FIGURE 2.2 Unconfined aquifer and its water table; confined aquifer and its potentiometric surface.

Water found in deep aquifers may have been stored over millions of years and is sometimes referred to as "fossil water." The natural rates of recharge to these deep aquifers, when recharge occurs at all, are quite low (Lloyd and Farag, 1978). Fetter (1994) notes that for practical purposes such aquifers are not recharged and any extractions are irreversible. The extraction and use of water from such aquifers is analogous to the mining of resources such as minerals that do not recur periodically on anything less than geologic time scales. Aquifers in arid regions are frequently characterized by very small rates of recharge that range from a few hundredths of a millimeter per year to perhaps 200 mm/yr (Heath, 1983). Aquifers characterized by either the total absence of recharge or by very low rates of recharge cannot be relied upon as a sustainable source of water supply. The Ogallala aquifer underlying parts of Texas, Oklahoma, and New Mexico is a good example of such an aquifer. The relatively high rates of extraction and use of water from the Ogallala aquifer for agricultural purposes over the past four decades has resulted in progressive increases in pumping depths. In many places the depth to ground water is so great that it is no longer economical to pump. In these areas irrigated agriculture that historically relied on waters from the Ogallala must be converted to dry land farming or other land uses.