Swath-altimetry measurements of the main stem Amazon River: measurement errors and hydraulic implications

Abstract

Abstract. The Surface Water and Ocean Topography (SWOT) mission, scheduled for launch in 2020, will provide a step-change improvement in the measurement of terrestrial surface-water storage and dynamics. In particular, it will provide the first, routine two-dimensional measurements of water-surface elevations. In this paper, we aimed to (i) characterise and illustrate in two dimensions the errors which may be found in SWOT swath measurements of terrestrial surface water, (ii) simulate the spatio-temporal sampling scheme of SWOT for the Amazon, and (iii) assess the impact of each of these on estimates of water-surface slope and river discharge which may be obtained from SWOT imagery. We based our analysis on a virtual mission for a ∼ 260 km reach of the central Amazon (Solimões) River, using a hydraulic model to provide water-surface elevations according to SWOT spatio-temporal sampling to which errors were added based on a two-dimensional height error spectrum derived from the SWOT design requirements. We thereby obtained water-surface elevation measurements for the Amazon main stem as may be observed by SWOT. Using these measurements, we derived estimates of river slope and discharge and compared them to those obtained directly from the hydraulic model. We found that cross-channel and along-reach averaging of SWOT measurements using reach lengths greater than 4 km for the Solimões and 7.5 km for Purus reduced the effect of systematic height errors, enabling discharge to be reproduced accurately from the water height, assuming known bathymetry and friction. Using cross-sectional averaging and 20 km reach lengths, resultsshow Nash-Sutcliffe model efficiency values of 0.99 for the Solimões and 0.88 for the Purus, with 2.6 and 19.1 % average overall error in discharge, respectively. We extend the results to other rivers worldwide and infer that SWOT-derived discharge estimates may be more accurate for rivers with larger channel widths (permitting a greater level of cross-sectional averaging and the use of shorter reach lengths) and higher water-surface slopes (reducing the proportional impact of slope errors on discharge calculation).

Flow Duration Curve from Satellite: Potential of a Lifetime SWOT Mission

“…Since investigative studies are needed before the planned launch of the satellite in 2021, and in line with previous studies that investigated SWOT potential [10,15,16,33], we reproduce the satellite observations by corrupting the simulated hydraulic variables (such as water level, water surface width, and slope) with errors consistent with the expected performance requirements [26]. In other words, for the scope of the analysis, water levels and other hydraulic variables obtained from the numerical simulations represent the synthetic reality that we refer to in order to simulate SWOT observations.…”

Characterizing water surface elevation under different flow conditions for the upcoming SWOT mission

“…In most countries, it is relatively sparsely monitored by means of ground-based stations, which measure the water surface height (referenced to some local datum) and estimate the river flows by means of rating curves. The result is a largely incomplete knowledge of river fluxes, with measurements provided by stream gauge networks of different density, accuracy and reliability over the globe (Biancamaria et al, 2010;Pavelsky et al, 2014;Wilson et al, 2015;Pena-Arancibia et al, 2015;Tomkins, 2014;Domeneghetti et al, 2012). The installation and maintenance costs required to sustain the monitoring networks constrain their installation mostly to highly-developed areas.…”

“…In the majority of existing studies about SWOT mission potential, satellite observations are synthetically reproduced by corrupting appropriate observed or simulated data with random errors defined according to the science requirements (see Table 1; e.g., Wilson et al, 2015;Andreadis et al, 2007).…”

Amazon Hydrology From Space: Scientific Advances and Future Challenges

“…Dedicated to sample all rivers wider than 100 m and lakes larger than 250 × 250 m, the mission will permit a consequent reduction of global and regional models, noteworthy through data assimilation (Emery et al, 2020;Wongchuig et al, 2020). The estimate of discharge from altimetry will benefit from SWOT data, both thanks to the global coverage and the observation of slopes, allowing a better constraining of uncertain hydraulics (Wilson et al, 2015).…”