Studying the Earth’s water from space with the newly launched SWOT satellite
The data analysis will involve some programming (Python using Jupyter Notebooks, with relevant site-packages) using data available within open cloud services. As SWOT is a near-global mission, the student can propose their own area of interest as well as focus on either discharge or lake/reservoir volume monitoring.
The new Surface Water and Ocean Topography (SWOT) satellite, launched in December 2022, aims to map Earth's water in unprecedented detail to help manage climate change and to share data openly to assist communities with managing this precious resource.
Figure 1: Two SWOT groundtracks and it's sideward looking KaRin observations cover almost the entire reservoir
The conventional method of representing river discharges is by indirect measurements in the form of a rating curve, which is a calibrated relationship between water levels and corresponding discharge. Due to low gauging station density, monitoring surface water resources with in-situ gauges is a difficult task next to the high continuous operational costs of gauges and the inability to access isolated locations. As a result, the majority of rivers, lakes, and reservoirs remain unmeasured. As a result of this, the primary constraint for hydrological studies related to reservoir monitoring, climate change impact and ecological assessment is the lack of data from ungauged water resources. Satellite radar altimetry has been used to monitor changes in water storage of substantial water bodies, lakes and the inundated surface water extent.
Satellite altimetry provides complementary information about river discharge by observing hydraulic variables such as, the width, slope, and surface water elevation of the river. The major drawback of nadir altimeters is that they can’t measure the extent of water bodies. This is due to the operational configuration of altimeters, which measures the elevation of objects beneath the satellite with a footprint of approximately 5 km wide and spacing of typically greater than 100 km between orbital tracks. The variation in the reservoir extent and height of the water is crucial information to estimate reservoir storage change. Radar nadir altimeters are useful for monitoring large surface water bodies such as reservoirs, but the water surface elevation information they provide must be supplemented by information about the water extent.
Besides a nadir radar altimeter SWOT also houses a sideward-looking Ka-band Radar Interferometer (KaRIn). The combination will enable observing both the extent and height of reservoirs in a single pass With accurate knowledge of reservoir bathymetry, the data from SWOT observations can be utilized directly to estimate reservoir storage. In absence of reservoir bathymetry, the information from SWOT like altimeters can provide only storage change. The SWOT mission has a return period of around 21 days which will be suitable to monitor the reservoir on a monthly scale (Akhilesh, et.al., 2022).
A: Reservoir volume determination:
The data recorded by the satellite can be used for discharge estimation as well as reservoir volume determination. The figure shows the changes in extent, resulting from the recent filling of the Grand Ethiopian Renaissance Dam (GERD) (https://en.wikipedia.org/wiki/Grand_Ethiopian_Renaissance_Dam). The research topic proposed here could focus on the use of SRTM, to determine the pre-dam elevation model of the reservoir area and deriving reservoir extent using Sentinel 1 during filling of the reservoir and monitoring the changes in reservoir volume over the last couple of years. This would be in conjunction with lake levels derived using conventional altimeters, e.g. from Jason-3, CryoSat-2, Sentinel 3 and Sentinel 6. Once this study is completed, additional information, as obtained from SWOT would be used to evaluate the added capability of this new satellite mission.
B: River discharge determination:
Another topic could be the evaluation if SWOT derived information can be used to determine river discharge, e.g. applying an approach as based on previous work like in the Amazon (LeFavour and Alsdorf, 2005) using SRTM derived slope for Amazon River to derive discharge applying Manning’s equation:
Some results obtained in this study are given in the table below.
Akhilesh S. Nair, Kaushlendra Verma, Subhankar Karmakar, Subimal Ghosh, J. Indu, Exploring the potential of SWOT mission for reservoir monitoring in Mahanadi basin, Advances in Space Research, Volume 69, Issue 3, 2022, Pages 1481-1493, ISSN 0273-1177, https://doi.org/10.1016/j.asr.2021.11.019.
Further references see SWOT EA papers: https://www.dropbox.com/sh/p4j8c7snyemilt1/AAC928nTxqVlZv81ExRH1ts9a?dl=0
Gina LeFavour and Doug Alsdorf (2005): Water slope and discharge in the Amazon River estimated using the shuttle radar topography mission digital elevation model GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L17404, doi:10.1029/ 2005GL023836, 2005 (https://research.byrd.osu.edu/water/publications/GRL_SRTMAmazonQ_2005.pdf)