Surface water flows originating from ground water also support riparian vegetation and play a major role in maintaining wetlands (NRC, 1995). Such support constitutes a vital ecological service. Ground water depletions are known to have eliminated surface water flows altogether in some areas. Many of the flowing streams in Arizona have disappeared because of the overpumping of ground water. High water tables may also support riparian species in areas where surface flows are ephemeral. The ecological services of ground water are particularly dramatic in cases where ground water supports habitat for endangered species. (An example of how ground water drawdown can affect stream flow appears in Figure 2.3.)
The availability of ground water is thus determined by the interaction of geological, hydrologic, and economic factors. The quantities of water available now and in the future depend upon the interaction of recharge and extraction. The cost of obtaining ground water is determined by pumping depths, energy costs, and the cost assigned to the opportunity foregone as a consequence of extracting ground water now rather than later. The value of ground water depends upon both the cost of obtaining it and the willingness of users to pay, and willingness to pay depends crucially on the quality of the water.