Daily water demand is people times a per-person budget, plus livestock, plus garden irrigation, plus a leakage allowance, and the source must supply that total in its driest month. For 20 people at 50-80 liters each, with two cows, six goats, 20 chickens and 100 square meters of beds, the sum comes to about 1,640-2,330 liters a day before leakage, which sits inside the 1,500-2,500 liters a day range for a group of that size.
Part of Community Water System
Demand calculation is the arithmetic that converts a head count, a herd and a garden into liters per day, the number every later choice of source, pipe and tank depends on.
Demand Is A Sum Of Separate Uses, Each Counted On Its Own
Quoting one figure per head hides the large uses. People, animals and plants each have their own budget, and the budgets differ by an order of magnitude, so they are added one line at a time. The result is a daily volume. Flow in liters per second comes later and is a different question.
What You Need
- A head count, including the growth you expect over the life of the pipe
- A per-person budget from Per Capita Use
- A herd list and a garden area (Livestock and Irrigation)
- A leakage allowance — Leak Detection treats under 10% loss as acceptable
Method: Four Lines And A Total
Step 1: People. Head count × liters per person per day. The carrier figures are a minimum of 20 and a comfortable 50-80.
Step 2: Livestock. A cow drinks 50-80 L/day, a goat 5-10 L/day, a chicken about 0.5 L/day.
Step 3: Garden. One millimeter of water over one square meter is one liter. Choose a daily depth for the crop and multiply by the bed area. The depth is an assumption you make and must test against the real season.
Step 4: Leakage. If you accept up to 10% loss, the source must supply demand ÷ 0.9, not demand × 1.1 the other way round: the loss is a share of what is put in.
Worked Example: A Group Of Twenty
Inputs: 20 people; 2 cows, 6 goats, 20 chickens; beds of 100 m² watered at an assumed 5 mm/day.
| Use | Low | High |
|---|---|---|
| People (20 × 50, 20 × 80) | 1,000 L | 1,600 L |
| Cows (2 × 50, 2 × 80) | 100 L | 160 L |
| Goats (6 × 5, 6 × 10) | 30 L | 60 L |
| Chickens (20 × 0.5) | 10 L | 10 L |
| Garden (100 m² × 5 mm = 100 × 5 L) | 500 L | 500 L |
| Total | 1,640 L | 2,330 L |
Allowing for 10% leakage: 1,640 ÷ 0.9 = 1,822 L/day and 2,330 ÷ 0.9 = 2,589 L/day. The high end slightly exceeds the carrier’s 1,500-2,500 L/day range, which is a summary figure; use your own sum.
Average flow for the low case: 1,640 L ÷ 86,400 s = 0.019 L/s. The peak hour runs 2.5-3 times the average hourly demand, so 1,640 ÷ 24 = 68 L/h average and 68 × 2.5 = 171 L/h = 0.047 L/s at peak.
That flow is tiny compared with what one open tap draws, 0.33 L/s from a 20 L bucket filling in 60 s. For a small group the pipe is sized by how many taps can run together, not by the daily total (Pipe Sizing). The daily total decides the source and the tank.
Common Mistakes
- Counting only people. In the example the garden alone is roughly a fifth to a third of the total (500 of 2,330 and 500 of 1,640).
- Using average demand to size pipe. Draw is concentrated in the morning and evening.
- Treating the leakage allowance as a free number. It is a promise to find and repair leaks; without that, loss grows.
- Using the garden depth as a fact. It depends on climate and soil; measure what the beds actually take.
What’s Next
- Learn next: Source Selection — checking that a source can supply this total in its worst month
- Combine with: Storage Tanks — holding at least one day of this demand