Perennial yield is estimated by plotting the change in ground water level versus the amount of ground water extracted each year over a period of years that are considered to be representative of the long-term average hydrology. For this analysis, data for 13 years were plotted for each basin analyzed. A "best fit" curve was drawn and the intersection of the best fit curve with the line showing zero ground water level change indicated the current estimated perennial yield of ground water in that basin. The perennial yield is similar to long-term sustained yield, assuming there are no changes in water management practices.
The procedure probably underestimates perennial yield, or may not work, in aquifers where extraction increases the ground water gradient and induces additional recharge. The perennial yield of these aquifers would increase as extraction increased so long as recharge was equal to, or greater than, the extraction. This procedure does not take into consideration either existing or potential problems with ground water quality.