed Using CubeSat Imagery

“…In total, these four study areas cover 226,553 km 2 of the western Arctic-Boreal region of North America (Figure 1, inset), and span Arctic tundra, boreal forest, peat plateau, lowland wetland, and upland shield terrains. The 33,383 km 2 YFB study area is characterized by fluvially disconnected wetlands surrounding a wide low-gradient reach of the Yukon River in north central Alaska (e.g., Anderson et al, 2013;Pitcher et al, 2019). The 82,200 km 2 MRV study area includes the Mackenzie River Delta, a vast, fluvially connected wetland, the Tuktoyaktuk Peninsula, a polar desert with continuous permafrost and thousands of thermokarst lakes, and boreal forest and plateaus further south along the Mackenzie and Peel Rivers (Burn & Kokelj, 2009;Plug et al, 2008).…”