Where the Rain Should Stop: Planning a 40 ha Catchment
You are with a watershed cell in a hilly block on the Baran–Jhalawar border of southern Rajasthan, where a 40 ha catchment feeds a check dam that is silting up. Slope is 8–12°, soil is 15–40 cm deep over quartzite, and the 850 mm annual rainfall falls in about 35 rainy days, often as 60 mm bursts. Two villages graze roughly 400 cattle on the upper slope. You have ₹6 lakh and sanctioned nursery stock of 20,000 seedlings from a list containing teak, bamboo, neem, aonla, anjan (Hardwickia binata), khair and a grass mix - but no khejri. The Divisional Forest Officer wants a one-page plan tomorrow, and the villagers have already objected to any fencing.
1. Where would you place soil and moisture conservation structures, and would you use continuous contour trenches? Justify from slope, rainfall intensity and soil depth.
Read the worked answer
No continuous contour trench on this site. On an 8–12° slope with 60 mm bursts, a continuous trench captures overland flow along its whole length, and one breach at a weak point concentrates water into a gully - which is exactly how the check dam is silting now.
Instead:
- Staggered contour trenches, 0.5 m × 0.5 m × 0.5 m, at 3–5 m spacing, only where soil depth is more than 30 cm; leave a gap between trenches in each row so water spills safely.
- Boulder check dams in the first- and second-order gullies, and a silt trap just above the check dam to catch the coarse load.
- No trenching on the ridge, where 15 cm soil over quartzite would be scraped to bedrock.
- Keep roughly 15% of the area untreated as a control, so the effect can be measured.
All earthwork must be finished before the monsoon, since a half-dug trench in June is worse than none.
2. Distribute the 20,000 seedlings across the slope and gullies, and explain why khejri is not on the list.
Read the worked answer
Match species to soil depth and moisture, not to availability:
- Gullies and gully heads (moist, silty): bamboo, which binds the gully bed and is harvested for baskets - a stake for villagers.
- Lower slope (30–40 cm soil, moisture received from above): teak and anjan.
- Mid-slope: neem and aonla, both hardy and useful for fruit and fodder.
- Ridge and shallow soil: the grass mix with khair, which survives on poor, dry sites and fixes nitrogen.
Khejri (Prosopis cineraria) is the state tree and a keystone of the arid west - deep sandy soils, 300–400 mm rainfall, a different rooting and water regime. Planted on a shallow quartzite slope with 850 mm rain it would fail, and because khejri felling is restricted in Rajasthan, a failing plantation would also create a compliance problem.
The rule: a plantation succeeds when the species matches the microsite, and a nursery list is not a plan.
3. How do you protect the plantation without fencing and without an enforcement confrontation over grazing?
Read the worked answer
Fencing 40 ha against 400 cattle is unaffordable and would harden the opposition. Use social fencing with material incentives.
- Convene the gram sabha first. In Scheduled Areas, and where a community forest resource right exists, grazing and collection are legitimate rights, not trespass; a guard who begins with fines loses the plantation.
- Agree a grazing calendar: keep the planted patches closed for three years, and in exchange assign a defined grazing block on the untreated 15% and allow grass harvest from the plantation area after the second year.
- Hand over protection to a village forest committee with two watchers per village, paid from the watershed works component.
- Employ villagers on the trenching and boulder work - the same households that would otherwise graze it will then guard it.
Write the agreement down and get it signed by the sarpanch and the committee, because an oral understanding collapses at the first dry year.