Rainwater Harvesting for Irrigation
Part of Irrigation
Rainwater harvesting captures precipitation from roofs and other surfaces, stores it, and delivers it to crops by gravity — providing irrigation water independent of rivers, wells, or springs.
Every rainstorm delivers free water across every surface in your settlement. A modest roof can capture thousands of liters per year — enough to sustain a substantial garden through dry spells. Rainwater harvesting requires no moving parts, no fuel, and minimal maintenance. It is one of the most accessible water supply technologies available, requiring only gutters, a storage vessel, and gravity.
Catchment Surfaces
Any impervious surface can serve as a catchment. The quality of the surface determines how much rain you actually capture (the runoff coefficient).
| Surface Type | Runoff Coefficient | Water Quality | Notes |
|---|---|---|---|
| Metal roof (tin, zinc) | 0.90-0.95 | Good | Best option — smooth, clean |
| Tile roof (clay, slate) | 0.80-0.90 | Good | Slightly more debris |
| Thatched roof | 0.20-0.40 | Fair — organic matter | Low yield, discolored water |
| Compacted earth/clay | 0.40-0.60 | Muddy | Needs settling/filtration |
| Rock outcrop | 0.70-0.85 | Good | Natural catchment, seal cracks |
| Paved courtyard | 0.70-0.85 | Fair — dust/debris | Ground-level collection |
| Plastic sheeting | 0.90-0.95 | Good | Temporary but effective |
Catchment Area Calculation
Catchment area is the horizontal footprint of the surface, not the sloped surface area. A roof 5 m × 8 m = 40 m² catchment, regardless of roof pitch. Each square meter captures 1 liter per millimeter of rainfall.
Calculating Annual Yield
Annual harvest (liters) = Catchment area (m²) × Annual rainfall (mm) × Runoff coefficient
Example: A 40 m² metal roof in an area receiving 800 mm annual rainfall:
- 40 × 800 × 0.90 = 28,800 liters per year
That is enough to irrigate roughly 100 m² of garden through a 4-month dry season at 5 mm/day application rate.
Gutter Systems
Gutters collect water from the roof edge and direct it to the storage vessel. They must be sloped, sized correctly, and sealed at joints.
Materials
| Material | Durability | Ease of Making | Cost |
|---|---|---|---|
| Split bamboo | 1-3 years | Easy | Free/very low |
| Hollowed half-log | 3-5 years | Moderate | Low |
| Bent sheet metal | 10-20 years | Requires tools | Moderate |
| Clay tile channel | 5-10 years | Pottery skills needed | Low |
| Bark trough | 1-2 seasons | Easy | Free |
Gutter Sizing
Gutters must handle peak rainfall intensity without overflowing. A general rule: 1 cm² of gutter cross-section per 1 m² of roof area.
For a 40 m² roof: minimum 40 cm² gutter cross-section. A half-round gutter 10 cm wide and 5 cm deep provides about 40 cm² — adequate.
Installation
- Slope: Gutters must slope toward the downspout at 1-2 cm per meter. Too little slope causes pooling; too much causes water to overshoot the end.
- Overhang: Position the gutter so its outer edge sits under the roof drip line. The inner edge should be higher than the outer edge so water sheets into the gutter rather than overshooting.
- Brackets: Support gutters every 60-100 cm with brackets attached to the roof fascia or rafters.
- Joints: Overlap sections by 10 cm minimum. Seal with pine pitch, beeswax, or clay slip.
The String Test
Before installing permanent gutters, tie a string along the planned gutter line during rain. Watch where water actually drips from the roof — it may not be where you expect. Adjust your gutter position accordingly.
Downspouts
Connect the gutter end to a vertical or angled pipe leading to the storage tank. A simple funnel shape made from bent bark or sheet metal transitions from gutter to downspout. Screen the downspout entrance with a mesh (woven grass or wire) to block leaves and debris.
First-Flush Diverters
The first water off any roof carries accumulated dust, bird droppings, pollen, and debris. Diverting this “first flush” improves water quality significantly.
Simple First-Flush System
Build a vertical standpipe (10-15 cm diameter, 50-100 cm tall) at the bottom of the downspout, before the storage tank inlet. The first water fills this standpipe. Once full, subsequent water flows past and into the tank.
Sizing the diverter: Allow 1-2 liters per square meter of roof area. For a 40 m² roof, a standpipe holding 40-80 liters diverts enough first flush. A 15 cm diameter pipe holds about 1.8 liters per 10 cm height — so a 40 cm tall pipe holds roughly 7 liters. For 40 liters, you would need a larger vessel or longer pipe.
Automatic Drain
Drill a 3 mm hole at the bottom of the first-flush standpipe. The dirty water slowly drains out between storms (taking roughly 12-24 hours), resetting the diverter automatically. Block the hole with a removable peg if you want to drain it manually.
Ball-Float Diverter
A more sophisticated approach: place a ball (or carved wooden sphere sealed with wax) inside the first-flush pipe. As the pipe fills with dirty water, the ball floats up and seals the inlet to the pipe. All subsequent water flows to the tank. The small drain hole empties the pipe between storms, and the ball drops back down.
Storage Options
Sizing Your Storage
Determine how much storage you need based on your dry season length and daily water use.
Storage needed (liters) = Daily irrigation need (liters) × Number of dry days × Safety factor (1.2)
Example: 100 m² garden needing 5 mm/day for 90 dry days:
- Daily need: 100 × 5 = 500 liters
- Storage: 500 × 90 × 1.2 = 54,000 liters
That is a large amount. In practice, most smallholders build what they can and accept partial dry-season coverage, supplementing from other sources.
Storage Vessel Comparison
| Type | Capacity Range | Lifespan | Skill Required | Materials |
|---|---|---|---|---|
| Clay pot/jar | 50-500 L | 5-20 years | Pottery | Clay, kiln |
| Ferro-cement tank | 1,000-50,000 L | 20-50 years | Masonry | Cement, wire, sand |
| Brick/stone cistern | 1,000-100,000 L | 50+ years | Masonry | Brick, mortar, plaster |
| Dug-out lined pit | 5,000-100,000 L | 10-20 years | Digging, lining | Clay, plastic, or cement lining |
| Wooden barrel | 100-500 L | 5-15 years | Cooperage | Staves, hoops |
| Earth dam/pond | 10,000+ L | Indefinite | Earthwork | Clay-rich soil |
Underground Cistern (Most Practical for Large Volume)
Dig a pit 2-3 meters deep, 3-4 meters across. Line with puddled clay (30 cm thick, wetted and compacted in layers) or plaster with cement if available. Cover with a timber and earth roof to reduce evaporation and contamination.
A 3 m × 3 m × 2 m cistern holds 18,000 liters — enough for 36 days of 500 L/day irrigation.
Evaporation Losses
Open-top storage can lose 5-10 mm per day to evaporation in hot climates. A 10 m² open pond loses 50-100 liters daily. Always cover storage tanks or build underground. Even a floating cover of boards or grass mats reduces evaporation by 50-70%.
Preventing Algae and Mosquitoes
- Keep tanks dark (covered, underground, or opaque) to prevent algae growth
- Screen all inlets and overflows with fine mesh to block mosquitoes
- For open ponds, stock with mosquito-eating fish (gambusia, guppies)
- A thin layer of vegetable oil on still water prevents mosquito breeding but is impractical for irrigation
Filtration for Sediment
Irrigation water does not need to be drinking-quality, but excessive sediment clogs delivery systems and can smother plant roots.
Gravity Sand Filter
Build a filter box between the catchment and the storage tank:
- Container: Wooden box, stone-lined pit, or large clay pot with a drain hole at the bottom
- Layers (bottom to top):
- 10 cm gravel (20-40 mm)
- 10 cm coarse sand (2-5 mm)
- 15 cm fine sand (0.5-1 mm)
- 5 cm gravel on top to prevent sand disturbance
- Flow rate: About 100-200 liters/hour per square meter of filter surface
Clean the filter by scraping off the top 2-3 cm of fine sand when flow rate drops. Replace the sand annually.
Settling Tank
A simpler alternative: route water through a settling tank (any large container) before the main storage. Water enters on one side, sits for 30-60 minutes, and exits through a pipe on the opposite side positioned 10 cm above the bottom. Sediment drops to the floor. Clean out sediment monthly.
Gravity Feed to Garden
Head Pressure
Place your storage tank uphill from your garden to use gravity feed. Every 1 meter of elevation difference (head) provides about 0.1 bar of pressure — enough to push water through pipes and basic drip lines.
| Elevation Difference | Pressure | Suitable Delivery Method |
|---|---|---|
| 0.5-1 m | Very low | Open channel, flood irrigation |
| 1-2 m | Low | Large pipe (5+ cm), furrow irrigation |
| 2-4 m | Moderate | Pipe to drip lines, sprinkler |
| 4+ m | Good | Pressurized drip, micro-sprinklers |
If the Tank is at Ground Level
If you cannot place the tank uphill, raise it on a platform. A tank elevated 2 meters on a timber frame provides adequate head for most garden irrigation. The platform must support the full weight of water — 1,000 liters weighs 1,000 kg (1 metric ton).
Platform Strength
Water is heavy. A 2,000-liter tank on a 2 m platform requires a structure capable of holding 2 tonnes. Use substantial timbers (15+ cm diameter) with cross-bracing. Anchor to the ground. Test by filling gradually over several days, watching for settlement or deflection.
Pipe and Channel Options
| Delivery Method | Materials | Flow Control | Best For |
|---|---|---|---|
| Open earth channel | None | Sluice gates, plugs | Large areas, flood irrigation |
| Bamboo pipe | Bamboo, pitch | Plugs at joints | Medium distances, moderate flow |
| Clay pipe | Fired clay, mortar | Fitted plugs | Permanent installations |
| Hollowed log pipe | Rot-resistant wood | Wooden plugs | Short to medium runs |
System Maintenance
Monthly
- Clean gutter screens and downspout filters
- Check gutter slope (sagging brackets cause pooling)
- Inspect tank for cracks, leaks, algae
- Empty and clean first-flush diverter if no auto-drain
Seasonally
- Clean or replace sand filter media
- Drain and scrub tank interior if algae present
- Check pipe/channel connections for leaks
- Repair any gutter joint separations
Annually
- Inspect roof catchment surface (repair holes, remove moss)
- Re-seal tank liner or plaster
- Replace deteriorated gutter sections
- Review yield records and adjust system if needed
Maximizing Your Harvest
| Strategy | Improvement | Effort |
|---|---|---|
| Increase catchment area (add roof, paved area) | Proportional to area added | High initial |
| Improve runoff coefficient (smooth surface) | 10-30% | Moderate |
| Reduce first-flush waste (smaller diverter) | 5-10% more stored | Low |
| Cover storage (reduce evaporation) | Save 50-100 L/day | Low-moderate |
| Mulch garden (reduce irrigation need) | 25-50% less water needed | Low, ongoing |
| Drip irrigation (reduce waste) | 30-50% less water needed | Moderate setup |
Rainwater Harvesting Essentials
Calculate your yield: catchment area (m²) x annual rainfall (mm) x runoff coefficient (0.40-0.95 depending on surface). A 40 m² metal roof in 800 mm rainfall zone captures about 29,000 liters/year. Install gutters sloped at 1-2 cm/m with leaf screens, add a first-flush diverter (1-2 L per m² of roof), and route clean water to covered storage. Underground cisterns are most practical for large volumes — a 3 m x 3 m x 2 m pit holds 18,000 liters. Place storage uphill or on a raised platform (minimum 1-2 m elevation) for gravity feed. Each meter of head provides 0.1 bar pressure. Filter through gravel and sand if sediment is present. Cover all storage to prevent evaporation (50-100 L/day losses in hot climates) and screen against mosquitoes.