Preliminary Characterization of SWOT Hydrology Error Budget and Global Capabilities

Abstract

River discharge and lake water storage are critical elements of land surface hydrology, but are poorly observed globally. The Surface Water and Ocean Topography (SWOT) satellite mission will provide high-resolution measurements of water surface elevations with global coverage. Feasibility studies have been undertaken to help define the orbit inclination and repeat period. Preliminary error budgets have been computed for estimating instantaneous and monthly river discharge from SWOT measurements (errors are assumed uncorrelated). Errors on monthly discharge due to SWOT temporal sampling were estimated using gauges and their observation times for two SWOT orbits with different inclinations (78° and 74°). These errors have then been extrapolated to rivers globally. The 78° and 74° orbital inclinations allow a good sampling frequency, avoid tidal aliasing and cover almost all the continental surface. For a 22-day repeat orbit, a single point at 72°N is sampled 11 and 16 times during one repeat period for the 78° and 74° inclination orbit, respectively. Errors in instantaneous discharge are below 25% for rivers wider than 50 m (48% of all rivers). Errors in monthly discharge are below 20% for rivers with drainage areas larger than 7000 km2 (34% of all rivers). A rough estimate of global lake storage change has been computed. Currently, available satellite nadir altimetry data can only monitor 15% of the global lake volume variation, whereas from 50% to more than 65% of this variation will be observed by SWOT, thus providing a significant increase in our knowledge of lake hydrology.