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Seasonal Monitoring

4 min read

Parent
📏
Flow Measurement
Quantifying spring output
Current
📅
Seasonal Monitoring
Tracking flow across seasons
Sub-Topics
📝
Flow Records
Keeping a measurement log

Measure a spring’s flow at least monthly for a full year, and weekly through the driest weeks, before designing a supply around it, because many springs that flow freely in spring barely trickle by late summer and the supply must be sized for the lowest reading, not the average. Record the date, the recent rain and the flow each time, size the pipework to the minimum and the overflow to the maximum, and treat a single year as a minimum, not a guarantee.

Part of Spring Capture

Seasonal monitoring is the repeated measurement of a spring’s flow across a full cycle of seasons, so that the lowest and highest flows and the response to rain are known before anything is built.

The Design Flow Is The Lowest Flow You Have Seen

A spring that gives 20 L/min in April can give a fraction of that in September. A system sized to the April flow is short of water in precisely the months when people need it most. The Spring Capture guide’s rule is to visit at least monthly for a full year, and to be sure to measure in the driest part of the year; the Source Development guide asks for weekly readings for a month, then across the full year (Source Development). Do both: monthly all year, weekly through the driest month or two.

Worked Example: One Year Of Readings

These readings are illustrative, to show the method. Each is a bucket timing converted to L/min (Bucket Timing).

MonthJanFebMarAprMayJunJulAugSepOctNovDec
Flow, L/min121418201612964.55811
  • Highest flow: 20 L/min in April. Per day: 20 × 1,440 = 28,800 L
  • Lowest flow: 4.5 L/min in September. Per day: 4.5 × 1,440 = 6,480 L
  • Ratio of highest to lowest: 20 ÷ 4.5 = 4.4. A system planned on the April flow would expect 4.4 times what the spring gives in September

Design the supply to the lowest reading: 6,480 L a day, divided by the per-person figure you plan on. Design the box overflow and the pipe capacity for the highest, so that the April flow leaves the box instead of backing up (Outlet & Overflow).

One Year Is A Minimum

A single year may not contain the driest year the spring has seen. Ask local people about the worst drought they remember, and what the spring did in it. If the answer is that it dried up, the measured minimum overstates the safe supply. Fit the recession curve to the dry-season readings to predict how low the flow would go if the dry season ran on (Flow Records), and plan storage to carry the community through.

What You Need

  • A calendar of visits — a fixed day each month, weekly in the driest weeks
  • A bucket, watch and channel — the same set-up each time so readings are comparable
  • A rain gauge — a straight-sided open container read with a ruler (Rainfall Data)
  • A notebook — a page per visit (Flow Records)
  • A marker at the spring — a stake or stone showing exactly where the flow is measured

Method: Same Place, Same Way, Every Time

Step 1: Fix the measuring point and the way the flow is gathered, and keep both unchanged.

Step 2: Measure on the planned day, in the same way, three fills averaged.

Step 3: Log the rain in the preceding week as well, so a high flow can be explained.

Step 4: Look and taste. Note clarity, colour, smell and any change after rain; cloudy water after rain means a surface connection.

Step 5: Tighten in the dry season. Move to weekly readings as the flow falls, and keep them going until it recovers.

Step 6: Review. At the end of the year, find the minimum, the maximum and the response to rain, and write the design flow on the first page of the log.

Common Mistakes

  • Averaging the year. The average hides the failure month.
  • Changing the measuring point. Readings from different places cannot be compared.
  • Building on three months of data. The dry season may not have come yet.
  • Skipping the rain log. A reading with no weather context is hard to interpret.

Safety

Isolated sites and weather

Springs are often remote and on steep, wet ground. Tell someone when you go, avoid visiting alone in storms, and keep clear of banks you have cut deeper than 1.2 m without support.

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