A yield test tells you how much water your borehole can safely deliver over time. Skipping it, or doing it badly, is one of the main reasons boreholes "dry up" or pumps burn out.
What a yield test is
After drilling, the contractor pumps water out at a steady rate for a set period, while measuring:
- Flow rate: litres per hour (l/h) being pumped;
- Drawdown: how far the water level drops while pumping;
- Recovery: how quickly the water level rises again after the pump stops.
Tests range from a few hours to 24 hours or longer. Longer tests give a more reliable picture, especially for smallholdings or irrigation.
What the results mean
- Sustainable yield: the rate the borehole can supply over the long term without the water level falling to the pump. The pump should be sized below this, not at the maximum the hole gave on day one.
- Pump depth: set so the pump stays under water at the lowest level during pumping, with a margin.
- Daily volume: yield × hours of pumping. This tells you whether the borehole can cover the house, garden and animals, or whether you need storage tanks.
Why it matters
- An oversized pump draws the water down too far, sucks sand and air, and runs dry. Dry-running protection helps, but the right size is better. See borehole pump problems.
- Yields can change with the seasons and with neighbours' boreholes. If the water starts running out late in winter, the pump rate or schedule may need adjusting.
Ask your driller
- How long will the yield test run, and will I get the results in writing?
- What sustainable yield do you recommend, and what pump size and depth follow from it?
- Is the yield test included in the quote? See what a borehole quote should include and borehole drilling costs.