A sneak peak at the first SWOT hydrology products

SWOT, the Surface Water and Ocean Topography mission was successfully launched more than a year ago (Dec 16, 2022). Hydrologists are eagerly awaiting the release of SWOT products, which will allow them to study for the first time the water levels variations of more than a million lakes and rivers around the globe!

A first batch of products from the SWOT mission has been released by NASA and CNES. As explained in the release note, these SWOT products are « at a very early stage, with known limitations ». The hydrology products span only two weeks in April 2023.  However, I could not resist to have a look at the data… Hence I downloaded some level 2 hydrology products via the CNES Hydroweb.next portal (https://hydroweb.next.theia-land.fr). The datasets are distributed as shapefile and convenient to use. The Level 2 High Rate River Single Pass Vector Product provides river data – including water surface elevation of the river. Here is an example of the surface water elevation of the Oranje river in southern Africa.

Water surface elevation of the Oranje river on April 07, 2023 from SWOT

It is quite amazing to be able to map a river’s elevation profile from satellite measurements. Such data is key to estimate the river discharge [Durand et al] and therefore should enable us to make considerable progress in our understanding of the terrestrial water cycle. SWOT is expected to measure river surface elevation with an accuracy of approximately 10 cm at 100–200 m along-stream resolution [Langhorst et al.]. Several teams are already working to evaluate the actual accuracy depending on the river morphology.

Similarly, the High Rate Lake Single Pass Vector Product provides the surface elevation of lakes. To begin with, I picked four artificial lakes in northern India near Varanasi (Benares). In this case, the lake polygons match well the water surfaces that can be seen in a Sentinel-2 image acquired four days earlier.

Selected lakes in northern India. Background image is a Sentinel-2 false color image captured on 2023 April 05. White lines indicate the lake polygons provided in the SWOT lake product of 2023 April 09.

In SWOT products, each lake has an ID. A lake’s attributes including its mean water surface elevation (wse) and uncertainty (wse_u) can be retrieved in a terminal using ogrinfo from a level 2 file:

file="SWOT_L2_HR_LakeSP_(...).zip"
id="4520103033" # lake ID
ogrinfo /vsizip/$file -nomd -al -geom=NO -where "lake_id='${id}'"

This is the output from all available preliminary SWOT products at these four Indian reservoirs.

The top panel corresponds to the largest lake (Pipari dam): the data indicate that the water level dropped by 2 meters between April 21 and April 26 (from 168.5  to 166.5 m). This agrees well with the time series of Sentinel-2 images below, which show a reduction of the surface water area from April 20 to May 05 after a period of constant lake area.

 

I am a bit more familiar with lakes in the French Pyrenees. There are many cases where SWOT lake polygons (red polygons in the map below) are off the actual lake position, both in the « obs » and « prior » products. Be careful if you use SWOT data in mountain regions!

Some lakes in the north of the map (Bassiès) are correctly geolocated, especially this one (Etang Majeur de Bassiès, 21 hectares):
Here, we see a quick increase of the water level after April 21, which is consistent with the snowmelt that happened over the same period as shown below from Sentinel-2 imagery.

Below is another example in western France near Pressac, where there are many small lakes and ponds. From the SWOT product, I selected 23 lakes with areas ranging from 3 to 19 hectares, all well located.

All lakes follow the same decreasing trend. This may reflect the evolution of the regional water table but a 2 meters drop in two weeks only? Its seems really big for natural lakes.
In conclusion, it is really exciting to have access to some SWOT products – even preliminary. Hydrology products are easy to obtain and to work with. This first release has indeed some limitations in lake geolocation but NASA and CNES engineers are probably working hard to improve the data. I look forward to the next release!
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Top picture: Oranje river by yakovlev.alexey, CC BY-SA 2.0 https://creativecommons.org/licenses/by-sa/2.0, via Wikimedia Commons

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