Abstract
Abstract. 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. Our findings provide important insight into the hydrological impacts of groundwater-fed irrigation and sea-level rise in other Asian mega-deltas where monitoring data are limited.
Highlights
Asian mega-deltas feature regionally extensive shallow aquifers within sedimentary sequences deposited over the last 10 ka (Benner et al, 2008)
Seasonal groundwater flow-fields at the regional scale are shown in Fig. 4 and Supplementary Fig. I; presented contours naturally mask localized groundwater flowfields as each monitoring well represents a mean area of 105 km2
Our analyses show that shallow groundwater in the GBM Delta is highly dynamic with strong seasonality and trends of variable magnitude
Summary
Asian mega-deltas feature regionally extensive shallow aquifers within sedimentary sequences deposited over the last 10 ka (Benner et al, 2008). Groundwater levels in shallow aquifers (depth
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