Whenever wells are drilled and pumped, groundwater will be affected as will any nearby surface stream fed by the groundwater. Describing and anticipating these hydrologic consequences should be a major task of the state Commission on Water Resource Management, the agency charged with protecting Hawai`i’s freshwater.
Ever since the commission was established, it has relied on a mathematical model to estimate how much water can be removed from a given underground water structure, or aquifer, without damaging the quantity or quality of water available. This estimate is called the sustainable yield.
Two recent studies of Maui’s important `Iao aquifer, however, suggest that the model employed by CWRM may lead to overestimates of the sustainable yield. These studies, conducted by the U.S. Geological Survey, have implications not only for Maui, which depends on the `Iao aquifer as its chief source of fresh water, but for all other islands as well.
Hydrologic Consequences
When groundwater is developed (that is, when wells are drilled and pumped), the first consequence will be a decline in water levels in some or all of the aquifer, with the greatest decline centered about the well. This decline will extend to at least one of the aquifer’s boundaries, where groundwater naturally discharges. This will generally be the ocean, a stream, spring, or some combination of the three.
Second, the amount of decline in the natural discharge will equal the amount of groundwater removed by pumping.
Third, in most areas of Hawai`i, the transition zone between freshwater and underlying salt water will rise and move inland. Generally, a one-foot decline in groundwater level results in a corresponding rise in the transition zone of 40 feet, which could result in the intrusion of saltwater into wells.
As significant as these consequences are, none is included in the definition of sustainable yield in the state Water Code nor are they necessarily considered when the Water Commission considers requests for drilling wells or installing pumps. In practice, these consequences are taken into account only as part of a prolonged contested case hearing – an exceptional occurrence.
Delineation of Aquifers
One of the major technical problems with the framework used by the Water Commission is the fact that it has delineated far more aquifers on each island than actually exist. For instance, the state identifies 16 aquifers on the island of Moloka`i. More logically, just one or at the most two aquifers – the East Volcano aquifer and the West Volcano aquifer – underlie the island. Because the rocks of these two volcanoes are similar in their physical characteristics, it is reasonable to say that the island is underlain by a single aquifer.
Because the decline in groundwater levels from pumping will extend to at least one of the aquifer’s boundaries, it becomes extremely important to define accurately the geographical extent of an aquifer. This all the more vital, given that the rise in the transition zone that occurs wherever groundwater levels decline. USGS studies have shown that the decline in water levels from pumping a single well on Moloka`i can spread through five or more of the aquifers delineated by the Water Commission.
So how does the commission define aquifer boundaries? Generally, it sets the boundaries at stream drainage divides. But aquifers and stream drainages are not the same. An aquifer extends as far as the rock unit in which the groundwater is found. A stream drainage divide defines the drainage area for a given stream. There is no correlation at all between aquifer boundaries and stream drainages, except in the rare event that the two happen to coincide.
Thus, the commission tends to underestimate the geographical range of the impacts of groundwater withdrawal and tends to limit its concerns to the immediate aquifer in question (however poorly that aquifer may be defined by the state).
Groundwater Recharge
Another major problem with the framework used by the Water Commission has to do with the value of recharge used to determine sustainable yield. It is not possible to measure groundwater recharge directly; instead, it must always be estimated. Many methods have been developed to do this, with some more sophisticated than others and the degree of reliability increasing with the level of sophistication.
One of the most common methods is a water budget. This is simply an equation that expresses the relationship between the processes by which water enters and leaves an area. In general, the equation states that precipitation falling within an area is lost by surface runoff, by evaporation, by plant transpiration, and by recharge to the underlying groundwater system. The first three of these can be estimated. Ground water recharge is thus estimated as whatever is left over when the first three are subtracted from total precipitation.
One of the simplest ways to solve the water budget equation is to use average annual values for precipitation, runoff, and evapo-transpiration for the entire aquifer area. This is the method employed by the Water Commission.
More sophisticated water budgets take into account variation in rainfall across the aquifer and the factors that affect the speed of runoff, evaporation, plant transpiration, or movement through the soil. The more specific the data plugged into the model, the more reliable the resulting estimates.
Time is another variable. Water budgets can be based on daily, monthly, or yearly values for precipitation. In general, those methods that use shorter time periods are likely to provide more realistic results.
Clearly, then, estimates of recharge will thus vary significantly depending on the method used to determine its value. This means that values of recharge used by the commission to calculate sustainable yield are subject to considerable error – and so, in turn, are the resulting values of sustainable yield.
Sustainable Yield
The concept of sustainable yield is a cornerstone of the state’s management of groundwater resources. Despite the apparent simplicity of the concept, its use is not in the best interests of the state and should be discontinued. No standard method exists to calculate its value. Modern texts on hydrology discuss sustainable yield only in the broadest sense and advise that, while the concept is good, actual determination of its value is not a simple task. As early as the 1950s, scientific literature began to call for abandonment of the term.
Sustainable yield was initially defined in 1915 as “the limit to the quantity of water which can be withdrawn regularly and permanently without dangerous depletion of the storage reserve.” At the time, the science of hydrology was young and there was a widespread belief that for each aquifer, there was a maximum quantity of water that could be withdrawn without permanently depleting it. This is true. It was also believed, however, that this quantity could be determined before drilling the first well. This is not true. It was many years before hydrologists came to understand that the amount of water that could be developed from an aquifer is highly dependent on the arrangement of wells within the aquifer. In other words, there is no single value for the amount of water that can be taken from an aquifer that is inherent to the aquifer.
Significant use of groundwater in the United States began in the early 1900s and, as usage grew over time, the hydrologic consequences of ground water development came to be better understood. It also became apparent that consequences of water development were often social, economic, and legal as well as hydrologic. For example, in some areas of the west, such as Arizona, withdrawal of groundwater for crop irrigation resulted in the decline of groundwater levels to the point that the cost of pumping finally made agriculture no longer economically feasible.
Over time, many definitions of sustainable yield have appeared in the scientific literature. All of the most recent ones include the ideas that sustainable yield is not a single value inherent in an aquifer, that its value depends on the arrangement of wells, and that sustainable yield is the amount of water that can be withdrawn from an aquifer for which the total range of consequences (including hydrologic, social, economic, legal, etc.) is acceptable.
The Water Code’s definition of sustainable yield does not reflect these more recent views. Instead, it assumes that there is a unique value for sustainable yield of each aquifer that is independent of the arrangement of wells. Further, as defined in the state Water Code, sustainable yield does not address the consequences of development beyond permanent depletion of the resource and degradation of water quality. Other hydrologic consequences resulting from development, such as the effect of development on other wells, streams, springs, and discharge to the ocean, are not identified. Social, economic, legal and other considerations are not discussed as part of sustainable yield for any aquifer and are not generally considered when the commission hears a request for a well. In practice, these concerns have only been addressed in the context of contested cases.
The failure of the commission’s view of sustainable yield to incorporate the consequences of development actually favors groundwater development over other considerations. This arrangement is not in the best interest of the public, the state, or of those individuals directly affected by the hydrologic consequences of a given development.
Robust Analytical Model
Probably the most significant problem concerning sustainable yield, as the term is used in Hawai`i, is the mathematical model used to estimate this value: the RAM, or robust analytical model. A recent report by the USGS demonstrates that RAM cannot account for spatial variations in recharge, discharge, and hydraulic properties of the aquifer
Spatial variations in recharge and discharge, the hydraulic properties of an aquifer, the variability of withdrawal – all these are variables that collectively control the decline in groundwater levels brought about by pumping. RAM cannot account for these variables so it will usually under-predict the decline in the water table and over-predict sustainable yield. RAM also tends to under-predict water level declines for wells that are widely scattered across an aquifer, with the greatest error occurring at the well sites. This is significant because the spatial distribution of wells is a key factor in the process of groundwater development, and protecting them from saltwater intrusion is vital in any regulatory regime. If RAM under-predicts water level declines at well locations, then those wells will not be protected from saltwater intrusion.
Another USGS report has described RAM’s shortcomings with respect to aquifers overlain by a dense caprock, such as the two major aquifers in the state: Pearl Harbor on O`ahu and `Iao on Maui. Here again, RAM tends to overestimate sustainable yield. If these aquifers are developed to their predicted sustainable yield, some or all of the wells will ultimately show saltwater intrusion.
In its recent decision on the Waiahole Ditch, the Supreme Court of Hawai`i held that the public trust doctrine applies to all water resources of the state and that the commission has an obligation to “protect, control and regulate the use of Hawai`i’s water resources for the benefit of its people.” For the reasons given above, then, it’s questionable whether the commission’s current use of sustainable yield as a tool for managing groundwater allows it to meet that public trust obligation.
Although it is not possible to predict sustainable yield, it is possible to predict the feasibility of groundwater development and associated hydrologic consequences on a well-by-well basis, as wells are proposed to be drilled. This, of course, is the manner in which aquifers are actually developed. The most widely recognized and scientifically accepted method for accomplishing this is a numeric groundwater model that simulates the movement of groundwater in an aquifer (including that of the underlying saltwater). The Water Commission began to move in this direction in the early and middle 1990s but has since backed away from it. This is unfortunate and should be changed.
William Meyer was chief hydrologist for the USGS in Hawai`i until his recent retirement.
References: “Water Budget for the Island of Moloka`i,” USGS WRI Report 97-4155.
“Analytical Versus Numerical Estimates of Water Level Declines Caused by Pumping, and a Case Study of the `Iao Aquifer, Maui, Hawai`i.” USGS WRI Report 00-4244
“The Response of the `Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices Between 1940 and 1998, Island of Maui, Hawai`i,” USGS WRI Report 00-4223.
— William Meyer
Volume 11, Number 11 May 2001
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