Development of a contextual (multi-waveform) retracker for estimating water levels in lakes and reservoirs
Event: 2016 Ocean Surface Topography Science Team Meeting
Session: Science III: Two decades of continental water's survey from satellite altimetry - From nadir low-resolution mode to SAR altimetry, new perspectives for hydrology
Presentation type: Type Oral
Contribution: PDF file
While using satellite altimetry data for water level measurements, it is commonly observed that the conventional retrackers (e.g. Ice1, Ice2, Sea-ice) often select a point within the waveform that comes from an off-nadir location. This phenomenon called "hooking" in certain situations (usually when it occurs for a series of consecutive points) can even happen in the middle of a lake or a large river. In lakes and reservoirs, we argue that this is probably due to waves that can return more energy than the water at nadir. However, it is our hypothesis that the nadir return is most probably present as an individual peak within the waveform and most probably one of the first ones (unless the point is located near the shore where the topography can produce the same effect). In this context, knowing that within a series of consecutive waveforms, all inside the lake limits, nadir return should be expressed as one of the first significant peaks, it should be possible to link these peaks by a straight line representing the flat water surface. To do so, we have developed an algorithm that first converts the waveforms in distance (hence elevation) and processes the lot by "forcing" a straight line through the points from top to bottom until some threshold pressure is obtained. Each waveform is represented by a series of vertical points each representing a peak in the waveform. Because these points have been previously ordered by order of amplitude, the resulting water level line also takes the strength of the return signal into account but as a whole instead of individually.
The retracker has been successively tested in the Sobradinho Reservoir in Brazil which is crossed by four different orbit tracks and for the period 2002-2016 with Envisat and Saral and being compared with in situ data from the CHESF hydro-electrical company (in progress). Results from the two satellite are also analysed independently in a comparison effort.