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

The 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 m 3 /s) using TOPEX‐Poseidon over the Brahmaputra to ∼36% (∼9000 m 3 /s) using ERS‐2 over the Ganga. Combined Ganga‐Brahmaputra monthly discharges for 1993–2008 are presented, showing a mean error of ∼17% (∼2700 m 3 /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 m 3 /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.