Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 13 12 2009 10.5194/hess-13-2373-2009 http://www.hydrol-earth-syst-sci.net/13/2373/2009/ http://www.hydrol-earth-syst-sci.net/13/2373/2009/hess-13-2373-2009.html http://www.hydrol-earth-syst-sci.net/13/2373/2009/hess-13-2373-2009.pdf 2373 2385 2009-12-10 Recent trends in groundwater levels in a highly seasonal hydrological system: the Ganges-Brahmaputra-Meghna Delta M. Shamsudduha m.shamsudduha@ucl.ac.uk R. E. Chandler R. G. Taylor K. M. Ahmed Department of Geography, University College London, London, UK Department of Statistical Science, University College London, London, UK Department of Geology, University of Dhaka, Dhaka, Bangladesh Groundwater levels in shallow aquifers underlying Asian mega-deltas are characterized by strong seasonal variations associated with monsoon rainfall. To resolve trend and seasonal components in weekly groundwater levels in the Ganges-Brahmaputra-Meghna (GBM) Delta, we apply a nonparametric seasonal-trend decomposition procedure (STL) to observations compiled from 1985–2005 in Bangladesh. Seasonality dominates observed variance in groundwater levels but declining groundwater levels (>1 m/yr) are detected in urban and peri-urban areas around Dhaka as well as in north-central, northwestern, and southwestern parts of the country (0.1–0.5 m/yr) where intensive abstraction of groundwater is conducted for dry-season rice cultivation. Rising groundwater levels (0.5–2.5 cm/yr) are observed in the estuarine and southern coastal regions. This novel application of the STL procedure reveals, for the first time, the unsustainability of irrigation supplied by shallow aquifers in some areas (e.g., High Barind Tract) of the GBM Delta and the hydrological impact of potential seawater intrusion of coastal aquifers associated with sea-level rise. 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