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, results show 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).
Highlights
The hydrological cycle is of fundamental importance to life and society and river gauges have long formed a basis our hydrological understanding, often providing real-time measurement capabilities of river stage or discharge and information for water management and flood warning
We aimed to (i) characterise and illustrate in two dimensions the errors which may be found in Surface Water and Ocean Topography (SWOT) swathaltimetry measurements of terrestrial surface water; (ii) simulate the spatio-temporal sampling scheme of SWOT for the Amazon; and (iii) assess the impact of each on estimates of water-surface slope and river discharge which may be obtained from SWOT imagery
We used a virtual mission study of twodimensional water-surface elevations which may be obtained by SWOT for a reach of the central Amazon River in Brazil and investigated the implications of errors in such measurements on the estimation of water-surface slope and channel discharge
Summary
The hydrological cycle is of fundamental importance to life and society and river gauges have long formed a basis our hydrological understanding, often providing real-time measurement capabilities of river stage or discharge and information for water management and flood warning. The United States has around 7000 stream gauges but, more than 20 % of basins are not gauged adequately (USGS, 1998), contributing to an insufficient knowledge of available national water resources (NSTC, 2004). Over the latter half of the 20th century, increasing numbers of gauging stations in the United States with 30 or more years. The gauge density in the Amazon, expressed as number of gauges per unit of discharge, is around 4 orders of magnitude less than what is typical in the eastern United States (Alsdorf et al, 2007b). Fekete and Vörösmarty (2007) indicated that the number of data available through the Global Runoff Data Centre (GRDC) is in sharp decline, and stands at less than 600 discharge monitoring stations, down from a peak of around 5000 in 1980
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