How Much Water Is There?

David Perry, PhD

The amount of water Hawai'i county believes can be sustainably pumped from the Hawi aquifer is based on two studies by the U.S. Geological Survey (USGS), one a basic water budget that predicts the rate at which water is replenished through rainfall, the other a more complex model of how water is distributed within the aquifer. In the course of reviewing these studies last year, the Kako'o board discovered several problems with the water budget, most notably the use of an incorrect procedure that resulted in an overestimate of replenishment rates. We subsequently asked four internationally known research hydrologists to review the USGS procedures, all of whom confirmed the water budget was done incorrectly.
Calculating a water budget is a simple accounting procedure in which rainfall is apportioned to either use by plants, runoff to streams, or movement down through the soil to groundwater storage. It is the last that recharges aquifers and provides the starting point for determining the amount of water that can be pumped from a given aquifer without drawing it down. The commonly accepted approach to modeling a water budget is to assume that rain is first stored in soil, and from there is taken up by plants and pumped back to the atmosphere. The other two routes water may take, runoff to streams and flow to aquifers, occur primarily during periods of heavy or rather constant precipitation, when rain is coming into the soil faster than plants can pump it back out. However, for reasons that are unclear, the author of the original North Kohala water budget study (who is no longer with the USGS) did not follow standard procedure and assumed that water flowed to the aquifers first, with the remainder going to the plants--the exact opposite of what happens in nature. As a result, water that was actually being transpired to the atmosphere was calculated as going to the aquifer.
A second problem with the existing water budget emerges from changes in global climate and how they may effect rainfall patterns in our area. The USGS study, like many others that attempt to project water budgets into the future, assumed past rainfall is a good guide to the future. However, the earth's changing climate raises significant questions about the validity of that approach. Recent research suggests global warming will shift patterns of precipitation in the Pacific and result in greater extremes of alternating wet and dry periods. In other words, past rainfall patterns may no longer be good guides to rainfall in the future.
In early Fall of last year we sent a detailed letter to the USGS in Honolulu pointing out the problems with the study, and within a couple of weeks received a reply acknowledging the water budget was flawed. In November, I met (at his request) with Gordon Tribble, the newly appointed District Chief of the USGS Water Resources Division in Honolulu. Over coffee in Waimea he told me his office had initiated work with USGS scientists in California to develop a more accurate approach. Presumably that work is underway. Tribble also told me he had informed Milton Pawao of the problems with the original study.
How much the original study overestimated the rate of aquifer recharge, and therefore sustainable water supply, won't be known until a new study is completed. However, to get a rough idea I made some calculations based on the distribution of rainfall in my own gauge over a 7-month period (fall and winter '98-99), which was very rainy and therefore a period of relatively high aquifer recharge. I live in a 60" zone. To simulate 90" and 120" zones, I multiplied my gauge amounts by 1.5 and 2, respectively. For each, I compared the procedure used in the original USGS study with the correct procedure. Here are the proportions of rainfall predicted to go to the aquifer by each procedure

Original Study Correct Approach
60" zone 46% 0
90" zone 46% 11%
120" zone 57% 37%

Though only a crude estimate, the magnitude of difference in procedures revealed by the above exercise, using rainfall distributions realistic for North Kohala, argues strongly that the USGS study has significantly overestimated rates of recharge for the North Kohala aquifers, and raises serious doubts that the amount of water the county proposes to pump from the Hawi aquifer is sustainable. Fortunately the USGS, a respected agency whose work is usually of high quality, readily admitted their mistake and is working to correct it.