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Specific Capacity

Definition

Specific Capacity is an assessment of the productivity of the well, and is defined as the well yield per unit drawdown* (i.e. Igpm/ft of drawdown). This translates into the amount of water level drop in a well at a designated pumping rate, over a specific time interval.

A general specific capacity measurement can be calculated using the maximum and minimum (i.e. non-pumping and pumping) water levels collected over a designated time interval. For future comparison purposes, these water levels should be collected over similar time intervals, with the same pumping rate.

A more accurate measurement of specific capacity can be made by conducting a pump test to determine any changes in the well's capacity. These tests can also be conducted about twice a year (e.g. spring and fall), to indicate if there are seasonal fluctuations in the specific capacity of the well. Pump tests to measure the specific capacity and overall well performance should also be conducted prior to any anticipated groundwater development or exploration activity in your area. This includes new commercial developments, irrigation well development, and drilling activity by the petroleum industry.

Follow these steps to calculate the Specific Capacity of your water well:

  1. Allow the water level in your well to recover to its highest point. The best way to do this is to shut off the pump at night and perform the test early in the morning.
  2. In the morning, measure the water level in the well to ensure that it has stabilized (i.e. not rising in the casing).
  3. Start the pump.
  4. Record the water level at 5 minute intervals.
  5. Measure the pumping rate (rate of discharge) at the beginning and near the end of the test.
  6. Run the test for a designated amount of time (e.g. 30 minutes or 1 hour).

Specific Capacity Calculation

Record:
  • 1. Pumping Rate
  • 2. Non-pumping water level at start of test (NPWL)
  • 3. Pumping Water level at end of test (PWL)
  • 4. Duration of test
Calculation:
  • Pumping rate (Igpm)
  • PWL - NPWL (ft)

*Note: Drawdown is the difference between the initial non-pumping water level and the pumping water level at a specified time interval.

Figure: Example specific capacity calculation