Water harvesting catches rain where it falls and keeps it for the crop, by slowing runoff so it soaks into the soil, by shaping the land to hold it, and by storing it in tanks and ponds. One millimetre of rain on one square metre is one litre, so a hectare receives 10 cubic metres for every millimetre that falls. Start with the cheapest step, getting water into the soil, before building storage.
Agricultural water harvesting is the collection and storage of rainfall and runoff, in soil, in earthworks and in tanks, to supply crops through dry periods.
The Arithmetic Of Rain Is Simple
1 mm of rain over 1 m² is 1 litre (1 m² × 0.001 m = 0.001 m³). A hectare is 10,000 m², so 1 mm over a hectare is 10,000 × 0.001 = 10 m³, and 500 mm over a hectare is 5,000 m³. Whether a given area can use that water depends on how much runs off, how much soaks in and how much evaporates.
Three Places To Keep Rain
| Where | How | Guide |
|---|---|---|
| In the soil | Open soil structure, organic matter, mulch, contour planting | Soil Organic Matter and Water Holding Capacity, Mulching & Soil Moisture |
| In earthworks | Swales, berms, terraces, basins | Swales and Berms on Contour, Terracing |
| In tanks and ponds | Roofs and catchments feeding cisterns | Rainwater Collection and Storage, Farm Ponds for Irrigation |
The carrier guide (Drought Resilience & Water-Efficient Agriculture) names swales on contour, rainwater collection from roofs and greywater reuse as its water-harvesting methods, and stresses capturing every drop to prevent runoff.
Start With The Soil
Water in the soil needs no tank, no pump and no evaporation loss from an open surface once it is below the top layer. Open, mulched, organic soil takes in rain that bare crusted soil sheds. Earthworks come next on slopes; tanks serve gardens, seedlings and stock where soil storage is not enough.
Method
Step 1: Walk the site in a heavy rain and see where water runs, ponds and carries soil.
Step 2: Improve infiltration first: mulch, cover, no bare crust.
Step 3: Slow the water on slopes with contour planting, swales and terraces.
Step 4: Store the surplus from roofs and hard surfaces.
Step 5: Plan the use of what you store (Scheduling: When and How Much).
Common Mistakes
- Building tanks before fixing infiltration — the field sheds rain.
- Capturing water without a use or an overflow.
- Taking all the runoff from land that others or streams depend on — water kept is water not delivered downstream.
What’s Next
- Learn next: Swales and Berms on Contour — the earthwork method
- Combine with: Rain Gardens and Swales — small-scale infiltration