Estimating groundwater resource and understanding recharge processes in the rapidly urbanizing Dhaka City, Bangladesh

Rapid urbanisation in Dhaka (Bangladesh) with high dependency on the groundwater have recently raised concern over the sustainability of the aquifers. The present study focusses on quantifying the dynamic groundwater resource vis-à-vis the status of groundwater development and understanding the different recharge processes in the Dhaka City, using the water balance based lumped and distributed approaches. In the lumped approach, the groundwater assessment is carried out using groundwater balance method following the methodology and norms recommended by the Groundwater Estimation Committee-2015 (referred hereafter as GEC). In the distributed approach, a physics-based hydrological model (Variable Infiltration Capacity, VIC) is utilised to model the baseflow (i.e., potential recharge). Then, an attempt is made to establish the relation between VIC-based baseflow and actual (GEC-based) groundwater recharge from rainfall. Remote sensing data derived spatio-temporal information along with field-based observations have been used in the process. Dhaka City is divided into 10 zones and the groundwater assessment is made individually in these zones as well as for the entire city during 2001–2015 period. The total annual recharge is estimated as 635.35 million m³ (or ~2065 mm/year per unit area). Lateral and unaccounted recharge is found to be the dominant (51.8%) process in the study area, followed by recharge from urban return flow (35.5%), rainfall (7.1%), and surface water bodies/wetlands (5.6%). The study underscores that the lateral and unaccounted recharge must not be ignored in the urban landscape underlain by alluvial aquifers and with non-hydrologic unit boundaries. The stage of groundwater development is found to vary from 117 to 320%, thus indicating overexploitation of aquifers in all the zones. The higher groundwater abstraction as compared to recharge in all the zones has led to consistent decline in groundwater level by 2–3.2 m/year across the study area. The VIC model, on the other hand, depicted that nearly 17.2% of the annual rainfall contributes towards baseflow. Further, it is observed that ~31% of baseflow contributes to groundwater recharge due to rainfall. Researchers can use this relationship for quick assessment of the groundwater recharge from rainfall in Dhaka, based on hydrological models with varying meteorological and land cover scenarios, especially in the light of climate change and rapid urbanisation.