Mapping Water Surface Elevation and Slope in the Mississippi River Delta Using the AirSWOT Ka-Band Interferometric Synthetic Aperture Radar

Michael Denbina,Tamlin Pavelsky,Albert Chen,Ernesto Rodriguez,Xiaoqing Wu,Marc Simard

doi:10.3390/rs11232739

Michael Denbina, Tamlin Pavelsky + Show 4 more

Open Access

https://doi.org/10.3390/rs11232739

Copy DOI

Abstract

AirSWOT is an airborne Ka-band synthetic aperture radar, capable of mapping water surface elevation (WSE) and water surface slope (WSS) using single-pass interferometry. AirSWOT was designed as a calibration and validation instrument for the forthcoming Surface Water and Ocean Topography (SWOT) mission, an international spaceborne synthetic aperture radar mission planned for launch in 2022 which will enable global mapping of WSE and WSS. As an airborne instrument, capable of quickly repeating overflights, AirSWOT enables measurement of high frequency and fine scale hydrological processes encountered in coastal regions. In this paper, we use data collected by AirSWOT in the Mississippi River Delta and surrounding wetlands of coastal Louisiana, USA, to investigate the capabilities of Ka-band interferometry for mapping WSE and WSS in coastal marsh environments. We introduce a data-driven method to estimate the time-varying interferometric phase drift resulting from radar hardware response to environmental conditions. A system of linear equations based on AirSWOT measurements is solved for elevation bias and time-varying phase calibration parameters using weighted least squares. We observed AirSWOT WSE uncertainty of 12 cm RMS compared to in situ water level measurements when averaged over an area of 0.5 km 2 at incidence angles below 15 ∘ . At higher incidence angles, the observed AirSWOT elevation bias is possibly due to residual phase calibration errors or radar backscatter from vegetation. Elevation profiles along the Wax Lake Outlet river channel indicate AirSWOT can measure WSS over a 24 km distance with uncertainty below 0.3 cm/km, 8% of the true water surface slope as measured by in situ data. The data analysis and results presented in this paper demonstrate the potential of AirSWOT to measure water surface elevation and slope within highly dynamic and spatially complex coastal environments.

Highlights

Quantifying and monitoring the volume and distribution of water is important, as climate change and other factors cause additional stress to Earth’s limited water resources [1,2]
We report the overall empirical uncertainty of the AirSWOT water levels in the form of mean absolute error (MAE) and root mean square error (RMSE) statistics
The MAE was calculated as follows: MAE = E hAirSWOT − hStation where E is the expectation operator, | | is the absolute value, hAirSWOT is a vector of the estimated AirSWOT water surface elevation (WSE) at each water level station, and hStation is a vector of in situ water levels measured at each station

Summary

Introduction

Quantifying and monitoring the volume and distribution of water is important, as climate change and other factors cause additional stress to Earth’s limited water resources [1,2] Accurate measurements of these water resources are important in planning for both overabundance and dearth of water, as in floods and droughts, respectively. In situ data regarding surface water is incomplete or unavailable in many parts of the world [3]. In response to this limitation, there has been an increased focus on using data from satellites to measure rivers and lakes [4]. Remote sensing measurements of water surface elevation (WSE) and water surface slope (WSS) can be used to estimate important hydrological parameters such as river discharge and fresh water storage at regional and global scales [4,5]

Methods

Results

Conclusion