Large‐scale controls on Ganges and Brahmaputra river discharge on intraseasonal and seasonal time‐scales

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Abstract
Reliable water supply from the Ganges and Brahmaputra is of critical importance to the sustainability of the agricultural societies of India and Bangladesh. But, the flow in both basins is highly variable on time‐scales ranging from days to years, creating challenges for the optimization of agricultural practices, water resource management and disaster mitigation. The following questions are addressed. Is intraseasonal monsoon variability related to the subseasonal variability of river flow? Do variations in the large‐scale tropical sea‐surface temperature (SST) located both regionally and remotely promote seasonal and interannual variations of river discharge? And, if these relationships do exist, are they determinable with sufficient lead‐times to allow useful predictions for user communities in South Asia? We examine these questions using 50 years of daily river discharge data for both rivers calculated at the points where they enter Bangladesh, and with SST data in the Indo‐Pacific region. We also examine the question of determining the impact of man‐made dams, diversions and barrages on the data record, especially that of the Ganges. A comparison of discharge prior to 1974 (the time of the construction of the largest barrage) shows no statistical difference that cannot be explained by basin‐wide rainfall distributions. Changes that do occur are restricted to the dry‐season months.Subseasonal river discharge is found to be strongly tied to the monsoon intraseasonal cycle resulting in a near‐in‐phase timing of Ganges and Brahmaputra discharge. A basin isochrone analysis is used to couple stream‐flow variability and intraseasonal precipitation during the different phases of the intraseasonal cycle. On longer time‐scales, statistically significant correlations are found between mean monthly equatorial Pacific SST and the boreal summer Ganges discharge with lead times of 2–3 months. These relationships are tied to El Niño–Southern Oscillation (ENSO) oscillations in addition to SST variability in the southwest and northwest Pacific that also seems to be related to ENSO. The Brahmaputra discharge, on the other hand, shows somewhat weaker relationships with tropical SST. Strong lagged correlation relationships are found with SST in the Bay of Bengal but these are the result of very warm SSTs and exceptional Brahmaputra discharge during the summer of 1998. When this year is removed from the time series, relationships with SST anomalies weaken everywhere except in the northwest Pacific for the June discharge and in areas of the central Pacific straddling the Equator for the July discharge. In addition, the northwest Pacific relationship changes polarity for June and July discharges. Although the relationships are weaker than those found for the Ganges, they are persistent from month to month and suggest that two different and sequential factors influence Brahmaputra river flow. Copyright © 2009 Royal Meteorological Society