The accurate yield of a borehole can only be determined by means of an aquifer test (pumping test). These tests involve installing a test pump and pumping the borehole for a period of time at a given rate (yield) whilst recording the drawdown (lowering of water level in the borehole as a result of abstraction). Simply put, the maximum yield of the borehole is achieved by increasing the abstraction rate to a yield, which results in the maximum drawdown of water in the pumping borehole without resulting in pump suction in a given period of time. Aquifer tests are valuable in determining optimum abstraction rates, which in turn allow for correct sizing of the pump (prevent over abstraction and pump burn out) and in determining optimum pumping schedules.
An alternative and less accurate method for determining the yield of a borehole is to quantify the BLOW YIELD whilst drilling or air flushing. A blow yield usually realizes a conservative estimate i.e. as a rule of thumb a blow yield represents approximately 60% of the maximum yield of the borehole. Unfortunately blow yield testing has limitations. Blow yields are not considered reliable if small (<1000 l/hr) volumes of water are intersected. The reason for this being that the volume and pressure of compressed air being introduced into the borehole partially inhibits water entering the borehole. A 4 hour test pump not only realizes an accurate water yield, but it also further cleans out debris from the borehole accumulated during the drilling process.