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Lake levels are sensitive to changes in precipitation and evaporation, which lead to changes in streamflow and groundwater flow. Changes in lake levels will depend on regional changes in temperature and precipitation (the latter of which is highly uncertain). In areas where climate change scenarios suggest that precipitation or soil moisture levels could decline, lake levels are likely to decline or to fluctuate more widely (IPCC 1996, WG II, Section 10.5.2). Decreases in precipitation and/or soil moisture are indicated for the southeastern United States and much of the midcontinental region of North America in several 2xCO₂ GCM simulations with only CO₂ forcing. Water-level declines would be most severe in lakes and streams in dry evaporative drainage basins and basins with small catchments. Semipermanent prairie sloughs are fed by groundwater in addition to precipitation and spring snowmelt. Severe droughts deplete groundwater storage and cause these sloughs to dry out-resulting in turn in a decline of bird habitats (Poiani and Johnson, 1991, 1993). In the north-central United States, some drainage lakes and seepage lakes are highly responsive to precipitation; lake levels declined substantially during the late-1980s drought (Eilers et al., 1988).

High-latitude lakes also may be particularly vulnerable to changes in precipitation and temperature. For a 2xCO₂ climate change scenario with temperature increases of 3-5�C and precipitation increases of 10-15%, lake levels in the Mackenzie delta of arctic Canada fluctuate more widely. If precipitation were to decline by 10% with these temperature increases, however, many lakes could disappear within a decade as a consequence of decreased flood frequency (Marsh and Lesack, 1996).

The Great Lakes of North America are a critically important resource, and potential climate change effects are of great concern. Based on 2xCO₂ scenarios from several GCMs that indicated seasonal temperature increases of 2.6-9.1�C and seasonal precipitation changes of -30% to +40% (generally summer/autumn declines and winter increases), the following lake level declines could occur: Lake Superior -0.2 to -0.5 m, Lakes Michigan and Huron -1.0 to -2.5 m, and Lake Erie -0.9 to -1.9 m; the regulation plan for Lake Ontario cannot meet the minimum downstream flow requirements and maintain lake levels (Croley, 1990; Hartmann, 1990; Mortsch and Quinn, 1996). Using the Canadian Climate Centre (CCC) GCM II scenario (which generally has drier summer and autumn conditions than other GCMs for this region), the surface area of Lake St. Clair decreases by 15%; its volume is reduced by 37%; the water level declines 1.6 m; and the shoreline may be displaced 1-6 km lakeward, exposing lake bottom (Lee et al., 1996). These Great Lakes water-level changes are based on climate change scenarios from models that produced global temperature increases that are at least twice as large and precipitation changes that generally are greater than the most recent climate change simulations with aerosols included. Nonetheless, although highly uncertain at this time, the potential declines in lake water levels shown in these analyses could have large effects on wetlands, fish spawning, recreational boating, commercial navigation, and municipal water supplies in the Great Lakes area. Also of concern is the exposure of toxic sediments and their remediation with declines in lake levels (Rhodes and Wiley, 1993).

Responses to adapt to these large changes in lake levels in developed areas could be costly. Changnon (1993) estimated the costs for dredging, changing slips and docks, relocating beach facilities, and extending and modifying water intake and sewage outfalls for a 110-km section of the Lake Michigan shoreline including Chicago to range from $298-401 million for a 1.3-m decline and $605-827 million (1988 dollars) for a 2.5-m decline.

Water quality could deteriorate during summer low flows in regions experiencing reduced summer runoff.

Changes in water quality as well as changes in hydrological regimes could occur as a result of climate warming. Increases in water temperature in streams and rivers reduce oxygen solubilities and increase biological respiration rates and thus may result in lower dissolved oxygen concentrations, particularly in summer low-flow periods in low- and mid-latitude areas (IPCC 1996, WG II, Section 10.5.4). Although temperature increases also may stimulate photosynthesis via increased nutrient cycling and thus prevent dissolved oxygen declines during the day, sharp nighttime declines could occur. Summer dissolved oxygen concentrations in the hypolimnion of lakes, particularly more eutrophic lakes, also may decline, and areas of anoxia may increase because of increased respiration rates in a warmer climate (IPCC 1996, WG II, Section 10.5.4). However, reduction in the length of winter ice cover may reduce the incidence of winter anoxia in more northerly lakes and rivers. Increases in water temperature also will impact industrial uses of water, primarily in the low and mid-latitudes, by reducing the efficiency of once-through cooling systems (IPCC 1996, WG II, Section 14.3.3). Increases in water temperature will have a positive impact on navigation in the mid- and high latitudes, especially in the Great Lakes, by increasing the length of the ice-free season (IPCC 1996, WG II, Section 14.3.4)-perhaps compensating for reduced cargo capacity due to low water levels.

Changes in the seasonality of runoff also may affect water quality. In the middle and high latitudes, the shift in the high-runoff period from late spring and summer to winter and early spring might reduce water quality in summer under low flows. Extended droughts in boreal regions have been shown to result in acidification of streams due to oxidation of organic sulfur pools in soils (Schindler, 1997). However, acidic episodes associated with spring snowmelt in streams and lakes in the northeastern United States and eastern Canada might be reduced under a warmer climate with lower snow accumulation and lower discharges during the spring melt (IPCC 1996, WG II, Section 10.5.3; Moore et al., 1997). In general, water-quality problems (particularly low dissolved oxygen levels and high contaminant concentrations) associated with human impacts on water resources (e.g., wastewater effluents) will be exacerbated more by reductions in annual runoff than by other changes in hydrological regimes (IPCC 1996, WG II, Section 14.2.4).

Increases in competition for limited water under a warmer climate could lead to supply shortfalls and water-quality problems, particularly in regions experiencing declines in runoff.

Under a warmer climate, more intensive water resource management will be required because population growth, economic development, and altered precipitation patterns will lead to more intense competition for available supplies (IPCC 1996, WG II, Sections 12.3.5 and 14.4). Managing increased and diversified water demands will be particularly problematic in regions that currently have the lowest water availability (e.g., western-central North America) and those that will experience declines in runoff with climate change.

National water summaries by the U.S. Geological Survey provide comprehensive data on water availability and demand. Agriculture and steam electric generation account for approximately 75% of total water withdrawals in the United States; agricultural uses are most dominant west of the 100th meridian, where evaporation generally exceeds precipitation. When seasonal and interannual variability of regional climates are considered, the most inadequate water supplies within the United States (70% depletion of available supplies by off-stream uses) are in the southWest-including the lower Colorado River basin, the southern half of California's Central Valley, and the Great Plains river basins south of the Platte River.

A warmer climate will likely increase the demand for irrigation water by agriculture (IPCC 1996, WG II, Section 14.3.1) and for industrial cooling water at the same time that urban growth will be increasing the demand for municipal water supplies. In addition, higher water temperatures will reduce the efficiency of cooling systems (Dobrowolski et al., 1995), and might make it increasingly difficult to meet regulatory constraints defining acceptable downstream water temperatures, particularly during extremely warm periods (IPCC 1996, WG II, Section 14.3.3). Furthermore, growing instream flow requirements to protect aquatic ecosystems also will reduce effective water supplies. However, improved management of water infrastructure, pricing policies, and demand-side management of supply have the potential to mitigate some of the impacts of increasing water demand (Frederick and Gleick, 1989; IPCC 1996, WG II, Section 12.5.5).

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