Size rainwater collection with two sums: supply is roof area times rainfall times a capture fraction, and storage is daily demand times the length of the longest dry spell plus a margin. The carrier guide’s figure of 600 gallons per inch of rain on a 1,000 square foot roof implies a capture fraction of 0.96, close to the physical ceiling, so a real system will collect less. Use your own rainfall record for the dry spell, and do not let the tank take water that the household needs to drink.
Rainwater collection for agriculture is catching runoff from roofs and hard surfaces in gutters and tanks or cisterns so it can be used to irrigate during dry periods.
Supply Is Area × Rain × Capture, Storage Is Demand × Dry Days
One millimetre of rain on one square metre is one litre, so the volume collected in litres is V = A × R × c, with A the catchment area in m², R the rainfall in mm and c the fraction of the rain that actually reaches the tank (less than 1, after first-flush diversion, splash, overflow and evaporation).
Check The Carrier’s Figure
The carrier guide (Drought Resilience & Water-Efficient Agriculture) says a 1,000 ft² roof collects about 600 gallons per inch of rain, so about 18,000 gallons a year in 30 inches.
- 1,000 ft² × 1 inch = 1,000 ÷ 12 = 83.3 ft³; × 7.481 gal/ft³ = 623 gallons.
- 600 ÷ 623 = 0.96.
- 30 inches: 623 × 30 = 18,700 gallons at c = 1; the carrier’s 18,000 matches c = 0.96.
- Metric: 18,000 gal × 3.785 = 68,100 L. The roof is 1,000 ft² = 92.9 m²; rain 30 in = 762 mm. 92.9 × 762 × 0.96 = 68,000 L, which matches.
The carrier’s capture fraction of 0.96 is the physical maximum minus a little. Real systems with gutters that overflow, a first-flush diverter, leaks and evaporation collect less. Measure yours: weigh or gauge a known volume of tank rise against a rain gauge reading (Rainfall Data).
Worked Example: Garden Tank
Illustrative inputs: 50 m² of beds needing 4 mm/day in the dry season; longest dry spell 90 days from your record; margin for evaporation, leaks and error 20%.
- Demand: 50 × 4 = 200 L/day (1 mm over 1 m² is 1 litre).
- Storage: 200 × 90 = 18,000 L; with the 20% margin, 18,000 ÷ 0.8 = 22,500 L = 22.5 m³.
- Roof needed to fill it in the wet season with 400 mm of rain and c = 0.8: A = 22,500 ÷ (400 × 0.8) = 70 m².
Weight: 22.5 m³ of water is 22,500 kg, and its weight is 22,500 × 9.81 = 221 kN. A tank that size sits on the ground on a prepared base, not on a roof. Pressure at the bottom of a tank of depth h is 9.81 kPa per metre of depth. Roofs, platforms and walls must carry the weight of the water plus the tank, so derive the load from the volume before placing a tank anywhere above ground. A full 1 m³ tank is 1,000 kg.
What the Rainwater Costs
Water that is caught for the garden is water that does not run downhill, recharge the ground or reach a stream. A tank that serves the garden also competes with the household’s drinking water if it is the only supply. Decide the order of use before building: drinking first, then the garden.
What You Need
- Catchment: a clean roof or hard surface with gutters.
- Screens and a first-flush diverter to keep debris and the dirtiest water out.
- A tank or cistern, with a lid that keeps out light, insects and animals (Cisterns and Rainwater Storage; Cistern Building; Rainwater Harvesting for Irrigation).
- A rain gauge and a record.
Method
Step 1: Record rainfall by month and the length of dry spells, for as many years as you can.
Step 2: Fix the demand (area × daily need).
Step 3: Compute storage = demand × longest dry spell ÷ (1 − margin).
Step 4: Compute the roof needed = storage ÷ (wet-season rain × c).
Step 5: Build in stages, and measure the actual catch.
Step 6: Keep it clean: clear gutters, flush the first runoff, screen openings, keep the tank dark and covered.
Common Mistakes
- Using c = 1 — nothing collects everything.
- Sizing for the average year — size for the dry spell.
- Putting a heavy tank on a weak support.
- Using roof water for drinking without treatment (Water Purification).
Safety
Underground tanks, pits and heavy water
Do not enter an underground tank or a dug pit deeper than 1.2 m without support and rescue arrangements: walls can collapse, and a covered tank can hold too little oxygen or build up gas. Never go in alone, and ventilate before entry. Cover every opening so no person or animal can fall in, and make lids strong enough to carry a person. A full tank is very heavy, and 1 m³ of water weighs 1,000 kg.
What’s Next
- Learn next: Greywater Reuse for Gardens — a second source
- Combine with: Scheduling: When and How Much — spending the stored water