Slope protection and dry-season irrigation for a women's vegetable plot
A women's self-help group in a village on the fringe of Karbi Anglong cultivates 0.8 ha of vegetables on a sandy loam plot with a 9% slope. The plot loses topsoil during the May–September monsoon, and spinach yields have fallen. A perennial stream flows at the bottom of the plot, 20 m below the field and 140 m away. In the dry season (November–March), stream flow is about 6 L/s. The group has a 3 hp diesel pump rated at 15 m³/h at 25 m total head and can invest ₹60,000. They want to grow tomatoes and French beans for the Kohima market, but water reaches the plot only by a loose earthen channel that loses about 30% of the flow. The group has asked for a soil-and-water conservation plan that also gives reliable dry-season irrigation.
1. Recommend a soil conservation measure for the 9% slope and justify it against bench terracing. Give approximate vertical interval and spacing for contour bunds.
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For a 9% slope growing vegetables, contour bunds or stone lines with a stable grassed outlet are more appropriate than bench terracing. Bench terracing requires cutting and filling, disturbs the topsoil, needs careful design and is expensive for 0.8 ha; it is normally considered on steeper slopes where annual crops cannot otherwise be grown. On 9%, the main loss is sheet and rill erosion, which contour bunds can interrupt.
Design assumptions:
- Vertical interval: about 1.0 m. A common field guide is VI = 0.3 × (slope % + 2) feet, but for a 9% slope a 1.0 m vertical interval is conservative and practical.
- Horizontal spacing = VI / slope = 1.0 / 0.09 ≈ 11 m.
- Bund section: 0.5 m top width, 0.4 m height, side slopes 1:1, with a 0.3 m deep grassed waterway or stone outlet at the lower end.
- Stabilise bunds with local grass or vetiver; do not leave loose soil.
Also adopt contour ploughing, a vegetative filter strip at the bottom, and a small sediment trap before the stream. These measures reduce soil loss and, importantly for the group, reduce the muddy runoff that blocks the pump suction.
2. Calculate the crop water requirement and pump hours for a 5 mm/day irrigation depth over 0.8 ha, allowing for the 30% channel loss. Is the 3 hp pump adequate?
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Plot area = 0.8 ha = 8,000 m².
- Net irrigation requirement = 5 mm/day × 8,000 m² = 0.005 m × 8,000 = 40 m³/day.
- With 30% loss in the earthen channel, the diversion required = 40 / 0.70 = 57.1 m³/day.
- Pump capacity is 15 m³/h at 25 m head, so pumping time = 57.1 / 15 = 3.8 h/day.
The pump is hydraulically adequate. The stream at 6 L/s = 21.6 m³/h, well above 15 m³/h, although the pump suction must be protected from silt and the stream may fall late in the season. The real constraint is the channel loss and the 20 m lift, not the pump size.
If the channel is replaced by a pipe with 5% loss, diversion falls to 40 / 0.95 = 42.1 m³/day and pumping time to 2.8 h/day. That saves about 15 m³/day, or 2,250 m³ over a 150-day season, plus diesel and labour. The group should also schedule irrigation early morning or late afternoon to reduce evaporation and avoid peak heat.
3. Would you recommend buried HDPE pipe or lining the earthen channel? Support the choice with a simple loss calculation and state one layout precaution on the slope.
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Buried HDPE pipe is the better choice. The earthen channel loses about 30% of 57 m³/day, i.e. about 17 m³/day. A well-jointed HDPE line can reduce loss to about 5%, giving a diversion of about 42 m³/day and a saving of about 15 m³/day. Over 150 days that is 2,250 m³ of water, plus roughly one hour per day of pumping time saved.
For 140 m of 63 mm HDPE, material and fittings may cost ₹20,000–25,000, within the ₹60,000 budget. Lining the channel with cement or plastic sheet is cheaper initially but is easily damaged by runoff, livestock and cultivation, and still loses water at joints and cracks.
Layout precautions:
- Bury the pipe at least 0.3 m deep across the slope, with a pressure rating of at least 2.5 kg/cm² and thrust blocks at bends.
- Install an air-release valve at the highest point and a drain valve at the lowest point; otherwise the line can air-lock or burst when the pump starts.
- Keep the pump suction in a small sediment pit with a screen, and use a foot valve.
- On the 9% slope, avoid long, steep pipe runs that create high static head; follow the contour where possible and use pressure-compensating drip tape if the group later converts to drip.