It is important to recognize the TEV of ground water even when one cannot develop specific quantitative separations of the various components. In fact, delineations of what can and cannot be quantified can be useful both to decision-makers for either development or remediation projects, and to researchers seeking to advance conceptual and methodological approaches. Descriptive information or surrogate quantitative measures that are not monetized may be the only information that can be assembled on some TEV components.
In many circumstances even a partial or inexact measurement of TEV can greatly aid decision-making by providing insight into how TEV changes with a policy or management decision. In some cases, the measurement of use values alone, or extractive services alone, can reveal substantial information on how the resource's TEV would be affected by a policy decision. In other circumstances, these limited measures may fail if they provide only a small portion of the components of TEV that would be altered.
Valuation of the extractive and in situ services of ground water requires an understanding of the hydrology and ecology of the ground water source. Hydrologic information includes numerous factors such as rainfall, runoff, infiltration, and water balance data; depth to ground water; whether the water-bearing zone is confined or unconfined; ground water flow rates and direction; and type of vadose and water-bearing zone materials. The contribution of ground water to stream base flow and the relationships between ground water and wetland and lake ecosystems are also important.
Knowing natural recharge rates and spatial locations, along with ground water usage rates and trends, is also necessary in water balance calculations and the consideration of ground water depletion. Depending upon the location, relationships between sea water or saline water intrusion and ground water use may also need to be established. Land subsidence can occur in some areas if ground water use is excessive, causing major problems with infrastructure components such as building foundations, roads, sewers, and water and utility lines. The effect of subsidence on flooding (especially) in coastal areas may also be significant. All these should be considered in valuing a ground water resource.
Some ground water supplies can be viewed as nonrenewable because of the long time-frame required to replenish them. Depletion of ground water (including overdrafting and mining) in deep aquifers, for instance, is essentially irreversible. Therefore, because ground water is a unique and potentially exhaustible resource vital to future generations, the costs of valuation studies may be recovered by assisting in the protection of ground water. Without planning and protection of ground water, the resource may not be available to support future generations.