Choose a community water source in this order of preference: a protected spring, then a stream or river, then a deep well with a hand pump, with rainwater as a supplement in most climates, and decide by the flow in the driest month rather than the wettest. Time how long a 20-liter bucket takes to fill: 20 liters in 60 seconds is 0.33 liters per second, 20 liters per minute, or about 28,800 liters a day.
Part of Community Water System
Source selection is the choice of where a shared supply takes its water, weighed on reliability, cleanliness, upkeep and the flow available in the worst month.
A Source Must Be Judged In Its Worst Month, Not Its Best
A source that fills a bucket in a minute in April and in two minutes in August is the August source. The comparison below follows the preference order in the community water system page.
| Source | Strength | Weakness | Read more |
|---|---|---|---|
| Protected spring | Most reliable, cleanest, lowest upkeep | Flow may fall in the dry season | Spring Capture |
| Stream or river | Abundant | Needs filtration; exposed to contamination upstream | Intake Structures |
| Deep well with hand pump | Reliable year-round if the aquifer is good | Needs a pump and its upkeep | Deep Wells |
| Rainwater | Clean at the roof, close to the user | Supplement in most climates; primary only in high-rainfall areas | Cisterns and Rainwater Storage |
A hand-dug well suits a settlement that has no spring or gravity head; the dug-well method is in Well Digging.
What You Need
- A container of known volume — a 20-liter bucket is convenient
- A watch
- A way to direct all the water into the container — a pipe, a trough, a clay or tarp funnel
- A notebook, to log the date, recent rain and result each time
Method: Measure Flow, Then Convert It
Step 1: Collect the whole flow in the container. Water that bypasses the funnel makes the result too low.
Step 2: Time the fill in seconds, and repeat three times.
Step 3: Convert. Liters ÷ seconds is liters per second. Multiply by 60 for liters per minute and by 86,400 for liters per day.
Step 4: Repeat on a schedule. The carrier recommends weekly measurements for a full season before building anything permanent. The dry-season figure is the design figure.
Worked Example: The Same Spring, Three Seasons
- 20 L in 60 s: 20 ÷ 60 = 0.33 L/s. × 60 = 20 L/min. × 1,440 min = 28,800 L/day.
- 20 L in 120 s: 20 ÷ 120 = 0.167 L/s = 10 L/min = 14,400 L/day.
- 20 L in 10 min: 20 ÷ 10 = 2 L/min = 2,880 L/day.
- The carrier’s dry-season case, 5 L/min in April falling to 0.5 L/min in August: 5 × 1,440 = 7,200 L/day, then 0.5 × 1,440 = 720 L/day.
Divide 720 L/day by 20 L (the minimum) and you serve 36 people with drinking, cooking and basic hygiene. Divide by 80 L (the comfortable top) and you serve 9. That one measurement decides how large a community the source supports, and it is the measurement most often skipped.
A flow measured in a single week of wet weather is not a design figure. A flow measured at the end of the longest dry spell is.
Common Mistakes
- Dividing by seconds when you meant minutes. 20 L in 60 s is 0.33 L per second, not per minute: the two differ by a factor of 60.
- Measuring after rain. The result overstates the dry-season flow.
- Choosing a stream because it is large. A large flow does not make it clean.
- Ignoring other users downstream. Taking all of a small stream leaves nothing for them.
Safety
Clear water is not clean water
A source can look clear and still carry pathogens, and a stream is exposed to everything upstream of the intake. Treat every source (Water Treatment at Scale) and protect the catchment (Source Protection). When temporarily damming a source to measure it, restore the flow at once.
What’s Next
- Learn next: Intake Structures — how water gets from the source into the pipe
- Combine with: Dry Season Backup — what to do when the measured flow falls short