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 earths 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 nonrenewable 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 current rainfall for refilling and so are vulnerable to changes in the quantity and quality of recharge water. For example, 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 withdrawal makes aquifers more susceptible to drought and other changes in weather and to contamination from pollutants and wastes that percolate into the ground water. Depletion of ground water can also cause land subsidence 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 human-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 surface 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 surface and ground waters alters the dissolved oxygen and nutrient concentrations of streams and dilutes concentrations of dissolved contaminants such as pesticides and volatile organic compounds. Because of such links, human development of either ground water or surface water 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 earths 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 worlds largest aquifers. Limited recharge makes such aquifers highly susceptible to groundwater depletion. For example, exploitation of the Northern 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 groundwater sources, discussing sustainable or appropriate rates of extraction is difficult. As with deposits of coal and oil, almost any extraction is non-sustainable. Important questions for society include at what rate groundwater pumping should be allowed, for what purpose, and who if anyone 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 consumption 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 worlds population of six billion people (Table 2). However, the distribution of this water, both geographically and temporally, is not well matched
Issues in Ecology Number 9 Spring 2001 Box I:A Case Study-the Middle Rio Grande Perhaps nowhere are man impacts on river and floodplain ecosystems greater than in arid and semi-arid regionso the world.The Middle Rio Grande Basin of central New Mexico is a rapidly growing area that holds more than half of the state's population.The desire to balance water needs there has led to development of a careful water budget for the basin (Table I).highlighting annual variability,measurement uncertainty,and conflicting water demands for the region.The goal of the water budget is to help design a sustainable water policy management has already greatly altered this floodplain ecosystem (Figure 4).Dams and constructed river channels zones, f I a mosaic of cottonwoo wet meado .once hosted with signific ant cotton wood establishment occurred in 1942,and cottonwoods are declining in most areas.Half of the wetlands in the drainage were lost in just 50 years.Invasion by nonnative deep-rooted trees such as saltcedar and Russian-olive has dramatically altered riparian forest composition.Without changes in water management.exotic species will likely dominate riparian zones within half a century The water budget of the Middle Rio Grande reflects recent changes in hydrology,riparian ecology,and ground epletions in the basin or dep etion The largest loss is open-water evaporation,comprising one-third of the total.This loss is large compared to pre-dam values-direct evaporation from Elephant Butte Reservoir alone ranges from 50 to 280 million cubic meters(m)per year depending on reservoir size and climate.The second largest depletion is riparian plant transpiration(135 to 340 million m/y). There is considerable uncertainty in this estimate because of the unknown effects of fluctuating river discharge on Table I Sources and average annual water depletion from Water Source 1972 to 1997 for the Middle Rio Grande reach(the 64.000 km- dra between Otowi Gage north of Santa Fe and El Average Otowi Flow 1360 ephant But m Flow records at the Otowi gage,the in San Juan-Chama Diversion 70 flow poin the Middle Rio Grande reach.are more than a entur ter supplemented from the San Juan Water Use Depletion 9/2 and inci sed Oto (106 m/vr) a89 pen-werevaporatio 165 on (ground water) Net aquifer recharg 85 ng prog y severe drought years 10 to human needs the laree river flows of the amazon other regions,spring floodwaters from snowmelt are and Zaire-Congo basins and the tier of undeveloped riv. captured in reservoirs for later use in tropical regions ers in the northern tundra and taiga regions of Eurasia a substantial share of annual runoff occurs during mon and north am soon flooding.In Asia,fore xample,80 Perc occurs between May and October. Although this flood gether.these remote rivers account for nearly one-fifth water provides a variety of ecological services,including of total global runoff. sustaining wetlands,it is not a practical supply for Approximately half of the global renewable water irrigation.industry,and household uses that need supply runs rapidly toward the sea in floods (Table 2). water to be delivered in controlled quantities at specific In managed river systems of North America and many times
6 Issues in Ecology Number 9 Spring 2001 Box 1: A Case Study the Middle Rio Grande Increasing water demands create potential conflicts between human needs and those of native ecosystems. Perhaps nowhere are human impacts on river and floodplain ecosystems greater than in arid and semi-arid regions of the world. The Middle Rio Grande Basin of central New Mexico is a rapidly growing area that holds more than half of the states population. The desire to balance water needs there has led to development of a careful water budget for the basin (Table 1), highlighting annual variability, measurement uncertainty, and conflicting water demands for the region. The goal of the water budget is to help design a sustainable water policy. Water management has already greatly altered this floodplain ecosystem (Figure 4). Dams and constructed river channels prevent spring floods. Riparian zones, now limited by a system of levees, once hosted a mosaic of cottonwood and willow woodlands, wet meadows, marshes, and ponds. The last major floods with significant cottonwood establishment occurred in 1942, and cottonwoods are declining in most areas. Half of the wetlands in the drainage were lost in just 50 years. Invasion by nonnative deep-rooted trees such as saltcedar and Russian-olive has dramatically altered riparian forest composition. Without changes in water management, exotic species will likely dominate riparian zones within half a century. The water budget of the Middle Rio Grande reflects recent changes in hydrology, riparian ecology, and groundwater pumping. Estimating all major water depletions in the basin is critical for managing its water. Major depletions include urban uses, irrigation, plant transpiration, open-water evaporation, and aquifer recharge. The largest loss is open-water evaporation, comprising one-third of the total. This loss is large compared to pre-dam values direct evaporation from Elephant Butte Reservoir alone ranges from 50 to 280 million cubic meters (m3 ) per year depending on reservoir size and climate. The second largest depletion is riparian plant transpiration (135 to 340 million m3/y). There is considerable uncertainty in this estimate because of the unknown effects of fluctuating river discharge on Table 1 Sources and average annual water depletion from 1972 to 1997 for the Middle Rio Grande reach (the 64,000 km2 drainage between Otowi Gage north of Santa Fe and Elephant Butte Dam). Flow records at the Otowi gage, the inflow point for the Middle Rio Grande reach, are more than a century old. Water supplemented from the San Juan-Chama diversion project began in 1972 and increased Otowi flow by 70 million m3 /y (average flow without this water was about 1400 million m3/y). Major municipal water systems in the basin currently pump ground water at a rate of 85 million m3 /y. Maximum allowable depletion for the reach is 500 million m3 /y when adjusted annual flow exceeds 1900 million m3 /y, decreasing progressively to 58 million m3 /y in severe drought years (inflows of 120 million m3 /y at Otowi Gage). Water Source Supply (106 m3 /yr) Average Otowi Flow 1360 San Juan-Chama Diversion 70 Water Use Depletion (106 m3/yr) Open-water evaporation 270 Riparian plant transpiration 220 Irrigated agriculture 165 Urban consumption (ground water) 85 Net aquifer recharge 85 to human needs. The large river flows of the Amazon and Zaire-Congo basins and the tier of undeveloped rivers in the northern tundra and taiga regions of Eurasia and North America are largely inaccessible for human uses and will likely remain so for the foreseeable future. Together, these remote rivers account for nearly one-fifth of total global runoff. Approximately half of the global renewable water supply runs rapidly toward the sea in floods (Table 2). In managed river systems of North America and many other regions, spring floodwaters from snowmelt are captured in reservoirs for later use. In tropical regions, a substantial share of annual runoff occurs during monsoon flooding. In Asia, for example, 80 percent of runoff occurs between May and October. Although this floodwater provides a variety of ecological services, including sustaining wetlands, it is not a practical supply for irrigation, industry, and household uses that need water to be delivered in controlled quantities at specific times.