The notion of sustainable yield might fairly be said to be the most important concept in the Hawai`i Water Plan. It is defined in the Water Code as “the maximum rate at which water may be withdrawn from a water source without impairing the utility or quality of the water source as determined by the commission.” Knowing sustainable yields for the aquifers of the state is thus a first step in knowing how much water can safely be pumped out.
But that first step has too often been confused with the last word. The Water Resources Protection Plan, which contains the sustainable yield estimates used in all the other components of the Water Plan, clearly states that the “confidence level” of sustainable yields for most aquifers in the state is very low. On a scale of one to three, with three being lowest, most sustainable yields are given a rating of three. The WRPP warns that “estimates of sustainable yields are not meant to be an exact number which could be used in final planning documents. The estimates should not be equated to developable groundwater.”
But most county water plans do precisely this. What’s more, they treat the sum total of sustainable yields for each island as a “global” resource. That is, all the water on a given island is considered potentially available to answer needs on any part of the island, with delivery and development of distant sources of water being regarded as ancillary technical problems.
The result is that county plans are “best-case” scenarios. If the sustainable yield estimates turn out to have been too high, the counties will be unable to deliver water to planned developments — unless they resort to desalination or other expensive water “augmentation” projects.
Over-optimistic estimates of sustainable yield have occurred in the past in Hawai`i. For years, the Pearl Harbor aquifer, one of the most thoroughly studied in the state, was thought to have a sustainable yield of 225 million gallons a day. By 1989, the figure was reduced by 13 percent to 195 mgd.
Finally, sustainable yields refer to ground water withdrawals, but in many places in Hawai`i, the relationship between ground and surface water makes it impossible to deal with these resources separately. For example, a study of Maunawili Stream on Windward O`ahu strongly suggests a one-to-one correspondence between ground-water withdrawals and reductions in instream flow. The almost exclusive focus on sustainable yield in the Hawai`i Water Plan thus risks ignoring the surface-water impact of developing ground water resources.
Volume 1, Number 2 August 1990