Calculation of river discharge and prediction of lake height from satellite radar altimetry: Example for the Lake Chad basin

The application of satellite radar altimetry to the determination of lake and river elevations has been used in numerous projects, and is well validated. Here we show that with the aid of ground-based information, this technique can be extended to determine river discharge and predict downstream lake and marsh height. The Lake Chad basin provides an ideal case study due to its well-known hydrology and complex lake and marsh morphology and because prediction of lake and marsh height has been identified as potentially useful to people living in the region. Altimetric stage measurements from the TOPEX/Poseidon satellite, at the Chari/Ouham confluence, estimate river discharge about 500 km downstream at N'Djamena 10 days in advance (r(2) = 0.9562). Via simple linear correlation methods, the stage measurements successfully estimate the height of the permanent waters of the lake (600 km downstream) 39 days in advance (r(2) = 0.9297). Predicting the water height on the western marshes of the lake bed is poorer (r(2) = 0.7958) due to a change in response time of the local stage to the seasonal floods coincident with an observed increase in mean water level in the latter half of the 1990s. Before 1997 a 96-day phase lag results in the best fit (r(2) = 0.6463). After 1997 the best fit is obtained with a 66-day phase lag (r(2) = 0.8139). The excellent river discharge and lake height predictions show that altimetry is a useful tool where ground-based data are difficult to obtain and where rapid water resource assessment is desirable.