Estimating River Depth from SWOT-Type Observables Obtained by Satellite Altimetry and Imagery

Mohammad Tourian,Nico Sneeuw,Omid Elmi,Abolfazl Mohammadnejad

doi:10.3390/w9100753

Abstract

The proposed Surface Water and Ocean Topography (SWOT) mission aims to improve spaceborne estimates of river discharge through its measurements of water surface elevation, river width and slope. SWOT, however, will not observe baseflow depth, which limits its value in estimating river discharge especially for those rivers with heterogeneous channel geometry. In this study, we aim to obtain river depths from spaceborne observations together with in situ data of river discharge. We first obtain SWOT-like observables from current satellite techniques. We obtain river water level and slope time series from multi-mission altimetry and effective river width from satellite imagery (MODIS). We then employ a Gauss–Helmert adjustment model to estimate average river depth for 16 defined reaches along the Po River in Italy, for which we use our spaceborne observations in two recognized models for discharge estimation. The average river depth estimates along the Po River are validated against surveyed cross-section information, which shows a generally good agreement in the range of ∼10% relative root mean squared error. Furthermore, we analyzed the sensitivity of error in the estimated river depth to errors of individual parameters. We show that the estimated river depth is less influenced by errors of river width and river discharge, while it is strongly influenced by errors in water level. This result gives a perspective to the SWOT mission to infer river depth by coarse estimates of river width and discharge.

Highlights

River discharge is perhaps the single most important hydrologic quantity representing the amount of available freshwater on landmasses [1]
The selected altimetric measurements are stacked at the center of each reach by shifting the water level hydrographs of all virtual stations according to time lag between different virtual stations
The stacked measurements are merged by normalizing the time series according to their statistical characteristics

Summary

Introduction

River discharge is perhaps the single most important hydrologic quantity representing the amount of available freshwater on landmasses [1]. The publicly available in situ river discharge database has been declining steadily over the past few years due mainly to economic and political reasons [2,3,4]. The number of available runoff gauging stations has gone down from about (pre-1970) to less than 1000 (around the year 2015). During this period, the total monitored annual stream flow has dropped by about 75%. Most of the active gauges are located over developed countries, and the density of stations is much sparser in the non-industrialized countries [5]

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Conclusion