Measuring Well Performance – A Vital Part of Irrigation Maintenance

When it comes to maintaining a productive irrigation system, knowing how your well is performing is just as important as knowing when to water. Over time, all wells naturally decline in performance, often unnoticed until a problem arises. That’s why accurate well performance records are essential. These records help detect changes early and guide timely rehabilitation before major losses occur.

Understanding Well Performance

After construction, every well has a baseline flow rate it can produce at a certain drawdown. This maximum rate is known as the well yield. As time passes, mineral scale, biological buildup, or physical obstructions can cause that yield to decline. To monitor this change, we use a performance measure called specific capacity.

What Is Specific Capacity?

Specific capacity is a standard metric that tells us how efficiently a well produces water. It’s calculated by dividing the flow rate (in gallons per minute) by the drawdown (in feet). The unit of measure is gallons per minute per foot of drawdown (gpm/ft).

• Well Yield:

This refers to the rate at which water is pumped from the well, typically measured in gallons per minute (gpm). 

• Drawdown:

This is the difference between the static water level (before pumping) and the pumping water level (during pumping). 

• Specific Capacity Calculation:

Specific capacity is calculated by dividing the well yield (gpm) by the drawdown (ft). For example, if a well yields 100 gpm with a drawdown of 10 feet, its specific capacity is 10 gpm/ft. 

The drawdown is the difference between the static water level (SWL) — the natural water level when the pump is off — and the pumping water level (PWL) — the water level while the pump is running.

Why Measure in August?

To accurately track well performance year to year, measurements should be taken at the same time each year. August is ideal in regions like Western Australia, as it is usually the driest month. By August, irrigation systems have been running regularly, and both the pumping and static water levels are typically at their lowest. This provides a consistent and realistic performance snapshot.

Sample Well Performance Record

Year	Flow Rate (gpm)	PWL (ft)	SWL (ft)	Drawdown (ft)	Specific Capacity (gpm/ft)	% of New
1	900	80	30	50	18	100%
2	900	73	22	51	17.6	98%
4	880	85	31	54	16.2	90%
6	850	89	30	59	14.4	80%

Notice how performance slowly drops — even a small increase in drawdown (which makes the pump work harder) leads to a lower specific capacity. By year six, this well is only producing 80% of its original capacity.

Tools for Accurate Measurement

To track specific capacity effectively, you need two things:

1. A Flow Meter: Installed on the well discharge line to measure gallons per minute.

2. An Access Port: A dedicated port to measure water levels and introduce treatment chemicals. It should be at least 1 inch in diameter, though a 2-inch port is preferred for easier access and accuracy.

   
   

This access point also allows for regular inspections and chemical treatments to remove mineral and biological buildup — keeping your well operating closer to peak performance.

Conclusion

Monitoring specific capacity annually is one of the most effective tools you have to extend the life of your irrigation well. It helps you identify problems early, plan maintenance strategically, and avoid emergency breakdowns during peak irrigation periods.

If you’re not already tracking your well’s specific capacity, now is the time to start. Ask your pump technician to install a proper flow meter and access port, and begin logging data each August.