Uncertainties in water stages by altmetry assessed by field measurements
Stephane Calmant(1) , Frederique Seyler(2) , Kevin Cheng(3) , Juan Leon(2) , frederique Frappart(4) , Marie Paule Bonnet(2) , Mathilde Cauhope(2) , Naziano Filizola(5) , and CK Shum(3)
14 av E belin,
(2) IRD/LMTG/OMP, 14 Av E. Belin, Toulouse, France
(3) OSU, Ohio State Univ, Columbus, United States
(4) LEGOS/OMP, 14 AV E. belin, Toulouse, France
(5) University of Amazonas, University of Amazonas, Manaus, Brazil
Although no satellite altimetry mission has been dedicated yet to continental hydrology, radar altimetry from space has been successfully used to monitor temporal variations in lake and river stages. Compared with ocean studies, large uncertainties are reported. Several factors can be inferred to explain these large uncertainties, as -among others- the land-water mixture in the radar beam footprint, tracking-retracking algorithms, missing or inadequate geophysical corrections…But, for the present, uncertainties of altimetric radar measurements have been only assessed by comparison with in-situ gauge stations, sometimes located at tens of kilometres away from the satellite tracks. Thus, these uncertainties encompass the changes in hydrological regime between the station and the satellite track. Indeed, when a satellite track is running along a large river, as it is the case for the Tapajos River (Amazon Basin), measurement noise appears to be ~10cm, far less than the 20 to >100cm commonly reported.
We have conducted field campaigns in the Amazon basin, in order to assess radar altimetry uncertainties in river stage estimates, for various morphological situations. We have realized altitude profiles for the Branco and upper Rio Negro rivers in GPS cinematic mode. These operations were conducted with GPS stations observing as fixed reference points every ~50 kilometres, and mobile GPS antennas set on top of a boat cruising down the rivers. ADCP (Acoustic Doppler Current Profiler) profiles were also performed under each crossing points between the T/P, JASON and ENVISAT tracks and the two rivers. These ADCP profiles provide cross-sections and flow velocity. Using the information from both GPS and ADCP, we run a 1D model to estimate the water stage at the crossing points. Comparison of modelled water stages and altimetric heights enables an evaluation of the uncertainties of radar measurements in different situations.