Ground Water Recharge through Percolation Tanks in the West Coast Region of Karnataka

The West Coast region of Karnataka is characterised by lateritic soils with a high coefficient of permeability. A large number of irrigation wells with good yields are found in the region. The water table remains high till around February because of the heavy rainfall during the monsoon. However, by March, a shortage of water is felt, probably because of heavy pumping and also due to groundwater drainage into the sea. A project was therefore initiated to study the feasibility of recharging the groundwater by constructing surface ponds and reducing run off during the monsoons.

Recharging of groundwater has been practiced in Maharastra since around 1950 through what are known as 'Percolation tanks' and in Rajasthan for even longer by menas of 'rapats'. The recharging involves building an earth dam (percolation tank) or a stone or concrete dam (rapat) across a stream. In the former case, a waste weir is provided to pass surplus discharge, and in the latter case, the surplus flows over the rapat itself. Unlike conventional tanks and dams, no irrigation canals are provided. Part of the water stored behind the dam evaporates and part of it percolates into the ground. As a result of the percolation, the water table as well as the well yield rises downstream of the dam. In order to facilitate the percolation, a full cut-off wall is not provided as in conventional dams.

In the first phase of the project various sand bed laboratory models for studying the percolation phenomenon were built. In these experiments two layers of sand of different permeability's were used to stimulate the ratio of permeability's (of the surface soil to the laterite at the water-bearing level) as found in the coastal region of Karnataka. The models built were capable of stimulating a dam at any one of the three positions along its length.

A two dimensional theory for explaining and predicting various percolation parameters was developed.

The designed storage capacity of the percolation tank is 20,000 cubic meters and the total submerged area would be 2 hectares of practically barren forest land. Using the theory for percolation developed in phase 1 of the project, it is predicted that the tank would improve the irrigation potential from well downstream in 40 hectares of cultivated land by making available 4,5000 cubic meters of additional water. Apart from increasing the irrigation potential, the tank could provide water for reforestation activities on the largely denuded upper slopes of the tank area as well as on some slopes downstream.

Another important benefit of the project is the reduction of reverse diffusion of seawater in the soil, occurring now as a result of over exploitation of wells and thereby prevent salinization of ground water and irrigated surface soils. It would also help in increasing drinking water supply during the lean summer season.