It is essential to have the yield of a borehole tested for numerous reasons:
(1) to establish the safe yield at which the borehole can safely be pumped.
(2) to determine the safe yield duty period that the borehole can be operated for indefinitely, without endangering the aquifer.
It is normal to undertake a six hour test and then install a pump with a capacity of around 50-60% of the flow at the end of the test. This 40-50% safety margin is sufficient in most cases, but not always. Unless absolutely necessary, never pump the borehole at its full capacity.
Boreholes to be subjected to a duty period of up to 10 hours per day should be tested for at least 24 hours. Water levels should be recorded at specific time intervals in the pumping borehole, together with the pump discharge for the duration of the test.
Water levels (draw-down) should be recorded from the start of the test until the pump is stopped. The recovery of the water level in the borehole should then be recorded until the borehole has recovered to within 10% of the initial static water level. Boreholes that are to be used every day and must supply a dependable daily water supply, for instance municipal, industrial or irrigation holes, should be test-pumped for longer periods of time. The test pumping should consist of a step draw-down test and a recovery test. This should be followed by a constant discharge test of at least 48 hours and a recovery measurement to within 5% of the original static water level.
A much asked questions is whether or not a borehole will always continue to have the same yield. The answer to this is “No”. The water yield can vary depending on the time of the year, the number of new boreholes in the vicinity, the yearly changes in the annual rainfall and the local detrimental effects of increased transpiration as a result of the planting of large numbers of trees.