2.5.6 Altimetry for wetland studies
by using hydrology dedicated products.

Satellite altimetry dedicated to hydrology.

Satellite altimetry is limited over continental areas due to a loss of quality in the measurements. These errors are caused by the heterogeneity of the reflecting surface (a mix of water and emerged land surfaces) in the altimetry and radiometric footprints (10 km and 50 km respectively) but also by inaccurate geophysical corrections. Despite this, the altimetric measurements are present and may contain useful information for hydrology studies.
This data use case proposes to give some instructions on how to use the Hydrology experimental products distributed by Aviso (PISTACH) products over a large wetland, the Sudd Marshes (southern Sudan).

Data used

Jason-2 along-track experimental Hydrology products produced by the PISTACH project, a hydrology (and coastal) -dedicated processing applied to the Jason-2 mission.
These products derive from the Jason-2 S-IGDR products and include new retracking solutions, several state-of-the-art geophysical corrections as well as higher resolution global/local models. Numerous extra fields derived from the various PISTACH processes are then added to build the products. These products are based on a 20-Hz along-track sampling rate (vs 1-Hz for the official Jason-2 IGDR) but the nomenclature of their variables and files is similar to Jason-2 IGDRs one.
Download freely coastal data files on the FTP server. Files are in the sub-directories named cycle_xxx/. Each cycle lasts about 10 days, the first one (in the cycle_001/ directory) was acquired early July 2008.

Methodology

We use the Basic Radar Altimetry Toolbox to observe the data and do some computation.
Over continental areas, the altimeter waveforms are highly perturbed by emerged land within the radar footprint. Numerous kind of echoes are encountered: Brown, peaky, Brown/peaky, multipeaked, ... Dedicated retracking algorithms are required to properly retrieve the altimetric range. First, we propose to localise the waveform classes onto Google Earth to appreciate their repartition according to surface type (land/water). Finally, we will be able to choose an appropriate retracker to plot the water level fluctuations.

Data chosen

To limit the volume of data to download, it is better to determine the ground tracks numbers over the area of interest (here, the Sudd Marshes). These ground track numbers are available in the pass locator on Aviso web site (download the .kml file for the Jason-2 referenced orbit to visualize it on Google Earth). Here, the Jason-2 interesting passes is: #120.
The time series used for this data use case streches out from the cycle_001 (July 2008) to cycle_094 (January 2011).
Although BRAT can handle numerous files, it appeared preferable in terms of efficiency for BRAT to perform an extraction of the files over a portion of track. We performed this 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 7°N and 9°N (download the script here).

Operation

to extract the waveforms classes

Once the relevant files are all downloaded, create a dedicated workspace and then, a new dataset in the "Dataset" tab: in dataset_tr120, we have added the whole time series defined above.
On "Operations" tab, we create one operation by waveform class mainly encountered on our area. Each waveform is classified according to its main shape and a number is assigned to each class. You can retrieve these classes in the Coastal and Hydrology Altimetry product (PISTACH) handbook in the session "Classification of the waveforms". In our case, four operations are created:

1) operation_120_wvf1 for ocean (Brown) echoes
2) operation_120_wvf2 for specular echoes
3) operation_120_wvf12 for Brown+peaky echoes
4) operation_120_wvf23 for multipeaked echoes

fig 1: Screen of the GE-Graph software to export from BRAT, the longitude/latitude points corresponding to each waveform class on Google Earth.

For the first operation above, drag and drop the longitude (ie the field "lon") on "X", the latitude (ie "lat") on "Y" and the field wf_class_ku on "Data". Define the selection criteria to only have the data for the considered waveform class (here for ocean echoes) over a narrow segment: (is_bounded(7.35, lat, 7.7)) && (wf_class_ku == 1).
Click on the "Export" button to obtain an ascii output file. Rename the .txt file as ExportAsciiOperations_120_wf_class1.txt. Then do the same previous steps for each waveform class by duplicating the first operation (rename it and change the waveform class value in the selection criteria).
Then, we use a free software to export on Google Earth, the longitude/latitude points corresponding to each waveform class. Download this GE-Graph sofware here and proceed as follow:

retrieve the output files in the Operation folder in your workpace and open them in excel. A three-row file (with latitude, longitude, waveform class) is opened per each waveform class file.
in excel, add a first row to insert an index corresponding to the value number and corresponding to the Place/WP name in GE-Graph. So in excel, four rows are created (per file) : index, lat, lon, waveform class value. Select and copy all the data.
open Ge-Graph, and click on "Paste grid from clipboard" button. Note that the row order is changed : Latitude, Longitude, Place/WP name and Value. Parameter the shape, size (constant), the color (different for each waveform class), the title.
Save the .kml file from GE-Graph and open it in Google Earth.


fig 2: LEFT: Waveform class = 2 corresponding to Specular echos. RIGHT: Waveform class = 1 (red), 12 (yellow) and 23 (green) respectively corresponding to Ocean(Brown), Brown+Peaky and Multipeaked echos (exported from Brat in GE-Graph and display on Google Earth).

to represent the water level variations

The distribution of echo shape over our selected area (7.35°<lat<7.7°) is dominated by echoes conforming to the Specular class with a high-power narrow echoes (almost 96%, in blue on fig.2). The Brown+Peaky echoes class (in yellow on Fig.3) is then represented (almost 3%). The two last waveform classes represented here are less represented (Brown echos and Multipeaked echoes). An appropriate retracker has to be choosen taking into account the specific narrow shape of the echoes. In "Operation" tab, create a new operation to plot the water level variations with all the available retracking algorithms (named Operations_p120_retrackers_vs_time) with the same Dataset, dataset_tr120. Drag and drop the field time on "X". The time is expressed in seconds, to convert it in day, write in the gray box: round(((time / 24) / 60) / 60).
Then insert nine new expressions in Data (an expression per RETRACKING): ice_range_ku, range_c, range_ku, range_ice3_c, range_ice3_ku, range_oce3_c, range_oce3_ku, range_red3_c, range_red3_ku. For each expression, insert the following operation to compute the Water Surface Altitude (WSA):

WSA=alt - RETRACKING - geoid_EGM2008 - model_dry_tropo_corr - model_wet_tropo_corr - pole_tide - solid_earth_tide - iono_corr_gim_ku

The typical computation of a Water Surface Altitude for hydrologic areas (height of the surface of a given water body above the geoid) is calculated by subtracting the corrected range from the satellite altitude. The corrections are: wet troposphere, dry troposphere, ionosphere corrections and geoid, tides (pole, solid earth). The radiometer and the difference between the altimeter dual-frequency are usually perturbed by emerged lands. Thus, the wet troposphere and the ionosphere corrections are computed from models.
Click on "Execute". In the "Views" tab, click on "New" and rename the plot in "View name". The previous operation (Operations_p120_retrackers_vs_time) is displayed on the left box; unfold it to see all the expressions and put all of them on the right. The mark "Group expressions on the same plot" is tickked off. Click on "Execute".

2.5.6 Altimetry for wetland studies by using hydrology dedicated products.