Groundwater Table

'Groundwater Table' the top zone of soil and rock that is saturated with water; ie it is the upper surface of a zone of saturation or the upper surface of the groundwater. Its level varies with topography and infiltration of water from rainfall or other sources. Under natural conditions water table is the subdued replica of the surface topography. Groundwater generally flows from areas where water table is higher to that of lower elevation. In the absence of groundwater flow, the water table will be flat and a sloping water table indicates that the groundwater is flowing. Natural springs are formed when the water table meets the ground surface.

In Bangladesh the depth of water tables varies from less than a meter to more than 30m. The shallowest water table occurs in the coastal region whereas the deepest water table occurs in the barind tract and Dhaka City (more than 30m from the ground surface). The depth to the water table moves seasonally with annual recharge and discharge conditions. Here, the water table is shallowest towards the end of monsoon and deepest in April-May. The amount of seasonal fluctuation varies from less than a meter to more than 10m depending on the local hydrogeological conditions, amount of groundwater abstraction and natural discharge of groundwater. In recent years there is a declining trend in the deepest water table due to larger amount of groundwater withdrawal, particularly for irrigation. The bangladesh water development board maintains a national network of observation wells all over the country and measures the depth of the water table once a week. [Kazi Matinuddin Ahmed and M Qumrul Hassan] [Ahmed, Kazi Matin Uddin  Professor of Geology, Dhaka University]