Where to Build the Percolation Tank? A Watershed Map of a Kolar Micro-Watershed
A watershed committee in a Kolar village has a sketch map of a 60-hectare micro-watershed. Four sites are marked with elevation and soil: A (812 m, upper slope, 1.2 m soil), B (805 m, mid-slope near a field bund, 2.5 m soil), C (798 m, valley bottom beside the stream, 3 m permeable soil), D (815 m, ridge, 0.5 m soil). The stream flows south to north. The village gets 620 mm rain, mostly July-October, and the open well downstream drops every summer. The panchayat has funds for one tank. Farmers near the ridge want it at A to “catch rain first”; farmers downstream want C. The committee must recommend one site and explain the trade-off. The map has no scale bar, but the distance from A to C is about 900 m. A percolation tank is meant to recharge groundwater, not to store water for irrigation.
1. Recommend one of the four sites for the percolation tank. Use the contour values, soil depth and stream direction in your reasoning.
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Site C is the best choice for recharge. It lies at 798 m, the lowest of the four, beside the stream that flows south to north, so water arriving from the 60-hectare catchment can be held there. The soil is 3 m deep and permeable, which allows water to move down into the aquifer rather than run off. Site B at 805 m is a second option, but a tank there would intercept water before it reaches the lower fields and would need a larger bund. A percolation tank is not a storage dam: its purpose is to slow water and let it seep, so the valley-bottom site matches the purpose. The recommendation should be conditional on an engineering survey and on the landowner's consent.
2. Farmers near the ridge prefer site A. Explain why their preference is understandable but not the best watershed decision.
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Ridge farmers want the tank close to them because they assume that water caught first will recharge their own wells. At A the elevation is 812 m and soil is only 1.2 m deep over rock, so a tank would hold little water and seepage would be limited; the bund would also need to be high. D is worse: 815 m on the ridge with 0.5 m soil. The ridge preference is understandable as a local claim on a common resource, but a watershed is connected: recharge at C benefits the whole aquifer, including upper wells over time. The committee should explain this with the contour map and offer ridge farmers a share in the maintenance and in the water-user association.
3. If site C is chosen, what two conditions would you attach to protect downstream users?
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First, the tank should not be allowed to store water permanently or block the stream in the monsoon; a controlled spillway and a maximum water level should protect the flow that downstream farmers depend on. Second, a water-user association with downstream representation should monitor the tank, and the panchayat should publish the water level and the social audit. A third condition could be compensatory work, such as field bunds on the upper slopes, so that the tank does not become the only intervention. These conditions turn a local construction decision into a shared watershed rule.