Data extraction | Computing water level variations

2.3 Temporal water surface height variations in enclosed areas:
the Amazon Basin

Methodology

Computing water surface heights

Water level h' is computed from GDR-M using:

h' = s - r

where s is the satellite's altitude (orbit) and r the range value.

Subtracting geoid and geophysical effects

Altimetry data must be corrected for geoid and propagation effects as follows:

h = h' - g - i - d - w

where g is the geoid value, i the ionospheric correction, d the dry tropospheric correction and w the wet tropospheric correction. NB: For T/P GDR-M, the wet tropospheric correction is not available for continents; thus in the present study we are unable to take this parameter into account [de Oliveira Campos et al., 2001]. Note also that depending on the area being studied, the dry tropospheric correction is fairly static, which means that sometimes it is possible to compute water surface heights without applying this correction.

Computing a mean water level

For each ground track, a mean water surface height is computed using

h_mean = (sum of h_i)/n

where h represents the value corresponding to the index i and n the number of altimetry measurements.

Mean water level variations in Manaus

Data series are shown in the following figures.
Left: Water level time series in the Amazon for T/P track 63 (3.21°S-3.14°S), in metres; dots represent in situ data from the Manau station. Right: Overview of the geographic window showing the stations; the backscatter coefficient and measurement density parameter have been computed along T/P track 63.