2.5.5 Altimetry for lake/reservoir studies
by using hydrology dedicated products.
Satellite altimetry dedicated to hydrology.
The purpose of this Data Use Case is twofold:
- Provide an overview of the information contained in Jason-2 experimental PISTACH products over non-ocean areas, based on the example of the monitoring of the Sathanur Reservoir (India),
- Provide newcomers to altimetry with a generic approach for studying a given water body using BRAT () and Level 2 (Geophysical Data Records) altimetry products. See the other Data Use Cases on the topic of hydrology.
Selection of the target area
First of all, and since satellites altimetry mission are optimized for open ocean studies with limited nadir footprint (a few kilometer ray circle) and repeat orbital cycles, one has to be lucky to have a suitable coverage over a specific inland water body of limited extent!
The pass locator on Aviso website (download the .kml file for the Jason-2 referenced orbit) that give quick and easy access to the precise ground track coverage of the various altimetry missions on Google Earth.
In our case, we have decided to look for a lake or reservoir in India. We found the Sathanur Reservoir located in the Tamil Nadu State, south-east India, around 100 km West of Pondicherry city. This reservoir is roughly located between 12.15°N and 12.25°N below Jason-2 track #079. This is an ascending track, the satellite flying from the south west to the north east.
While on GoogleEarth, it is usefull to fly over and around the lake with the mouse to get an idea of the topography in the surroundings of the lake. It is important to realize that the surrounding terrain of an inland water body can have a significant impact on the accuracy of the altimetric over the lake itself: for example, reflections of the radar signal over surrounding mountains can strongly alter the shape of the waveforms and induce inaccurate or even erroneous range measurements. For the Sathanur Reservoir, we notice that the topography is rather flat around the water body, with altitude of:
Some hills nevertheless appear on a 3D view on GoogleEarth, both in the South and North directions.
- ~235 m below the track a few kilometers south west (i.e. “before”) of the lake,
- ~221 m over the lake,
- ~250 m below the track a few kilometers north east (i.e. “after”) of the lake.
|fig 1: Jason-2 pass 079 overlayed on Google Earth in the vicinity of Sathanur Reservoir, India. Different views on Google Earth give important details on topography.
In addition, in GoogleEarth, the urbanization of the immediate surroundings of the lake seems rather limited (hydro power plant), and therefore, polluted waveforms should be limited. So, we reasonably expect acceptable quality of the altimetric data over the reservoir.
Download and preparation of the data
In order to study inland water bodies with the PISTACH products, one has to download the “hydro” product found (free access) on the FTP server. After reading the “Readme.txt” file, then go to the J2/IGDR/hydro/ directory.
The PISTACH products have the same organization (and format) as the standard official Jason-2 Level Products: one file per track and per cycle. Therefore, 254 files are found in each of the “cycle_CCC” directories. For that example, we only downloaded the files corresponding to the track #079 (example: the file ftp://ftpsedr.cls.fr/pub/oceano/pistach/J2/IGDR/hydro/cycle_009/JA2_IPH_2PTP009_079_20081002_102440_20081002_110916.nc.gz corresponds to the track #079 of orbital cycle #009, corresponding to an acquisition date between 10h24 and 11h09 on the 2nd of October 2008).
A shell script (download it here) can be used on a Unix/Linux computer to download a series of files for a given track number (#079 here) and several cycles.
Of course, the files need to be unzipped before using them within BRAT.
To elaborate this Data Use Case, we downloaded track #079 for the first 100 cycles. Although BRAT can handle 100 files or more, it appeared preferable in terms of efficiency for BRAT to perform an extraction of the files over a portion of track. We performed that extraction with the following script, based on the use the “ncdump”, “ncgen” and “ncea” commands. The latter needs the installation of NCO (NetCdf operators) on the computer. The NCO homepage contains more information. Here, the extraction is done between 12.15°N and 12.25°N, i.e. over the Sathanur Reservoir (download the script here).
First and quick look at the data with BRAT
The first objective is to check whether the quality of the data is acceptable to perform the monitoring of the level of Sathanur Reservoir.
In BRAT, performed the following actions:
We then obtain the following screen, with:
- Creation of a new workspace
- In "Datasets" tab, creation of a new dataset that we name “PISTACH_IGDR_Hydro_T079”
- Addition of the PISTACH product netcdf files in the dataset
The most simple operation to do for checking the data is to visualize the “orbit – range” difference along the track. To do so:
- the list of data files in the “dataset” window,
- the display of the header of the selected file in the “File description” window,
- the list of the variables of the selected file in the “product” window (upper right),
- the description of the selected field in “Fields description” window (lower right),
If the data files have not been “reduced” in the preparation step, it is useful to introduce a selection criterion (copy in the Expression grey box: is_bounded(12.15,lat,12.25). On the example below, only the data points with latitudes between 12.15°N and 12.25°N are selected.
- In "Operations" tab, create a new operation (named “OrbitMinusRange” for example)
- Select the latitude field (“lat”) and drop it on the “X” in the “Data expression frame”
- Select the orbit height field (“alt”) and drop it on the “Data” in the “Data expression frame”
- Type the “-“ operator sign after “alt” in the “Expression” frame
- Select the range field (“range_ku”) and drop it in the “Expression” frame” to form the “alt-range_ku” expression.
- Rename the “alt” name in the “Data Expression” frame with “AltMinusRangeKu”
Now click on “Execute”. BRAT computes this operation on each data point of each cycle for track #079 on the selected track portion.
Once the computation is completed (a few seconds to a few minutes, depending on your computer…), go to the “Views” menu to create a graphical representation of the result of this operation:
- Creation and naming of a new view
- Selection of the data to draw among the “Available” operation results (only one choice at this stage of the Data Use Case)
- Click on the “==>” button to introduce it in the “Selected” frame (or drag&drop)
- Click on “Execute”.
The figure 2 appears (after masking the lines and drawing the points). Interpretation: no evident information can be extracted from that plot!
Does it mean that the PISTACH products don’t contain any useful information over the Sathanur Reservoir?
In fact, this plot mainly illustrates that the “range_ku” field (i.e. the “ocean” range field of the standard Jason-2 products) is not the most appropriate for that purpose, because the waveform over that area are not ocean-like waveforms.
fig 2: First look on data, computation of the "orbit – range" with the "range_ku" (i.e. the "ocean" range).
Let us then plot the “Ice1” range. It is necessary to create a new operation (or use the “duplicate” button), and replace the “range_ku” field with the “ice_range_ku” (results of the “Ice1” retracking). After creating a new plot with both operations, we obtain the figure 3 (left). Here, the “alt-ice_range_ku” values are in red and the “alt-range_ku” in blue. We now observe a coherent and dense set of points (with some outliers). The “cloud” seems particularly coherent between 12.18°N and 12.21°N, i.e. over the reservoir.
Let us now add the result of the PISTACH Ice3 retracking (field: “range_ice3_ku”) on that plot (after creating once again a new operation and a new view…). The result appears on figure 3 (right).
|fig 3: Computation of the "orbit - range" with the Ice1 retracking (left, in red) and with the Ice3 retracking (right, in green). The "orbit - range" with the ocean retracking is plotted in blue on both.
The data set now appears even more coherent, with very few outliers. This indicates that a coherent set of data is present in the PISTACH products over the Sathanur Reservoir, when using the results of the Ice3 retracking algorithm.
A representation as a function of time (figure 4) should also give a coherent signal: this is verified with a new operation (replacing “lat” with “time” in the X coordinate).
We can easily distinguish a temporal fluctuation of more than 10 m peak to peak amplitude. Outliers on that representation probably correspond to measurement acquired on emerged lands surrounding the reservoir.
Now that we are sure that exploitable information is contained in the PISTACH data set, it is necessary to precisely compute the water surface height and to refine the data selection.
fig 4: Computation of the "orbit - range" with the Ice3 retracking vs time.
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