Spending ₹18 Lakh on Water in a Jalna Watershed
The watershed committee of a village in the rain-shadow belt of Jalna district must spend ₹18 lakh on water conservation across 640 ha. Rainfall averages 610 mm and has failed in three of the last seven years. Cotton and jowar cover about 70% of the cultivated area. The contour map shows three candidate sites. Site A: a narrow first-order valley, gradient about 8%, catchment 90 ha, 1.2 km from the main fields, black cotton soil that cracks deeply when dry. Site B: a broader second-order valley, gradient about 1.5%, catchment 210 ha, only 300 m from the fields, sandy loam over weathered basalt. Site C: on a stream villagers say runs for about 20 days a year, catchment 40 ha, 100 m from the fields. A check dam costs ₹5-7 lakh depending on width, a farm pond ₹3 lakh. Forty wells in a village 2 km downstream of Site B depend on the same stream. The committee may build at most three structures and must keep ₹1.5 lakh for repairs.
1. Allocate the money. State which structures you would build and why you rejected the others.
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Work from the constraint: ₹18 lakh minus ₹1.5 lakh for repairs leaves ₹16.5 lakh, which is roughly two check dams plus two farm ponds, or three check dams if all are narrow.
Site C should be dropped first. A stream that flows 20 days a year gives a storage structure almost nothing to capture; the structure would sit dry and would silt up during the few flood days. Catchment size is not the test; reliable inflow is.
Site A is technically attractive for storage per rupee because a narrow valley with a steep gradient lets a short, cheap wall hold a long, thin tank, and the distance from the fields is a labour cost, not a hydrological one. But black cotton soil cracks when dry, so seepage and, more importantly, rapid siltation from the steep catchment are real risks; it should be built only with a silt trap and a waste weir.
Site B has the largest catchment, the best soil for infiltration and the shortest walking distance, so it gives the best recharge per rupee for the 40 downstream wells. Its weakness is the low gradient: a long, shallow tank loses more to evaporation.
Recommendation: one check dam at B with a regulated outlet, one at A with a silt trap, one farm pond near the fields, and the repair fund kept intact.
2. A member says, 'Site B has the biggest catchment, so put all the money there.' Explain why catchment area alone is a poor basis for choosing a site.
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Catchment area tells you the potential supply, not the dependable supply and not the net benefit.
The yield also depends on rainfall, on how much of the catchment is bare and erosion-prone, on slope and soil, and on what is already stored upstream. A 210 ha catchment of badly eroded black soil will deliver a heavy silt load, so the tank loses capacity faster and the money spent on de-silting recurs every few years.
A single large structure also concentrates risk. One design flood, one breach or one bad monsoon and the whole investment is idle. Spreading structures across the watershed slows the water, recharges a wider area of aquifers and gives partial returns even in a poor year.
Finally, the forty downstream wells are a political and equity constraint. A large dam at B without a regulated outlet could reduce their recharge and convert a technical success into a village conflict. The committee should therefore judge sites on dependable inflow, silt risk, cost per cubic metre stored, and who gains and who loses, not on catchment size alone.
3. Explain how a student reading this contour map should interpret the spacing of contour lines, and connect that reading to the storage decision at Sites A and B.
Read the worked answer
Where contour lines crowd together, the ground rises steeply over a short horizontal distance; where they are widely spaced, the slope is gentle. The gradient figures in the problem (8% at A, 1.5% at B) are exactly this reading expressed as a number: rise over run.
For a check dam this matters in two opposite ways. In a steep, narrow valley like A, a modest wall height backs water up a long distance, so you get a large volume for a short, cheap wall - good economics. But the same steepness means the stream has high energy in flood, so it carries sand and gravel and can damage the structure; hence the silt trap and the spillway.
In a gentle, broad valley like B, water spreads sideways quickly, so a low wall floods a wide, shallow area. That is excellent for slow recharge into the sandy loam, but the large water surface evaporates more, and a wide, low embankment needs a longer wall for the same height, which is why cost depends on width.