How Well Will the Surface Water and Ocean Topography Mission Measure Water Surface Heights and Slopes in Complex Terrain?
Abstract
The Surface Water and Ocean Topography (SWOT) mission will measure water surface heights, widths, and slopes of rivers wider than 100 m and possibly as narrow as 50 m. Unlike traditional altimeters, SWOT's main payload uses near nadir radar interferometry to observe water bodies inside its measurement swath, allowing more spatially continuous observations. However, due to its measurement principle and viewing geometry, observations may be affected by terrain layover, a condition that happens when the radar returns from water and land targets reach the satellite simultaneously, causing them to become indistinguishable, which may lead to stage-dependent and spatially correlated biases. We applied a simplified height and slope error model capable of estimating systematic and random errors to all rivers in the Global River Widths from Landsat database with the intent of estimating SWOT height and slope uncertainties and evaluate the effect of terrain layover. The error model considers river width, orientation and location with respect to the satellite ground track, and the topography roughness to estimate height uncertainty at the node scale, i.e. equally spaced points located every 200 m over the river centerline, which are later aggregated into 10 km reaches. Our results showed that overall, the impact of terrain layover on reach height and slope uncertainties is relatively small, leading to an increase of the 68th height error percentile from 9.4 cm to 10.4 cm and an increase of the 68th slope error percentile from 1 cm/km to 1.7 cm/km. Nevertheless, the impact of layover is not equally distributed over the globe, depending on width, topographic roughness, and location, being more influential for narrower rivers flowing through complex terrain. When height and slope uncertainties are propagated through Manning's equation, we see that the impact of layover induced errors will lead to a marginal increase of 68th percentile of discharge uncertainty (due to height and slope uncertainties alone) from 12% to 13%. Finally, our analysis shows the unexpected result that the extent of layover is rather large, affecting even rivers that are not surrounded by complex topography, nevertheless, the magnitude of the introduced bias is rather small.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMOS51A..04F
- Keywords:
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- 1845 Limnology;
- HYDROLOGYDE: 1856 River channels;
- HYDROLOGYDE: 4520 Eddies and mesoscale processes;
- OCEANOGRAPHY: PHYSICALDE: 4544 Internal and inertial waves;
- OCEANOGRAPHY: PHYSICAL