Satellite altimeter-derived monthly discharge of the Ganga-Brahmaputra River and its seasonal to interannual variations from 1993 to 2008

International audienceThe Ganga-Brahmaputra accounts for ∼25% of the total amount of freshwater received by the Bay of Bengal. Using daily in situ river discharge data along with altimetry-derived river heights, the present study aims to produce a monthly data set of altimetry-derived Ganga-Brahmaputra River discharge at the river mouths for 1993-2008. First, we estimate the standard error of ENVISAT-derived water levels over the Ganga to be 0.26 m, much smaller than the range of variability of ∼7 m, and consistent with the accuracy of altimeter measurements over large rivers. We then establish rating curves between altimetry-derived water levels and in situ river discharges and show that TOPEX-Poseidon, ERS-2, and ENVISAT data can successfully be used to infer Ganga and Brahmaputra discharge. The mean error on the estimated daily discharge derived from altimetry ranges from ∼15% (∼4700 m3/s) using TOPEX-Poseidon over the Brahmaputra to ∼36% (∼9000 m3/s) using ERS-2 over the Ganga. Combined Ganga-Brahmaputra monthly discharges for 1993-2008 are presented, showing a mean error of ∼17% (∼2700 m3/s), within the range (15%-20%) of acceptable accuracy for discharge measurements. During 2004-2008, we assess the variability of the estimate against precipitation and river heights records. Finally, we present a basic approach to infer Ganga-Brahmaputra monthly discharge at the river mouths. The upscaled discharge exhibits a marked interannual variability with a standard deviation in excess of ∼12,500 m3/s, much larger than the data set uncertainty. This new data set represents an unprecedented source of information to quantify continental freshwater forcing flux into Indian Ocean circulation models