5.1.3 Percolation tanks

The percolation tanks are mostly earthen dams with masonry structure only for spillway. These are the most prevalent structures in India as a measure to recharge the groundwater reservoir both in alluvial as well as hard rock formations.
Percolation tank is an artificially created surface water body, submerging in its reservoir a highly permeable land so that surface runoff is made to percolate and recharge the ground water storage.
Percolation tank should be constructed preferably on second to third order streams, located on highly fractured and weathered rocks, which have lateral continuity downstream.
Percolation tank should be located on highly fractured and weathered rock for speedy recharge. In case of alluvium, the bouldary formations are ideal for locating percolation tanks.
The aquifer to be recharged should have sufficient thickness of permeable vadose zone to accommodate recharge.
The downstream of recharge area should have sufficient number of wells and cultivable land to get benefit from the augmented ground water.
In Peninsular India with semi arid climate, the storage capacity of percolation tank should be designed such that the water percolates to ground water reservoir by January/February, since the evaporation losses would be high subsequently. • The size of a percolation tank should be governed by the percolation capacity of the strata in the tank bed rather than yield of the catchment. In case, the percolation rate is not adequate, the impounded water is locked up and wasted more through evaporation losses, thus depriving the downstream area from the valuable water resource.
Detailed analysis of the rainfall pattern, number of rainy days, dry spells, evaporation rate and detailed hydrogeological studies are necessary to demarcate suitable percolation tank sites.
Detailed hydrological studies should be done for runoff assessment and designed capacity should normally not be more than 50 percent of the total quantum of utilizable runoff from the catchment.

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