Issues in Ecology Number 9 Spring 2001 hundreds to tens of thousands of years.although the range face and ground waters for water supply water quality in turnover rates is large.Indeed,a majority of grou nd and aquatic habitats where extraction of s und wate water is not actively tur xceeds recharge rates.the o vater tables the earth's surface at all fosi water. In summer when a high water table is needed to sustair relic of wetter ancient climatic conditions and melting minimum flows in rivers and streams.low groundwater Pleistocene ice sheets that accumulated over tens of levels can decrease low-flow rates.reduce perennial thousands of years.Once used,it cannot readily be stream habitat.increase summer stream temperatures replenished. and impair water quality.trout and salmon species selec The distinction between renewable and non areas of groundwater upwelling in streams to moderate renewable iscritica for water managemen extreme s al tem to keer egg and policy. an three-quarters of un ergroun ange of sur water is non-renewable,meaning it has a replenishment face and ground waters alters the dissolved oxygen and period of centuries or more(Figure 3).The High Plains or nutrient concentrations of streams and dilutes concen Ogallala Aquifer that underlies half a million km2 of the trations of dissolved contaminants such as pesticides and central United States is arguably the largest aquifer in the volatile organic compounds.Because of such links.hu elopment of either ground water or surface wa 20.000%e % tity and quality of the othe The links between surface ar ater the Ogalla ala the primary water source for a fifth of especially important in regions with low rainfall (see Box irrigated U.S.farmland.The extent of irrigated cropland 1,Table I,and Figure 4).Arid and semi-arid regions in the region peaked around 1980 at 5.6 million hectares cover a third of the earth's lands and hold a fifth of the and at pumping rates of about 6 trillion gallons of water a global population.Ground water is the primary source of year.That has since declined somewhat due water for drinking and irrigation in these regions.which gro ndwater depletion ic ch possess m the orld's la est aqu limited re region.However. the average thickness of the charge n aquifers high le to groun declined by more than 5 percent across a fifth of its area water depletion.For example,explo tation of the North in the 1980s alone. em Sahara Basin Aquifer in the I990s was almost twic In contrast,renewable aquifers depend on cur. the rate of replenishment,and many springs associated rent rainfall for refilling and so are vulnerable to changes with this aquifer are drying up.For non-renewable ground- water.For water sources,discussing Edwards Aqu stainabler appropriate rates ction is difficult As with s much, of coal and oil suppl ng water amost any extraction nable portant que has increased four-fold since the tions for society include at what rate groundwater pump exceeds annual recharge rates. Increased water with ing should be allowed,for what purpose,and who if any drawal makes aquifers more susceptible to drought and one will safeguard the needs of future generations in other changes in weather and to contamination from pol the Ogallala Aquifer,for example,the water may be gone lutants and wastes that percolate into the ground wa in as little as a century. Depletion of ground wa can als caus land subs porous sand. gravel or rock HUMAN APPROPRIATION OF FRESHWATER SUPPLY water.The Central Valley of California has lost about 25 Global Renewable Water Supplies km3 of storage in this way,a capacity equal to more than 40 percent of the combined storage capacity of all hu- Growth in global population and water consump man-made reservoirs in the state tion will place additional pr essure on freshwater resources s hav in the cc available several nty the wa im s more sh wa er each tha legally.This view is changing.however.as studies in needed to sustain the world ds population of six billior streams,rivers,reservoirs,wetlands.and estuaries show people(Table 2).However,the distribution of this water the importance of interactions between renewable sur- both geographically and temporally,is not well matched 5 Issues in Ecology Number 9 Spring 2001 hundreds to tens of thousands of years, although the range in turnover rates is large. Indeed, a majority of ground water is not actively turning over or being recharged from the earth’s surface at all. Instead, it is fossil water, a relic of wetter ancient climatic conditions and melting Pleistocene ice sheets that accumulated over tens of thousands of years. Once used, it cannot readily be replenished. The distinction between renewable and non￾renewable ground water is critical for water management and policy. More than three-quarters of underground water is non-renewable, meaning it has a replenishment period of centuries or more (Figure 3). The High Plains or Ogallala Aquifer that underlies half a million km2 of the central United States is arguably the largest aquifer in the world. The availability of turbine pumps and relatively inexpensive energy has spurred the drilling of about 200,000 wells into the aquifer since the 1940s, making the Ogallala the primary water source for a fifth of irrigated U.S. farmland. The extent of irrigated cropland in the region peaked around 1980 at 5.6 million hectares and at pumping rates of about 6 trillion gallons of water a year. That has since declined somewhat due to groundwater depletion and socioeconomic changes in the region. However, the average thickness of the Ogallala declined by more than 5 percent across a fifth of its area in the 1980s alone. In contrast, renewable aquifers depend on cur￾rent rainfall for refilling and so are vulnerable to changes in the quantity and quality of recharge water. For ex￾ample, groundwater pumping of the Edwards Aquifer, which supplies much of central Texas with drinking water, has increased four-fold since the 1930s and at times now exceeds annual recharge rates. Increased water with￾drawal makes aquifers more susceptible to drought and other changes in weather and to contamination from pol￾lutants and wastes that percolate into the ground water. Depletion of ground water can also cause land subsid￾ence and compaction of the porous sand, gravel, or rock of the aquifer, permanently reducing its capacity to store water. The Central Valley of California has lost about 25 km3 of storage in this way, a capacity equal to more than 40 percent of the combined storage capacity of all hu￾man-made reservoirs in the state. Renewable ground water and surface waters have commonly been viewed separately, both scientifically and legally. This view is changing, however, as studies in streams, rivers, reservoirs, wetlands, and estuaries show the importance of interactions between renewable sur￾face and ground waters for water supply, water quality, and aquatic habitats. Where extraction of ground water exceeds recharge rates, the result is lower water tables. In summer, when a high water table is needed to sustain minimum flows in rivers and streams, low groundwater levels can decrease low-flow rates, reduce perennial stream habitat, increase summer stream temperatures, and impair water quality. Trout and salmon species select areas of groundwater upwelling in streams to moderate extreme seasonal temperatures and to keep their eggs from overheating or freezing. Dynamic exchange of sur￾face and ground waters alters the dissolved oxygen and nutrient concentrations of streams and dilutes concen￾trations of dissolved contaminants such as pesticides and volatile organic compounds. Because of such links, hu￾man development of either ground water or surface wa￾ter often affects the quantity and quality of the other. The links between surface and ground waters are especially important in regions with low rainfall (see Box 1, Table 1, and Figure 4). Arid and semi-arid regions cover a third of the earth’s lands and hold a fifth of the global population. Ground water is the primary source of water for drinking and irrigation in these regions, which possess many of the world’s largest aquifers. Limited re￾charge makes such aquifers highly susceptible to ground￾water depletion. For example, exploitation of the North￾ern Sahara Basin Aquifer in the 1990s was almost twice the rate of replenishment, and many springs associated with this aquifer are drying up. For non-renewable ground￾water sources, discussing sustainable or appropriate rates of extraction is difficult. As with deposits of coal and oil, almost any extraction is non-sustainable. Important ques￾tions for society include at what rate groundwater pump￾ing should be allowed, for what purpose, and who if any￾one will safeguard the needs of future generations. In the Ogallala Aquifer, for example, the water may be gone in as little as a century. HUMAN APPROPRIATION OF FRESHWATER SUPPLY Global Renewable Water Supplies Growth in global population and water consump￾tion will place additional pressure on freshwater resources in the coming century. Currently, the water cycle makes available several times more fresh water each year than is needed to sustain the world’s population of six billion people (Table 2). However, the distribution of this water, both geographically and temporally, is not well matched