Potential Interactions with Water Resources

Water is a central resource supporting human activities and ecosystems, and adaptive management of this resource has been an essential aspect of societal development. Increases in global temperatures during the 20th century have been accompanied by more precipitation in the middle and high latitudes in many regions of North America. For example, U.S. precipitation increased by 5 -- 10 percent, predominantly from the spring through the autumn. Much of this increase resulted from a rise in locally heavy and very heavy precipitation events, which has led to the observed increases in low to moderate stream flow that have been characteristic of the warm season across most of the contiguous United States.

Local to global aspects of the hydrologic cycle, which determine the availability of water resources, are likely to be altered in important ways by climate change (NWAG 2000). Because higher concentrations of CO₂ and other greenhouse gases tend to warm the surface, all models project that the global totals of both evaporation and precipitation will continue to increase, with increases particularly likely in middle and high latitudes.

The regional patterns of the projected changes in precipitation remain uncertain, however, although there are some indications that changes in atmospheric circulation brought on by such factors as increasing Pacific Ocean temperatures may bring more precipitation to the Southwest and more winter precipitation to the West. Continuing trends first evident during the 20th century, model simulations project that increases in precipitation are likely to be most evident in the most intense rainfall categories typical of various regions. To the extent such increases occur during the warm season when stream flows are typically low to moderate, they could augment available water resources. If increases in precipitation occur during high stream flow or saturated soil conditions, the results suggest a greater potential for flooding in susceptible areas where additional control measures are not taken, especially because under these conditions the relative increase in runoff is generally observed to be greater than the relative increase in precipitation.

Effects on Available Water Supplies

Water is a critical national resource, providing services to society for refreshment, irrigation of crops, nourishment of ecosystems, creation of hydroelectric power, industrial processing, and more. Many U.S. rivers and streams do not have enough water to satisfy existing water rights and claims. Changing public values about preserving in-stream flows, protecting endangered species, and settling Indian water rights claims have made competition for water supplies increasingly intense. Depending on how water managers are able to take adaptive measures, the potential impacts of climate change could include increased competition for water supplies, stresses on water quality in areas where flows are diminished, adverse impacts on ground-water quantity and quality, an increased possibility of flooding in the winter and early spring, a reduced possibility of flooding later in the spring, and more water shortages in the summer. In some areas, however, an increase in precipitation could outweigh these factors and increase available supplies.

Significant changes in average temperature, precipitation, and soil moisture resulting from climate change are also likely to affect water demand in most sectors. For example, demand for water associated with electric power generation is projected to increase due to the increasing demand for air conditioning with higher summer temperatures.

Climate change is also likely to reduce water levels in the Great Lakes and summertime river levels in the central United States, thereby adversely affecting navigation, general water supplies, and populations of aquatic species.

Effects on Water Quality

Increases in heavy precipitation events are likely to flush more contaminants and sediments into lakes and rivers, degrading water quality. Where uptake of agricultural chemicals and other non-point sources could be exacerbated, steps to limit water pollution are likely to be needed. In some regions, however, higher average flows will likely dilute pollutants and, thus, improve water quality. In coastal regions where river flows are reduced, increased salinity could also become more of a problem. Flooding can also cause overloading of storm-water and wastewater systems, and can damage water and sewage treatment facilities, mine tailing impoundments, and landfills, thereby increasing the risks of contamination and toxicity.

Because the warmer temperatures will lead to increased evaporation, soil moisture is likely to be reduced during the warm season. Although this effect is likely be alleviated somewhat by increased efficiency in water use and reduced demand by native plants for water, the drying is likely to create a greater susceptibility to fire and then loss of the vegetation that helps to control erosion and sediment flows. In agricultural areas, the CO₂-induced improvement of water-use efficiency by crops is likely to decrease demands for water, particularly for irrigation water. In addition, in some regions, increasing no-till or reduced-till agriculture is likely to improve the water-holding capacity of soils, regardless of whether climate changes, thereby reducing the susceptibility of agricultural lands to erosion from intensified heavy rains (NAAG 2002, NWAG 2000).

Effects on Snowpack

Rising temperatures are very likely to affect snowfall and increase snowmelt conditions in much of the western and northern portions of the country that depend on winter snowpack for runoff. This is particularly important because snowpack provides a natural reservoir for water storage in mountainous areas, gradually releasing its water in spring and even summer under current climate conditions.

Model simulations project that snowpack in western mountain regions is likely to decrease as U.S. climate warms (Figure 6-9). These reductions are projected, despite an overall increase in precipitation, because (1) a larger fraction of precipitation will fall as rain, rather than snow; and (2) the snowpack is likely to develop later and melt earlier. The resulting changes in the amount and timing of runoff are very likely to have significant implications in some basins for water management, flood protection, power production, water quality, and the availability of water resources for irrigation, hydropower, communities, industry, and the sustainability of natural habitats and species.

Effects on Ground-Water Quantity and Quality

Several U.S. regions, including parts of California and the Great Plains, are dependent on dwindling ground-water supplies. Although ground-water supplies are less susceptible to short-term climate variability than surface-water supplies, they are more affected by long-term trends. Ground water serves as the base flow for many streams and rivers. Especially in areas where springtime snow cover is reduced and where higher summer temperatures increase evaporation and use of ground water for irrigation, ground-water levels are very likely to fall, thus reducing seasonal stream flows. River and stream temperatures fluctuate more rapidly with reduced volumes of water, affecting fresh-water and estuarine habitats. Small streams that are heavily influenced by ground water are more likely to have reduced flows and changes in seasonality of flows, which in turn is likely to damage existing wetland habitats.

Pumping ground water at a faster rate than it can be recharged is already a major concern, especially in parts of the country where other water resources are limited. In the Great Plains, for example, model projections indicate that drought is likely to be more frequent and intense, which will create additional stresses because ground-water levels are already dropping in parts of important aquifers, such as the Ogallala.

The quality of ground water is being diminished by a variety of factors, including chemical contamination. Salt-water intrusion is another key ground-water quality concern, particularly in coastal areas where changes in fresh-water flows and increases in sea level will both occur. As ground-water pumping increases to serve municipal demand along the coast and less recharge occurs, coastal ground-water aquifers are increasingly being affected by sea-water intrusion. Because the ground-water resource has been compromised by many factors, managers are increasingly looking to surface-water supplies, which are more sensitive to climate change and variability.

Effects on Floods, Droughts, and Heavy Precipitation Events

Projected changes in the amount, timing, and distribution of rainfall and snowfall are likely to lead to changes in the amount and timing of high and low water flows -- although the relationships of changes in precipitation rate to changes in flood frequency and intensity are uncertain, especially due to uncertainties in the timing and persistence of rainfall events and river levels and capacities. Because changes in climate extremes are more likely than changes in climate averages to affect the magnitude of damages and raise the need for adaptive measures at the regional level, changes in the timing of precipitation events, as well as increases in the intensity of precipitation events, are likely to become increasingly important considerations.

Climate change is likely to affect the frequency and amplitude of high stream flows, with major implications for infrastructure and emergency management in areas vulnerable to flooding. Although projections of the number of hurricanes that may develop remain uncertain, model simulations indicate that, in a warmer climate, hurricanes that do develop are likely to have higher wind speeds and produce more rainfall. As a result, they are likely to cause more damage, unless more extensive (and therefore more costly) adaptive measures are taken, including reducing the increasing exposure of property to such extreme events. Historical records indicate that improved warning has been a major factor in reducing the annual number of deaths due to storms, and that the primary cause of the increasing property damage in recent decades has been the increase in at-risk structures, such as widespread construction of vacation homes on barrier islands.

Despite the overall increase in precipitation and past trends indicating an increase in low to moderate stream flow, model simulations suggest that increased air temperatures and more intense evaporation are likely to cause many interior portions of the country to experience more frequent and longer dry conditions. To the extent that the frequency and intensity of these conditions lead to an increase in droughts, some areas are likely to experience wide-ranging impacts on agriculture, water-based transportation, and ecosystems, although the effects on vegetation (including crops and forests) are likely to be mitigated under some conditions by increased efficiency in water use due to higher CO₂ levels.

Water-driven Effects on Ecosystems

Species live in the larger context of ecosystems and have differing environmental needs. In some ecosystems, existing stresses could be reduced if increases in soil moisture or the incidence of freezing conditions are reduced. Other ecosystems, including some for which extreme conditions are critical, are likely to be most affected by changes in the frequency and intensity of flood, drought, or fire events. For example, model projections indicate that changes in temperature, moisture availability, and the water demand from vegetation are likely to lead to significant changes in some ecosystems in the coming decades (NAST 2000). As specific examples, the natural ecosystems of the Arctic, Great Lakes, Great Basin, and Southeast, and the prairie potholes of the Great Plains appear highly vulnerable to the projected changes in climate (see Figures 6-6 and 6-8).

The effects of changes in water temperatures are also important. For example, rising water temperatures are likely to force out some cold-water fish species (such as salmon and trout) that are already near the threshold of their viable habitat, while opening up additional areas for warm-water species. Increasing temperatures are also likely to decrease dissolved oxygen in water, degrade the health of ecosystems, reduce ice cover, and alter the mixing and stratification of water in lakes -- all of which are key to maintaining optimal habitat and suitable nutrient levels. In addition, warmer lake waters combining with excess nutrients from agricultural fertilizers (washed into lakes by heavy rains) would be likely to create algal blooms on the lake surfaces, further depleting some lake ecosystems of life-sustaining oxygen.

Potential Adaptation Options to Ensure Adequate Water Resources

In contrast to the vulnerability of natural ecosystems, humans have exhibited a significant ability to adapt to the availability of different amounts of water. There are many types of water basins across the country, and many approaches are already in use to ensure careful management of water resources. For example, more than 80,000 dams and reservoirs and millions of miles of canals, pipes, and tunnels have been developed to store and transport water. Some types of approaches that studies have indicated might prove useful are highlighted in this section (see also box, Potential Adaptation Options for Water Management).

Strategies for adapting to climate change and other stresses include changing the operation of dams and reservoirs, re-evaluating basic engineering assumptions used in facility construction, and building new infrastructure (although for a variety of reasons, large dams are no longer generally viewed as a cost-effective or environmentally acceptable solution to water supply problems). Other potentially available options include conserving water; changing water pricing; using reclaimed wastewater; using water transfers; and developing markets for water, which can lead to increased prices that discourage wasteful practices.

Existing or new infrastructure can also be used to dampen the impacts of climate-induced influences on flow regimes and aquatic ecosystems of many of our nation's rivers. While significant adaptation is possible, its cost could be reduced if the probable effects of climate change are factored in before making major long-term investments in repairing, maintaining, expanding, and operating existing water supply and management infrastructure.

Because of the uncertainties associated with the magnitude and direction of changes in precipitation and runoff due to climate change, more flexible institutional arrangements may be needed to ensure optimal availability of water as supplies and demand change. Although social, equity, and environmental considerations must be addressed, market solutions offer the potential for resolving supply problems in some parts of the country. However, because water rights systems vary from state to state and even locally, water managers will need to take the lead in selecting the most appropriate adaptive responses.

Because the United States shares water resources with Canada and Mexico, it participates in a number of institutions designed to address common water issues. These institutions, which include the U.S. -- Canada Great Lakes Commission and joint commissions and agreements covering the Colorado and Rio Grande rivers, could provide the framework for designing adaptive measures for responding to the effects of climate change. For example, the U.S. -- Canada Great Lakes Commission has already conducted studies to evaluate options for dealing with the potential for increased evaporation, shorter duration of lake ice, and other climate changes that are projected to affect the Great Lakes -- St. Lawrence River basin. Close coordination will be needed to efficiently manage the levels of these crucial water resources to ensure adequate water supplies for communities and irrigation, high water quality, needed hydroelectric power, high enough levels for recreation and shipping, low enough levels to protect communities and shorelines from flooding and wave-induced erosion, and more.