Satellite monitoring of the water cycle over the Amazon using upstream/downstream dependency: Mass-conserved interpolation of the river discharge at 0.25° along the river.

Abstract:

River discharge over land is a major component of the global water cycle, with a large influence on water resources availability for human life. No direct large-scale observations exist for the river discharge, and monitoring is limited to a few stations. The hydrological community is still awaiting observations from the Surface Water and Ocean Topography (SWOT) mission, which is being jointly developed by NASA and CNES and will be launched in 2021. In the meantime, estimation of a continuous river network relies only on hydrological models. Despite recent advances in LSMs, simulations of hydrodynamic processes are still limited, and large discrepancies exist among models. Furthermore, simulations can be calibrated/validated at only a few in situ discharge stations due to the scarcity of these measurements. In this study, a scheme was developed to continuously estimate R along the river: it indirectly interpolates discharge station measurements by using satellite estimates of groundwater change, precipitation, and evaporation by using the water mass conservation law.

Our improved combination/optimization methodology called SAtellite Water Cycle (SAWC) analysis imposes spatial inter-basin constraints on regional terrestrial budget closure by considering the horizontal water exchanges such as river discharge. This simultaneous optimization at the sub-basin scale allows for correction of satellite datasets for the various hydrological regimes that occur across the Amazon basin. Once all water components have been updated, SAWC analysis can provide mass-conserved spatiotemporal variation of continuous discharge along a river, at high resolution, based on satellite and in situ measurements, using only topography for the drainage area, independently from any hydrological model.

These discharge estimates along the river were evaluated with 80 independent measurements and had a correlation of 0.86 and a Kling–Gupta efficiency index of 0.71. A comprehensive monthly river discharge estimate was proposed for the first time across the Amazon basin for a 13-year period (2002–2015) at a 0.25° resolution. Extreme events of the 2009-flood and 2010-drought were analyzed at spatial scale over the river network.

The SAWC framework is an observation tool for integrating a large range of satellite observations.