Rainwater harvesting structures
Collecting rainwater for farms and homes: rooftop systems, micro-catchments, farm ponds, check dams, tanks and sand dams, with runoff and storage sums.
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In many dry regions, the problem is not only how little rain falls, but how much of it runs away in a few intense storms, leaving crops and people short of water for the rest of the year. Rainwater harvesting collects runoff from roofs, slopes, roads or streams and either stores it, in tanks, ponds and reservoirs, or helps it soak into the soil and aquifers. It is among the oldest water technologies, from the cisterns and runoff farms of the ancient Negev to the tanks of South Asia, and remains one of the cheapest ways to improve water security in rain-fed farming.
Water harvesting types, design and calculations are standard topics in soil and water conservation, dryland agriculture and agricultural engineering.
Classifying water harvesting
- Rooftop harvesting: water from roofs led to tanks or cisterns for drinking, livestock and kitchen gardens.
- In-situ (within-field) harvesting: holding rain where it falls, using contour ridges, tied ridges, zaï planting pits, half-moons and conservation tillage, covered in the article on dryland farming.
- Micro-catchment harvesting: runoff from a small area flows to an adjacent cropped area or tree basin, such as negarim (diamond-shaped) micro-catchments for trees.
- Macro-catchment (external catchment) harvesting: runoff from a larger area, such as a hillside or road, is diverted to fields or stored in ponds and tanks.
- Floodwater harvesting: spreading seasonal floods from ephemeral streams onto fields, such as spate irrigation in Yemen, Pakistan and the Horn of Africa.
- Groundwater recharge: percolation ponds, recharge wells and check dams that raise water tables.
Common structures
- Farm ponds: dug or embankment ponds on individual farms, storing runoff for supplementary (life-saving) irrigation at critical crop stages, livestock and fish. Plastic or clay linings reduce seepage in permeable soils.
- Check dams: small barriers across streams and gullies that slow flow, trap silt and store or recharge water.
- Percolation tanks: shallow reservoirs on permeable ground, designed to recharge wells downstream.
- Village tanks: traditional reservoirs, such as the cascades of tanks in South India and Sri Lanka, many centuries old.
- Sand dams: concrete walls across seasonal sandy rivers, common in Kenya; sand accumulates behind the wall and stores water within it, protected from evaporation.
- Cisterns and underground tanks: for rooftop and road runoff.
- Traditional systems such as the khadin of Rajasthan (runoff from rocky catchments collected in valley fields), the jessour of Tunisia and the tanka underground tanks of the Thar desert.
Design steps
- Estimate runoff from the catchment, using rainfall records and a runoff coefficient or the SCS curve number method.
- Determine demand: crop water for supplementary irrigation, livestock or household use.
- Size storage, allowing for seepage and evaporation losses and for sediment.
- Design the inlet, silt trap and spillway to handle peak flows safely.
- Plan use and maintenance: desilting, repairing embankments, and protecting water quality.
The SCS (NRCS) curve number method estimates storm runoff depth Q from rainfall P:
CN, the curve number, ranges from about 30 for permeable, well-vegetated land to over 90 for paved or very compacted surfaces. No runoff occurs until P exceeds 0.2S, the initial abstraction.
Sizing a farm pond and a rooftop tank
Pond. A 2 ha cultivated catchment with a curve number of 80 receives a design storm of 80 mm.
- S = 25,400 ÷ 80 − 254 = 317.5 − 254 = 63.5 mm; initial abstraction 0.2S = 12.7 mm.
- Q = (80 − 12.7)2 ÷ (80 + 0.8 × 63.5) = 67.32 ÷ 130.8 = 4,529 ÷ 130.8 ≈ 34.6 mm.
- Runoff volume = 0.0346 m × 20,000 m2 ≈ 692 m3.
- If seepage and evaporation take 30 per cent before the water is needed, usable water ≈ 484 m3: enough for about one 50 mm supplementary irrigation of 1 ha (500 m3) at flowering.
Rooftop tank. A 100 m2 roof in a place with 600 mm of annual rain and a runoff coefficient of 0.8 yields 100 × 0.6 × 0.8 = 48 m3 (48,000 litres) a year. For a family using 100 litres a day for drinking and cooking through a 6-month dry season (about 18,000 litres), a tank of about 18 to 20 m3 is sufficient if the dry season begins with it full.
Benefits and cautions
Benefits: water for supplementary irrigation at critical stages, which can double yields in drought years; drinking water; groundwater recharge; reduced erosion and floods; livestock and fish production.
Cautions:
- Upstream–downstream trade-offs: intensive harvesting upstream can reduce flows to downstream users and reservoirs.
- Sedimentation shortens the life of ponds and dams without catchment treatment.
- Water quality and mosquito breeding must be managed.
- Maintenance is often neglected once structures are built.
- Evaporation from shallow open ponds in hot climates can be very high.
Key points
- Water harvesting collects runoff from roofs, slopes, roads or streams for storage or infiltration.
- Types: rooftop, in-situ, micro-catchment, macro-catchment, floodwater harvesting and groundwater recharge.
- Structures: farm ponds, check dams, percolation tanks, village tanks, sand dams, cisterns and traditional systems.
- SCS method: Q = (P − 0.2S)2 ÷ (P + 0.8S), S = 25,400 ÷ CN − 254.
- Size storage for demand plus losses; include silt traps and spillways; plan maintenance and consider downstream users.
What the exams ask
- School-leaving and entrance papers: need for water harvesting, and types of structures.
- Degree and research-fellowship entrances: classification, traditional systems, runoff estimation by runoff coefficient and curve number, and design of farm ponds and check dams.
- Agricultural engineering and professional examinations: hydrological design, spillway sizing, seepage control, and watershed-level planning.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
How much can a farm pond help?
In semi-arid areas, a single supplementary irrigation at a critical stage, such as flowering, can save a crop in a dry spell. Even a small pond can make the difference between a failed and a fair harvest.
Why do ponds dry up so quickly?
Seepage through permeable soils and evaporation in hot weather. Linings, compacting the floor, reducing surface area relative to depth, and shade or covers reduce losses.
Is harvested rainwater safe to drink?
Rooftop water stored in clean, covered tanks is often good, but it should be kept free of debris and animals, the first flush of each storm diverted, and water treated or boiled if contamination is possible.
Sources
- Critchley, W. and Siegert, K. Water Harvesting: A Manual for the Design and Construction of Water Harvesting Schemes for Plant Production. FAO AGL/MISC/17/91, 1991.
- Oweis, T., Prinz, D. and Hachum, A. Rainwater Harvesting for Agriculture in the Dry Areas. CRC Press and ICARDA, 2012.
- USDA Natural Resources Conservation Service. National Engineering Handbook, Part 630: Hydrology, chapters 9 and 10. USDA.
- Agarwal, A. and Narain, S. (eds) Dying Wisdom: Rise, Fall and Potential of India's Traditional Water Harvesting Systems. Centre for Science and Environment, 1997.
- Rockström, J. et al. "Managing water in rainfed agriculture: the need for a paradigm shift." Agricultural Water Management 97, 543–550 (2010).
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