Size a rainwater tank for the longest stretch without enough rain: daily demand times the number of dry days, less whatever the roof still collects in that time. A group of 20 people at 20 L/person/day uses 400 L/day, so 30 days needs 12,000 L, and at 50 L/person/day over a 60-day spell the figure is 60,000 L. Check first that a year of roof yield exceeds a year of demand, because storage moves water through time but cannot create any.
Part of Cisterns and Rainwater Storage
Dry-spell bridging is sizing a cistern so stored water lasts from the end of one rainy period to the start of the next. It combines the group’s daily demand, the length of the dry spell and what the roof still collects.
A Tank Can Move Water Through Time But Cannot Make More
Two checks come before any tank size. The first is whether the year’s yield covers the year’s demand. The second is whether the tank can carry the group through the gap.
Supply ceiling. The example roof yields 144,000 L a year (Rainfall Data). Spread over 365 days that is 144,000 ÷ 365 = 394 L/day, which for 20 people is 394 ÷ 20 = 19.7 L/person/day. So that roof can supply the 20 L/person/day (drinking, cooking, basic hygiene) tier and nothing like the 50 L/person/day tier. At 50 L/person/day the year’s demand is 20 × 50 × 365 = 365,000 L, more than twice the yield. For that, you need more catchment (Catchment Area) or another source.
Dry-spell storage. Demand per day × dry days − collection during the dry days = the storage needed.
What You Need
- Daily demand per person — 20 L for the basic tier, 50 L for comfort
- Group size
- Your longest dry spell — from the rain log; use the worst year, not the average
- Yield during the dry months — roof area × dry-month rainfall × coefficient
- A margin for evaporation, leaks and a tank that is not full at the start
Method: Demand, Gap, Then Tank
Step 1: Daily demand. People × litres per person.
Step 2: Dry days. The longest spell with little or no rain.
Step 3: Subtract dry-season collection, if any.
Step 4: Add margin. The cisterns page says to build bigger if you can: “you’ll never regret having too much”.
Step 5: Check the tank can be filled by the wet season. A tank that is never full at the start of the dry months does not carry what you calculated.
Worked Example
Zero-inflow spells (demand only). 20 people × 20 L = 400 L/day.
- 30 days: 400 × 30 = 12,000 L
- 50 days: 400 × 50 = 20,000 L, so a 20,000 L tank lasts 50 days
- 75 days: 400 × 75 = 30,000 L, so a 30,000 L tank lasts 75 days
- At 50 L/person/day, 60 days: 20 × 50 × 60 = 60,000 L
This is why the example tank of 20,000–30,000 L handles most dry spells at the basic tier. It carries 50–75 days with no rain at all.
A whole 6-month dry season (the roof still trickles). 180 dry days at 400 L/day need 72,000 L. The roof collects 200 × 200 × 0.9 = 36,000 L in those months. Deficit: 72,000 − 36,000 = 36,000 L. A tank that starts full at the start of the dry season needs about 36,000 L, so 30,000 L falls 6,000 L short, and the group must ration in the last weeks. This ignores tank losses, so allow more.
Wet-season overflow. In the wet half the roof yields 108,000 L; a 36,000 L tank fills early and the rest overflows. Spare water above that is lost unless a second tank or a use for it (garden, animals) exists.
Common Mistakes
- Sizing storage for demand without checking yield. The 50 L/person/day tier runs out regardless of tank size.
- Using the average year. Plan for the dry one.
- Assuming a full tank at the start. Check it is filled before the dry season begins.
- Forgetting tank losses. Evaporation, seepage through the lining and leaks all subtract.
What’s Next
- Learn next: Underground Cisterns — the cool, space-saving option for large volumes
- Combine with: Storage Tanks — how the same logic works in a piped system