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Understanding of Ground Water-Surface Water Relationship through the Analysis of Ground Water Flow System of Part of Bengal Delta

Sheikh Mahabub Alam

The study area is part of Ganges-Meghna Deltaic Plain of the largest in the world, enjoys monsoonal climatic conditions. The main aquifer (aquifer-1) is made up of unconsolidated mostly sandy materials with some finer sediment and is gradually coarsening downwards with gravels and pebbles at the bottom. Hydraulic characteristics indicator confined conditions for the smaller eastern part while leaky-confined conditions prevail elsewhere. The ground water elevation is highest in the east-central (Comilla town) region and this is primarily due to maximum withdrawal and partly due to partial blockage of ground water flow by the north-south trending Lalmai Hills located to the west of Comilla town. The complicated flow net structure and its behaviour suggest that all perennial rivers maintain direct hydraulic continuity with Ground Water. Flow-net analysis reveals that most rivers in its downstream show the characteristics of a gaining stream conversely upstream river sections feeds the ground water with few exceptions. The surface water contribution of Gumti River to ground water is estimated to be about =707616000 m³/ annum at Comilla and Jibanpur station for the year 1979-1980 and 1980-1981. The Muhuri River discharge measured at Parshuram shows a baseflow component of 38 percent. The general ground water flow directions are mainly towards west, south-west, north, north-west and south. This unique flow pattern has developed because of the variable sloping direction. The close correlation in hydraulic gradient suggests a similar distribution of resource potential in different regions and also indicates that most parts of the aquifer are well connected and the permeability does not change abruptly. A sparse distribution of equipotential line and small hydraulic gradient is indicative of higher resource potential. The pattern of change of fluctuation in the north decreases radially from the central maximum in contrast with the southern region where the pattern of change is almost opposite. The total amount of recharge in the form of effective infiltration (Ie = 560 mm/annum or 5.71 x 10⁹ m³/annum) calculated from the annual volumetric changes (Wy = 2.85 x 10¹⁰ m³/annum).