Fluctuations in water surface elevation (WSE) along rivers have important implications for water resources, flood hazards, and biogeochemical cycling. However, current in situ and remote sensing methods exhibit key limitations in characterizing spatiotemporal hydraulics of many of the world's river systems. Here we analyze new measurements of river WSE and slope from AirSWOT, an airborne analogue to the Surface Water and Ocean Topography (SWOT) mission aimed at addressing limitations in current remotely sensed observations of surface water. To evaluate its capabilities, we compare AirSWOT WSEs and slopes to in situ measurements along the Tanana River, Alaska. Root‐mean‐square error is 9.0 cm for WSEs averaged over 1 km2 areas and 1.0 cm/km for slopes along 10 km reaches. Results indicate that AirSWOT can accurately reproduce the spatial variations in slope critical for characterizing reach‐scale hydraulics. AirSWOT's high‐precision measurements are valuable for hydrologic analysis, flood modeling studies, and for validating future SWOT measurements.

Scheduled to launch in 2022, the Surface Water and Ocean Topography (SWOT) mission is the first satellite designed with a specific objective of observing earth's rivers (Biancamaria et al., 2016;Durand et al., 2010). During the 3 years that it will spend in its primary orbit, SWOT is designed to provide measurements of water surface elevation (WSE), width, and slope, along with estimates of discharge for global rivers wider than 100 m (Rodriguez et al., 2018). In addition to this science requirement, SWOT also has a science goal of providing these data for rivers as narrow as 50 m. Based on its orbit configuration, SWOT will observe

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.

Rivers are among the most degraded ecosystems on earth (Best, 2019). Water quality is impaired due to human activities such as agriculture and urbanization (Foley et al., 2005; Meybeck et al., 1990), and currently only 23% of earth's largest rivers flow uninterrupted to the ocean (Grill et al., 2019; Nilsson et al., 2005). Because large rivers integrate millions of kilometers of land area, understanding rivers and their impairments is inherently macroscale: both distant and local impacts generate the patterns we observe (Heffernan et al., 2014; McCluney et al., 2014). There is a profound need for integrative water quality measurements that are spatially explicit and globally scalable, as local and global changes impairing Earth's rivers cannot be fully understood using sparse, ground-based measurements (Stanley et al., 2019; Stets et al., 2020). Remote sensing enables spatially explicit, global observations of large rivers (Palmer et al., 2015). Satellite missions, such as the joint NASA/USGS Landsat mission, have been used for decades to measure river and lake water quality (Brezonik et al., 2005; Carpenter & Carpenter, 1983). However, measuring water quality at continental to global scales remains challenging over inland waters due to optical complexity, or the presence of multiple water quality constituents (Ross et al., 2019; Topp et al., 2020). The three main constituents are chlorophyll-a (chl-a), suspended sediment, and colored dissolved organic matter (CDOM) (Davies-Colley et al., 2003; Ritchie et al., 2003). These water quality constituents are ecologically important, control light availability for photosynthesis and photodegradation, and together determine the color of water which is an integrative measure of water quality (

25The Surface Water and Ocean Topography (SWOT) satellite mission aims to improve the 26 frequency and accuracy of global observations of river water surface elevations (WSEs) and 27 slopes. As part of the SWOT mission, an airborne analog, AirSWOT, provides spatially-28 distributed measurements of WSEs for river reaches tens to hundreds of kilometers in length. For 29 the first time, we demonstrate the ability of AirSWOT to consistently measure temporal 30 dynamics in river WSE and slope. We evaluate data from six AirSWOT flights conducted 31 between June 7-22, 2015 along a ~90 km reach of the Tanana River, AK. To validate AirSWOT 32 measurements, we compare AirSWOT WSEs and slopes against an in situ network of 12 33 pressure transducers (PTs). Assuming error-free in situ data, AirSWOT measurements of river 34 WSEs have an overall root mean square difference (RMSD) of 11.8 cm when averaged over 1 35 km 2 areas whilst measurements of river surface slope have an RMSD of 1.6 cm/km for reach 36 lengths >5 km. AirSWOT is also capable of recording accurate river WSE changes between 37 flight dates, with an RMSD of 9.8 cm. Regrettably, observed in situ slope changes that transpired 38 between the six flights are well below AirSWOT's accuracy, limiting the evaluation of 39 AirSWOT's ability to capture temporal changes in slope. In addition to validating the direct 40 AirSWOT measurements, we compare discharge values calculated via Manning's equation using 41AirSWOT WSEs and slopes to discharge values calculated using PT WSEs and slopes. We 42 define or calibrate the remaining discharge parameters using a combination of in situ and 43 remotely sensed observations, and we hold these remaining parameters constant between the two 44 types of calculations to evaluate the impact of using AirSWOT versus the PT observations of 45 WSE and slope. Results indicate that AirSWOT-derived discharge estimates are similar to the 46 PT-derived discharge estimates, with an RMSD of 13.8%. Additionally, 42% of the AirSWOT-47 based discharge estimates fall within the PT discharge estimates' uncertainty bounds. We 48 conclude that AirSWOT can measure multitemporal variations in river WSE and spatial 49 variations in slope with both high accuracy and spatial sampling, providing a compelling 50 alternative to in situ measurements of regional-scale, spatiotemporal fluvial dynamics. 51