ICESat‐2 Based River Surface Slope and Its Impact on Water Level Time Series From Satellite Altimetry

Abstract

The water surface slope (WSS), hydraulic gradient, or flow gradient of a river is the slope of the hydraulic grade line, that is, the change of the pressure head per distance unit (Gliński et al., 2011;Herrmann & Bucksch, 2014;Julien, 2018b). It is typically defined positive for an decreasing water surface elevation (WSE) in downstream direction (Julien, 2018b). WSS is not stationary but changes over time and space. Especially in natural rivers that are non-uniform and unsteady, the WSS is variable over time because of morphological changes of the river bed and flood waves (Julien, 2018a). Locally, WSS may differ from larger-scale averages and change with every reach because of local characteristics like cascades, pools, or tributary estuaries (Rhoads, 2020;Schumm, 2005). While the WSS of an alluvial river is gradual, bedrock causes natural discontinuities in semi-alluvial rivers (Julien, 2018b). In hydrology, WSS is a critical parameter required to calculate flow velocity and discharge (Manning, 1891;Rhoads, 2020). The flow velocity derived from WSS is also essential for densifying spatial or temporal low-resolution water level measurements from non-repeating satellite altimetry missions such as Cryosat-2 (Tourian et al., 2016). Generally, the WSS can be used to correct any satellite altimetry mission to compensate for the satellites' ground track variability when calculating long-term water level time series at fixed locations, so-called virtual stations (VS). WSE measurements of the "Shuttle Radar Topography Mission" (SRTM) are regularly used to derive WSS (

Stage‐Slope‐Discharge Relationships Upstream of River Confluences Revealed by Satellite Altimetry

“…Using the latest generation of satellite altimetry missions, we can, for the first time, observe local water surface slope (WSS) along with WSE. This has been demonstrated for the ICESat-2 mission (Christoffersen et al, 2023;Scherer et al, 2022), and similar capabilities are also expected for the recently launched Surface Water and Ocean Topography (SWOT) mission (Biancamaria et al, 2016). WSE is an important parameter for hydrological applications, particularly in the flooding context.…”

“…Paris et al (2016) estimated a monthly average slope from the interpolated WSE series for one specific VS located at the mouth of the Negro River, showing that a WSE-WSS-discharge rating curve outperformed a WSE-discharge rating curve for discharge estimation. Accurate observation of WSS for a broad range of river reaches has only recently become possible due to the availability of ICESat-2 data sets (Christoffersen et al, 2023;Scherer et al, 2022). In the near future, WSS observations from the recently launched SWOT mission will also become available.…”

Hydraulics of time-variable water surface slope in rivers observed by satellite altimetry

“…Currently, WSS can be observed with a standard error of ca 2 cm/km in rivers using ICESat-2 data 12,13 . Similar accuracy is expected from SWOT.…”

ICESat-2 river surface slope (IRIS): A global reach-scale water surface slope dataset

“…River discharge critically contributes to the characterization of the Earth’s hydrological cycle and climate and, thus, its determination on a global scale is of great scientific relevance. Additionally, correcting water surface elevation (WSE) observations from satellite altimetry for WSS can significantly improve the accuracy of the resulting water level time series 2 , 3 . Depending on the river’s morphology, regulation, bed material, and basin size, the WSS can be highly variable in both space and time 4 .…”

“…Due to its dense ground track pattern, ICESat-2 is well suited for global studies of the Earth’s hydrosphere 11 . ICESat-2 WSE observations have already been used to derive WSS in small study areas 2 , 12 , but not at the global scale. WSS datasets at the global scale so far exist only on the basis of DEM data such as the “Global River-Slope” (GloRS 13 ) or as part of the “SWOT Mission River Database” (SWORD 14 ).…”