Reading a basalt watershed's falling discharge and judging a silted check dam
A 210 sq km watershed near Koregaon in Satara district lies on Deccan basalt covered by only 4–6 m of weathered mantle, with a few laterite-capped plateaus. Mean annual rainfall is 1,420 mm, of which 1,180 mm falls on just 34 rainy days between June and September; August 2018 contained a 22-day dry spell. The irrigation department records the main stream's discharge at the outlet as 1.35 cumec on 20 July, 0.90 cumec on 5 September, 0.42 cumec on 15 November and 0.11 cumec on 20 February. A check dam built in 2015 across a third-order tributary has trapped sediment to within 0.8 m of its crest. Wells within 300 m of the dam show nitrate of 52 mg/l; wells on the plateau show 18 mg/l. Farmers downstream of the dam complain that February flow has fallen since 2016, while farmers just upstream report higher well yields and are asking for two more check dams.
1. Explain the shape of the recession curve - why discharge falls so steeply - using the rainfall pattern and the geology.
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The numbers: 1.35 cumec (20 July) → 0.90 (5 September) → 0.42 (15 November) → 0.11 (20 February). Discharge loses about two-thirds of its July value by 5 September, then a further 88% by February; that is a steep, early recession, not a slow one.
Rainfall control. 1,180 mm on 34 rainy days averages about 35 mm per rainy day, i.e. high-intensity storms. Intensity exceeds infiltration capacity, so a large share leaves as surface runoff in a few days, and the 22-day August dry spell interrupts recharge exactly when the soil is wettest. Rainfall is concentrated, not spread, so there is little slow top-up later.
Geology control. Deccan basalt with only 4–6 m of weathered mantle is a low-storage, hard-rock aquifer. The weathered zone saturates quickly and drains within weeks - that is the July–September decline. After it empties, flow must come from fractures and joints, which store little and transmit slowly, so by February only a thin base flow of 0.11 cumec survives. A thick alluvial aquifer would show a much flatter curve.
Conclusion: the curve is the signature of high-intensity monsoon rain on a thin, hard-rock mantle - flashy runoff, fast depletion, and a February flow that is almost entirely base flow and therefore very vulnerable to any upstream abstraction or storage.
2. Has the check dam worked? Evaluate it on storage, groundwater, water quality and downstream flow, and state what a fair verdict requires.
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Local groundwater: yes. Upstream farmers report higher well yields, which is what a check dam is designed to do - slow the stream, spread the water, and give the weathered zone more time to recharge.
Storage: badly degraded. Sediment has filled the reservoir to within 0.8 m of the crest, so live storage is almost gone. A structure that has silted to this level within a few years is now mainly a sediment trap, not a recharge structure, and its recharge benefit will decline as the bed rises.
Water quality: warning sign. Nitrate of 52 mg/l near the dam versus 18 mg/l on the plateau cannot come from the basalt. It indicates irrigation return flow, fertiliser and cattle waste entering the shallow, now-permanent water table near the dam - an unintended consequence of raising the water table in a cultivated, sugarcane-and-onion belt.
Downstream flow: plausible but not yet proven. A reduction in February flow is exactly what upstream storage plus local recharge and evaporation would cause, but the 2016-onwards comparison may also reflect poorer rainfall years. The verdict needs a before-and-after comparison normalised by rainfall, not one dry year.
Fair verdict: a locally successful recharge structure that has failed as storage, is degrading water quality, and has redistributed water from downstream to upstream users. Desilt it, and do not sanction two more until the downstream share is formally allocated.
3. Recommend three watershed measures, specifying where each should go and why that location.
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1. Continuous contour trenches and compartment bunding on the laterite-capped plateaus. The plateau tops generate the most intense runoff and currently send it sheet-flowing off the edge. Trenches there intercept rain before it becomes stream flow, and the laterite cap is where soil moisture is most limiting for cashew and fodder. This treats the source, not the channel.
2. Farm ponds in the upper reaches of second-order gullies, above the check dam. A farm pond in a gully head captures the early, silt-laden flush before it reaches the dam, protecting the dam's remaining 0.8 m of storage and giving a protective irrigation dose in the August dry spell. Placing it above the dam, not below, is what reduces silt.
3. Recharge shafts or defunct dug wells in the weathered zone along fracture lines in the middle reaches. With only 4–6 m of mantle, surface spreading reaches a thin layer; a shaft that penetrates the weathered zone into the jointed basalt connects monsoon water to the fracture storage that sustains February base flow. It must be sited on a mapped joint/fracture line, not arbitrarily.
Supporting rules: desilt the existing dam and use the silt on fields; keep a fertiliser and cattle-shed buffer within 100 m of the dam to control nitrate; and make any new structure conditional on a written downstream release arrangement, since the present dispute is about distribution, not about the technology.