Each 1 mm of rain falling on 1 m² of roof gives 1 L of water before losses, so the water a cistern can collect is roof area in m² times rainfall in mm times a runoff coefficient. Measure rain with a straight-sided open container read with a ruler, log it daily, and plan on the dry months: in the worked example 600 mm falls in 6 wet months and only 200 mm in 6 dry ones, which is why storage is needed at all.
Part of Cisterns and Rainwater Storage
Rainfall data is the record of how much rain falls and when, expressed as a depth in millimetres. It is the first input to any estimate of rainwater available for storage.
One Millimetre Of Rain Is One Litre On Every Square Metre
The conversion is plain geometry. 1 m² of surface under 1 mm (0.001 m) of water holds 1 m² × 0.001 m = 0.001 m³, which is 1 L. Nothing in the unit needs a table, so any rainfall figure in millimetres converts straight to litres per square metre.
The cisterns page turns that into the working formula: liters collected = roof area (m²) × rainfall (mm) × runoff coefficient. The coefficient, covered in Roof Coefficient, allows for water the roof does not deliver. The roof area is covered in Catchment Area.
What You Need
- A straight-sided container — a tin, a bottle with the top cut off, a section of pipe with a sealed bottom; the sides must be vertical so depth does not depend on how full it is
- A ruler marked in millimetres, or a stick with notches
- An open site — clear of overhanging trees and walls, with the rim level and about 1 m above the ground to cut splash
- A notebook — date, depth read, container emptied
Method: Read The Same Time Every Day
Step 1: Set the gauge upright and level in the open and brace it against the wind.
Step 2: Read it at the same time each day with the ruler to the nearest millimetre, then empty it. The depth in a straight-sided container equals the depth of rain that fell; no conversion is needed.
Step 3: Magnify small falls (optional). If the collecting funnel has 10 times the area of the measuring tube, then 1 mm of rain stands 10 mm deep in the tube. Divide the reading by 10.
Step 4: Add up each month. Keep a column of monthly totals and a row for the year. A short record misleads: a single wet year hides the dry ones, so keep it going and give weight to your driest year when sizing the tank.
Step 5: Separate wet from dry. The number that sizes storage is not the annual total. It is the shape of the year: how much falls in the dry months against the demand in those months.
Worked Example
The cisterns page uses 200 m² of metal roof (coefficient 0.9) and 800 mm of rain a year, which gives 200 × 800 × 0.9 = 144,000 L per year. With 600 mm in the 6 wet months and 200 mm in the 6 dry ones:
- Wet half: 200 × 600 × 0.9 = 108,000 L
- Dry half: 200 × 200 × 0.9 = 36,000 L
- Sum: 108,000 + 36,000 = 144,000 L, which matches
Three-quarters of the year’s water arrives in the wet half. A group using 400 L/day for 180 dry days needs 72,000 L but collects only 36,000 L in that time. That gap is the storage requirement, worked out in Dry Spell Bridging.
Common Mistakes
- Sizing to the average year. The dry year is the one that empties the tank.
- Gauge under a tree. Drip from leaves and blocked rain both distort the reading.
- Reading a tapering container. A bottle that narrows at the bottom shows the wrong depth.
- Forgetting to empty it. The next reading then includes old rain.
What’s Next
- Learn next: Catchment Area — how much surface the rain falls on
- Combine with: Rain Harvesting: Catchment and Storage — the short emergency version of the same estimate