When a source’s flow drops below demand in the dry season, a community has three levers: a larger tank to buffer within the day, a backup source such as a well or stored rainwater, and rules that cut demand in a fixed order, drinking first. For a spring giving 720 liters a day against 1,500 liters of demand, storage cannot cover a gap of 780 liters a day that lasts weeks, so the fix is rationing down to the 20-liter minimum per person, 400 liters a day for 20 people, or a second source.
Part of Community Water System
Dry-season backup is the set of measures a water system uses when its source’s flow falls below demand: extra storage, an alternative source and demand-reduction rules.
Storage Moves Water Between Hours; It Cannot Create It
The community water system page names three responses when spring flow drops below demand: a larger storage tank to buffer overnight, a backup source such as a well or rainwater, and demand reduction rules such as rationing schedules. They solve different problems.
A tank smooths the daily cycle: the spring fills it overnight and people draw it down in the morning and evening (Tank Sizing). It cannot supply a shortfall that continues day after day, because the tank is emptied faster than it fills.
What You Need
- Dry-season flow, measured (Source Selection)
- A priority list of uses, agreed in advance
- A second source identified and, ideally, developed (Deep Wells, Cisterns and Rainwater Storage)
- Authority to enforce the rules (Water Committee)
Method: Close The Gap In This Order
Step 1: Quantify the gap. Demand minus dry-season flow, in liters per day, and how many days it lasts.
Step 2: Cut demand in order. Garden first, then laundry, then hygiene beyond the basic level, with drinking and cooking last. Animals need their own allowance: they cannot be rationed like people.
Step 3: Test what remains against the minimum. If the source covers 20 L per person per day, the people are served and only comfort is lost.
Step 4: Add storage or a second source only for the part still missing.
Step 5: Announce the rules before the dry season, not during it, and review them when the flow is re-measured.
Worked Example: A 780 Liter Daily Gap
Inputs: 20 people; demand 1,500 L/day; spring 0.5 L/min in August (the carrier’s example).
- Spring: 0.5 × 1,440 = 720 L/day.
- Gap: 1,500 - 720 = 780 L/day.
- Over 60 dry days: 780 × 60 = 46,800 L of storage would be needed. At 5,000 L per ferrocement tank (Ferrocement Tanks) that is 46,800 ÷ 5,000 = 9.4, so 10 tanks, filled in the wet season from a source that must have the surplus to fill them. Rarely practical.
- Minimum demand: 20 × 20 = 400 L/day. The spring covers it with 720 - 400 = 320 L/day to spare.
So the practical plan is rationing to the minimum plus a small second source for the garden or animals. The spare 320 L/day could supply livestock or a part of the beds, but not both in full: livestock alone need 140-230 L (Livestock and Irrigation).
Common Mistakes
- Treating a bigger tank as the cure. It only helps a shortage measured in hours.
- Agreeing the rules in the middle of the shortage. Disputes follow.
- Opening an unprotected backup source without treatment. It is typically less clean than the main source.
- Ignoring the animals. They cannot be rationed like people.
Safety
A backup source is usually a worse source
A shallow well, a pond or a stream that people turn to in drought is rarely protected, and at low water its contamination is more concentrated. Treat it with the full chain (Water Treatment at Scale) and test it. When a tank runs low, do not draw from the bottom: sediment and slime collect there.
What’s Next
- Learn next: Dry Spell Bridging — sizing rainwater storage to cover a dry period
- Combine with: Repair Schedule — fixing leaks first, since lost water is the cheapest water to recover