William H. McAnally, Phillip H. Burgi, Richard H. French, Jeffery P. Holland, James R. Houston, Igor Linkov, William D. Martin, Bernard Hsieh, Barbara Miller, Jim Thomas, James R. Tuttle, Darryl Calkins, Jose E. Sanchez, Stacy E. Howington, William R. Curtis, and Matteo Convertino

Current estimates of climatic change have significant implications for the hydrologic cycle and water resource system. This chapter provides an overview of the problems and issues likely to result from potential changes in the magnitude and distribution of water as a result of global climate change. Potential response strategies to cope with current hydrologic variability, as well as projected changes, are also examined. Finally, research and development needs are outlined to improve the flexibility, resiliency, and robustness of water resource systems to deal with the projected, but uncertain, climatic changes and present variability that stresses existing systems.

Global climate change is projected to alter the temporal and spatial patterns and ranges of temperature, precipitation, and evaporation. Changes in other important climate variables, such as solar radiation, cloud cover, wind, and humidity, are also predicted. These changes and their interactions could significantly alter key components of the hydrologic cycle, including runoff, soil moisture, groundwater recharge, and snowmelt patterns. The magnitude, distribution, and frequency of extreme climatic events, such as droughts and floods, may also be modified.

Currently, water resources projects are planned, designed, and operated based on historical patterns of water availability, quality, and demand, assuming the climate is statistically stationary. Changes outside the expected normal range of variation in these parameters can stress the ability of hydraulic structures to perform safely as designed and the ability of water resource systems to meet designated, and often competing, uses. Some examples of the potential effects of climatic change on water resources are described in Sections 5.2.1 through 5.2.2.

Any discussion of water resources must recognize the substantial differences between the consumptive use of water, in which little or no flow returns directly to the environment, and the nonconsumptive uses, in which most of the water is returned to the local environment. Agriculture is primarily consumptive, since crops transpire water to the atmosphere. The water is still in the system, but is not locally available for reuse. Municipal use is primarily nonconsumptive, as most drinking, bathing, and cleaning water is returned and available as treated effluent.

Water resources are typically defined by use sectors—water supply for municipal, industrial, and agricultural uses; transportation by waterborne commerce; power generation; flood and storm damage reduction; recreation; water quality; and habitat preservation. Figure 5.1 illustrates these sometimes competing uses. Two other cross-cutting sectors influence our use of water—the infrastructure that contains, conveys, and otherwise makes water useful, and the institutions that regulate, preserve, and provide water quantity and quality. Each is examined briefly in this section.

Climate change will affect water resources through its impact on the quantity, variability, timing, form, and intensity of precipitation. There is already evidence that this is happening in some regions through the increased frequency of floods, droughts, and changes in long-term precipitation trend Page 182  |  Top of Article( U. N. 2011 ). The ability to anticipate and efficiently prepare for future water resource management challenges is currently limited by imprecise regional climate change models and long-term weather forecasts. Uncertainty about future climate conditions and our will to adapt also make it difficult to prepare for and adapt to changes in water availability and quality.

 
FIGURE 5.1The many competing uses of water. (Photos courtesy of U.S. Bureau of Reclamation, Geological Survey, Corps of Engineers, Department of Agriculture, National Park Service, and Transportation Security Administration.) (Photos courtesy of U.S. Bureau of Reclamation, Geological Survey, Corps of Engineers, Department of Agriculture, National Park Service, and Transportation Security Administration.)

Although increasing human demand for freshwater is the largest challenge facing water resource managers, substantially altered hydrologic cycles as a result of future climate change can make their task even more difficult and uncertain. Under most climate change scenarios, both the supply and demand of water will change, creating potential imbalances between the two or exacerbating existing imbalances. Reservoirs that store surface runoff from snowmelt or precipitation input may be either too small to accommodate the increase in demand or oversized for the change in requirements. The recharge and withdrawals from groundwater basins may be altered. Transmission and distribution systems (canals and conduits) that convey the water to the end user may be similarly impacted. Spillways that protect reservoirs may also need to be resized to accommodate the additional inflow Page 183  |  Top of Articleor changes in the timing of the inflow hydrograph. Water users will need to adapt to more frequent and severe droughts, such as by shifting limited water supplies towards higher-value uses. Such shifts could be from low- to high-value crops, or from agricultural and industrial to environmental and municipal uses. Gradual changes in the frequency and severity of drought will be difficult to distinguish from normal variations in precipitation. Therefore, climate change adaption will likely be delayed.

Global food production depends on water not only in the form of precipitation, but also, and critically so, in the form of available water resources for irrigation. Indeed, irrigated land, representing a mere 18% of global agricultural land, produces 1 billion tons of grain annually, or about half the world's total supply; this is because irrigated crops yield, on average, two to three times more than their rain-fed counterparts ( IPCC 2011 ).

The projected increase in world population, combined with a change in the climate, will challenge existing food production regions to increase their output. This may result in an increased demand for irrigation in some regions. If the climate shifts or changes, existing irrigation systems may not be adequate to accommodate the increased demand. Resource allocations to these systems will have to be adjusted, which will be difficult without accurate hydrologic predictions. In addition, agricultural areas that have traditionally not required irrigation may, under changed climatic conditions, require irrigation. Water banking—using groundwater storage or water transfer banks—will become more popular and necessary in mitigating economic impacts by increasing the reliability of water supply or facilitating the short term reallocation of water.

Existing conjunctive use of ground and surface water may be altered. Groundwater withdrawal in many areas has been shown to cause dramatic ground subsidence. High water withdrawal rates near the coastal regions often induce salinity intrusion, and any sea-level rise, combined with increased groundwater pumping, will magnify these problems. Groundwater recharge areas will also be affected by changes in the amount of precipitation, its form, and distribution.

Land transportation is vulnerable to water impacts at crossings and near the margins of seas and lakes. Flooding due to sea-level rise and increases in the intensity of extreme weather events will pose threats to land transportation networks in some areas. These include localized street flooding, flooding of subway systems, and flood and landslide-related damages to bridges, roads, and railways ( IPCC 2011 ).

Waterborne transportation relies on an inland and coastal network of waterways and structures that facilitate the passage of people and goods. Navigation channels and ports have been designed with channel depths and widths based on historical water levels, flows, and sedimentation rates. Page 184  |  Top of ArticleClimate-induced changes could potentially alter these physical conditions, and thus affect channel and port navigational capacity. The result would be an alteration of operation and maintenance costs and a reduction of the confident use of the port, in addition to making the identification and justification of new facilities difficult.

See Chapter 9 for a further discussion of transportation impacts.

Flood control structures (channels, dams and spillways, levees, retention and detention basins, and floodwalls) are designed to provide protection against events with an occurrence frequency based on statistics derived from historical data. For example, many levee systems are designed to contain a flood that happens, on average, every 100 years. However, potential climate changes may alter both the frequency distribution of storms that produce flood events and the magnitude of probable maximum floods; and thus, existing projects may have future protection requirements that are quite different from the initial design requirements.

Ongoing research suggests that flood event magnitude and frequency in a given location may be so altered by climate change as to render the period of record non-homogeneous and, therefore, of little use in predicting the future magnitude and frequency of extreme and non extreme events. It is predicted that climate change will increase the frequency and magnitude of droughts, floods, and destructive storms in specific regions. Floods are likely to become more problematic in many temperate and humid regions, necessitating advanced planning, flood forecasting, and even greater attention to well-developed emergency response networks to avoid significant loss of life and property.

Risk reduction not only saves lives; it is also less expensive than responding to a disaster. A number of countries have reduced the impact of disasters by investing in measures such as flood control, hurricane-proof building design, and protection of coastal ecosystems, including mangroves and coral reefs.

Society has become more vulnerable to natural hazards. Although floods are natural phenomena, human activities and human interventions into the processes of nature, such as alterations in the drainage patterns from urbanization, agricultural practices, and deforestation, have considerably changed the situation in whole river basins. At the same time, exposition to risk and vulnerability in flood-prone areas have been growing ( U. N. 2000 ).

Hydropower generation is likely to be directly affected by climate change because it is sensitive to the amount, timing, and geographical pattern of precipitation and temperature. Further, hydropower needs may increasingly Page 185  |  Top of Articleconflict with other priorities, such as salmon restoration goals in the Pacific Northwest. However, changes in precipitation are difficult to project at the regional scale, which means that climate change may affect hydropower either positively or negatively, depending on the region. Hydropower is a nonconsumptive use in that the water used immediately returns to the stream. Fossil fuel and nuclear power are consumptive uses in which much of the cooling water is evaporated into the atmosphere.

Infrastructure for energy production, transmission, and distribution could be affected by climate change. For example, if a warmer climate is characterized by more extreme weather events, such as windstorms, ice storms, floods, tornadoes and hail, the transmission systems of electric utilities may experience a higher rate of failure, with attendant costs ( IPCC 2007 ).

Power plant operations can be affected by extreme heat waves. For example, intake water that is normally used to cool power plants can become warm enough during extreme heat events that it compromises power plant operations.

Renewable energy sources will likely be affected by climate change, although these changes are very difficult to predict. If climate change leads to increased cloudiness, solar energy production could be reduced. Wind energy production would be reduced if wind speeds increase above or fall below the acceptable operating range of the technology. Changes in growing conditions could affect biomass production—a transportation and power plant fuel source that is starting to receive more attention ( CCSP 2008 ).

Freshwater bodies have a limited capacity to process the pollution stemming from expanding urban, industrial, and agricultural uses. Climate-induced changes in the magnitude, timing, and quality of runoff will impact waste assimilative capacity, nutrient levels, water temperature, salinity, turbidity, and dissolved oxygen levels, which in turn affect riverine, reservoir, estuarine, wetland, and lake environments. The balance in the environmental conditions for some sensitive biota is very delicate. Waterfowl, raptors, and other higher trophic species are affected by water quality conditions.

One of the most significant sources of water quality degradation results from an increase in water temperature. The increase in water temperatures can lead to a bloom in microbial populations, which can have a negative impact on human health. Additionally, the rise in water temperature can adversely affect the different inhabitants of the ecosystem due to a species' sensitivity to temperature. The health of a body of water—such as a river—is dependent upon its ability to effectively self-purify through biodegradation, which is hindered when there is reduced dissolved oxygen. This occurs when water warms and its ability to hold oxygen decreases.

Habitat, a species-dependent variable, is defined as the portion of landscape that is potentially suitable for the species considered ( Storch et al. 2007 ; Convertino et al. 2009 ). A large number of studies have been published on the influence of climatological variables on ecosystems and singles species, but in this chapter, we focus on the effect of hydrological variables on biodiversity patterns for three water-controlled ecosystems.

Climate change will have a substantial effect on the habitats of species for a variety of taxa ( UNESCO 2010 ; Westervelt and Hargrove 2010 ; Lozar et al. 2010a, b ). Every species on earth is affected by the water cycle, either directly or indirectly, in that the entire food chain is directly affected by changes in water resources. For example, heavy rain associated with strong tropical cyclones will affect bird metapopulations ( Convertino et al. 2010, 2011 ) and alter vegetation and pool-dependent invertebrate communities. Moreover, the portion of the rain that infiltrates underground will potentially affect bacteria communities. The connections and feedback within the food chain are enormous.

While changes in temperature are causing the expansion and contraction of many species' habitats ( UNESCO 2010 ), together with phonological changes like breeding and flowering time, long-term changes in water resources are much more dangerous for species and biodiversity ( Rodríguez-Iturbe et al. 2009 ). Changes in water resources have proven to modify species' composition and abundance at the landscape scale ( Convertino et al. 2009 ). On the contrary, temperature variations do not produce perennial changes for species, and these changes determine spatiotemporal shifts that do not influence the whole ecosystem.

Water-controlled ecosystems ( Figure 5.2 ) are complex, evolving structures whose characteristics and dynamic properties depend on many interrelated links between climate, soil, and vegetation. Different water variables are important for different ecosystems and species habitats ( Rodríguez-Iturbe et al. 2009 ). To illustrate the relationship between water resources and habitat characteristics, we provide the examples of river networks, forests, and arid ecosystems, in which river runoff, rainfall, and soil moisture, are respectively the key water variables shaping habitats and biodiversity patterns.

In river networks ( Figure 5.2a ), the local habitat capacity (i.e., the number of individuals at each link) is directly proportional to the river runoff ( Muneepeerakul et al., 2008 ). Though the runoff is certainly not solely responsible for all the changes in the species habitat, the topology of the species dispersal and the network connectivity are also key variables; variations in the river runoff consistently change species' habitat extent and abundance. For example, this has been verified for the entire Mississippi-Missouri River System (MMRS), where biodiversity patterns have been faithfully reproduced. For a more uniform river runoff, the local species richness (of freshwater fish and riparian vegetation) is, on average, very homogeneous, while for a very heterogeneous runoff, the local species richness is smaller and the species' habitats are highly fragmented. Fragmentation causes an increase in the risk of extirpation ( Akcakaya et al. 2004 ), increasing the probability that an epidemic, or even an extreme discharge will completely destroy the habitat of a species.

 
FIGURE 5.2Water-controlled ecosystems: (a) River basin ecosystem in which the controlling driver for species (e.g., freshwater fishes and riparian vegetation) is the river runoff (Muneepeerakul et al. 2008) (Ichetucknee River). (b) Forest ecosystem in which the controlling variable is the average annual rainfall (Konar et al. 2010) (Pennsylvania forest new Delaware Water Gap). (c) Arid ecosystem in which the main controlling variable is the soil moisture (Rodríguez-Iturbe et al. 2001, 2009) (PaloDuroCanyonStatePark). (Photos courtesy of M. Convertino). (Photos courtesy of M. Convertino).

For forest ecosystems ( Figure 5.2b ), the average annual precipitation is the hydrological variable controlling biodiversity patterns ( Konar et al. 2010 ; Convertino 2011 ). In forests, species dispersal is an intrinsic biological property that is unlikely to change as a function of climate change because evolutionary changes like these usually occur at much slower speeds than that of climate change. The change of habitat for species in a forest ecosystem has been simulated in the Mississippi-Missouri River System. Spatially explicit estimates of the impact of climate change under “species-poor scenarios” on region-averaged local species richness were calculated and resulted in a decreasing trend in the percentage of species lost from west to east of the MMRS. However, regions west of the 100° W meridian are composed of species-poor sub-basins, while those to the east encompass species-rich sub-basins, resulting in the increasing trend from west to east in the absolute loss in mean local species richness. The largest decrease in region-averaged local species richness occurs in the south, with 6.3 species, on average, lost per sub-basin across the region.

For arid ecosystems ( Figure 5.2c ), soil moisture is the key variable that synthesizes the action of climate, soil, and vegetation on the water balance and the dynamic impact of the water balance on plants ( Rodríguez-Iturbe et al. 2001, 2009 ). Moreover, many ecosystems of tropical and subtropical latitudes suffer water stress, which is, in turn, controlled by the temporal fluctuations of soil moisture. Although other sources of stress (fire, grazing, nutrient availability, etc.) are certainly present, in many of the world's arid ecosystems, soil moisture is the most important resource affecting vegetation structure and organization. Changes in soil moisture patterns have proven to dramatically change patterns of plant biodiversity (e.g., from uniform to clustered patterns), thus having a profound impact on the spatial organization of single species habitats and their abundance. Like river and forest ecosystems, the fragmentation of arid ecosystems due to more frequent droughts increases the risk of species extirpation.

These three examples underline the importance of water resources on single species' habitats and ecosystem composition. Thus, in a changing climate, it is important to predict the potential effects of changes in water-controlling variables on species. While it is impossible to substantially modify the climate, it is possible to adopt sustainable plans that mitigate the effects of changing water resources on the earth's ecosystems. We reviewed the cases of river, forest, and arid ecosystems; however, coastal (affected by sea-level rise), sub-surface (affected by base flow), and many other types of ecosystems are also affected by changes in water-controlling variables that need to be managed.

Water is critical for many recreation activities—from boating and fishing to mountain biking and backpacking. Impacts on water resources from climate change are pervasive and will vary by region and by season. They will likely alter winter snowpack, impact fishing and boating due to higher or lower stream flows and reservoir levels (depending on region), and change hunting and wildlife viewing opportunities.

In recent years, public demands have required those agencies with water control authority to allocate water resources for boating, fishing, rafting, swimming, and other recreational uses in addition to the original water use requirement of the designed facility. With climate changes, additional pressures for continued or increased recreational uses will create more demand for sustainable water supplies.

An increase or decrease in the demand for water from surface supplies will impact both the water supply facilities and wastewater treatment facilities. Changes in water quantity and quality could require modified or new strategies for treatment.

Cities and towns are very vulnerable to climate change. Hundreds of millions of people in urban areas across the world will be affected by rising sea levels, increased precipitation (in some areas), inland floods, more frequent and stronger cyclones and storms, periods of more extreme heat and cold, and the spread of diseases. Climate change will likely have a negative impact on the sustainability of infrastructure and worsen access to basic urban services and the quality of life in cities.

The last century has seen the rapid urbanization of the world's population, as the global proportion of urban population rose from 13% (220 million) in 1900, to 29% (732 million) in 1950, to 49% (3.2 billion) in 2005. By 2050, over 6 billion people—two-thirds of humanity—will be living in towns and cities.

The changes in the average precipitation and seasonal distribution of rainfall and runoff due to climate change make the job of urban planners very difficult. In some places, the dry months get even drier and the wet months get even wetter. The engineering profession has serious challenges as it faces an uncertain future in how to adapt the infrastructure system to the long-term effects of climate change and the short-term shocks of extreme weather.

Today, the cities with the highest value of property and infrastructure assets exposed to coastal flooding caused by storm surge and damage from high winds are primarily in developed countries. However, the rapid economic development expected in the developing nations means that, in the future, the highest exposure becomes more concentrated in Asian cities, with eight of the top ten situated in this region. Over the coming decades, the unprecedented growth and development of the Asian megacities will Page 190  |  Top of Articlebe a key factor in driving the increase in coastal flood risk globally. Climate change could triple the population at risk from coastal flooding by 2070 ( Nicholls and Hanson 2007 ).

The increasingly extreme weather—in particular, heavy storms and flooding—is severely affecting the living standards of millions globally. In 2011 alone, megafloods inundated one-fifth of the total land area in Pakistan and vast stretches in Queensland and Victoria in Australia, and the overflowing Mississippi River in the United States. It is estimated that since 1970, storms and floods were responsible for more than 90% of the economic costs of extreme weather-related events worldwide.

It is clear that if such “unusual” climatic events are visited upon us ever more regularly, then there will be practical limits to the adaptation, or at least exponentially rising costs involved in coping. There are a few general statements that have become true in today's climate uncertainty ( Khor 2010 ):

The competitive use of water is increasing, and with a change in climatic conditions, institutions and the legal and social framework for allocating water and resolving conflict may be forced to change. The pressure to meet competing interests and supply water for multiple uses can increase as a result of any climate change, producing contention and water wars, such as the challenges faced by the California Department of Water Resources in diverting water from the Sacramento-San Joaquin Delta to southern California, the Federal Bureau of Reclamation in apportioning Colorado River among the several western states that depend on it, and Corps of Engineers' management of Lake Lanier, which provides water supply to Atlanta and Page 191  |  Top of Articleenvironmental flows to Georgia, Alabama, and Florida. Thousands of local water management districts and water utilities must deal with such challenges within a changing environment.

In a few places, basin-wide institutions are in place to manage water across U.S. political boundaries—the Tennessee Valley Authority system crosses seven states and the Delaware Basin Commission four states. State compacts cover a number of interstate waters, such as the Great Lakes Compact, without full management authority, but a great improvement over management by litigation.

Multiple organizations are trying to move beyond the single project, single sector perspectives that have too long dominated water resources management. Projects have consequences at great distances, as demonstrated by disputes over Lake Lanier water among the states of Georgia, Alabama, and Florida, and federal agencies ranging from the Corps of Engineers to Fish and Wildlife Service. Disputes between advocates for economic development and the endangered pallid sturgeon of the southeast provide grist for multiple court disputes and public debates as advocates for single sectors make their respective cases.

As encouraging as these efforts to use a broader, more inclusive perspective are, they also raise the question of where to draw the boundaries of our broader examination. Aside from questions of scalability, we must balance our need to see the bigger picture with our ability to properly grasp what we are seeing. If we begin by paraphrasing Jacob Marley's cry * to say that the “whole earth is our business,” we have properly recognized that the earth is a interconnected ecosystem, but have also overstepped our abilities to manage or even fully understand how it works.

The proper perspective for water-related management is the hydrologic footprint, or aquascape—the watershed plus water spread in the ocean, the landscape over which water flows to the ocean, and the coastal and ocean zone over which that water spreads and carries the material acquired during that journey. An aquascape perspective supports and reinforces integrated watershed management in its many forms, plus marine spatial planning, and the ecosystem approach to management, as discussed below. We use the phrase “holistic aquascape management” to denote the practice and process of achieving sustainable water resources use for the benefit of humans and the natural environment throughout the hydrologic footprint.

The word “holistic” has often been misused, but is so uniquely descriptive of the need that we are compelled to use it here. It is derived from the Greek holos, meaning “altogether” or “entire,” which was defined Page 192  |  Top of Articleby Aristotle (350 BCE) as, “the whole is greater than the sum of the parts.” Jan Smuts * (1926) is credited with coining the English term “holism,” which he described as “the tendency in nature to form wholes that are greater than the sum of the parts through creative evolution.” The definition has been refined and applied in diverse fields, most vividly by Douglas Adams (1987) as the “fundamental interconnectedness of all things.” Adams' definition helps to remind us, first, that economic development and a healthy ecosystem are fundamentally connected as interacting contributions to the quality of life, and second, that what happens in one part of an aquascape affects other, often unseen aspects and areas of the aquascape.

Holistic management of water resources is related to concepts and terms such as “integrated water resources management,” “total water management,” “watershed management,” and “regional management.” Total water management is defined by the American Water Works Association Research Foundation ( AWRA 1996 ) as

The AWRA definition includes the concept of sustainability, which the American Society of Civil Engineers defines for water resources as, “Sustainable water resource systems are those designed and managed to meet the needs of people living in the future as well as those of us living today.”

A frequent criticism of the sustainability concept is that it is idealistic and impossible—any use of resources is bound to decrease the amount available to future generations. However, that criticism is no more valid than saying that we need not strive for safety, since perfect safety is never achieved. Absolute environmental sustainability can be an ideal goal that is balanced with economic development and the cultural fabric of a region, which are implicitly included in the above sustainability definition.

The Corps of Engineers ( USACE 2000 ) defines watershed perspective planning as

The Tennessee Valley Authority (TVA) is often cited as the model for managing a watershed for multiple purposes. Chartered by the federal government in 1933, its intended purpose was “… in the interest of the national defense and for agricultural and industrial development, and to improve navigation in the Tennessee River and to control the destructive flood waters in the Tennessee River and Mississippi River Basins, …” ( U.S. 1933 ). TVA became an engine for not just economic growth, but also education, cultural preservation, and environmental stewardship, all centered around water management.

Another term that leads to many of the same conclusions as the watershed perspective is “systems,” sometimes expressed as systems thinking, systems engineering, and so on, and appears in the Corps of Engineers' definition above.

As the atmosphere warms, its ability to hold water vapor will increase at an approximate rate of 5%–6% per °C ( Rosenberg et al. 1989 ). Higher air temperatures will also increase surface evaporation. To maintain equilibrium in the moisture budget, the precipitation rate must also increase ( MacCracken and Luther 1985 ).

The global average increases in precipitation and evaporation are presently estimated at 7%–15% for a doubling in CO₂ concentrations. These increases, however, will vary both spatially and temporally. Individual regions may actually experience reduced precipitation. The changes in the seasonal distribution of precipitation will also vary from region to region ( Rind and Lebedeff 1984 ; Waggoner 1990 ). Moreover, recent studies indicate that when complex plant-climate interactions are considered, changes in evapotranspiration can vary from −20 to +40% in specific river basins ( Martin et al. 1989 ).

The predicted changes in precipitation and evaporation are likely to be coupled with changes in other climatic variables, such as solar radiation, cloud cover, wind, and humidity. The range of these changes on a global basis are shown in Figure 5.3 . Projected changes in these meteorological variables could significantly alter key components of the hydrologic cycle, including runoff, soil moisture, groundwater recharge and discharge, and snowmelt patterns. Due to the nonlinear relationship between precipitation and runoff, small changes in rainfall and evaporation can produce significant changes in runoff and regional water availability ( Gleick 1986, 1989 ). Nemec and Schaake (1982) used hydrologic watershed models to show that a 10°C temperature increase, combined with a 10% decrease in precipitation, can cause a 25% reduction in average annual runoff in a humid basin and a 50% reduction in an arid basin. Using statistical correlations in the Colorado River Basin, Revelle and Waggoner (1983) estimated that a 20°C rise in temperature, even when coupled with a 10% increase in average annual precipitation, could still produce an 18% reduction in annual runoff due to increased evapotranspiration. Although more complex studies and further refinements in hydrologic analysis procedures may alter the magnitude of these results, the sensitivity of runoff to changes in meteorology remains an important consideration.

 
FIGURE 5.3Changes in temperature and precipitation for 2080–2099 from two GCM. (Adapted from Solomon, S., et al., Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, 2007.) (Adapted from Solomon, S., et al., Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, 2007.)

Groundwater, in general, exists as either a perched water table or in an aquifer connected by some geologic means to the surface. Perched water typically has infiltrated deep into the ground and encountered an impermeable rock formation, properly formed to act as a large receptacle. There, the water collects and can remain for hundreds of years. The recharge of such formations is so slow as to be almost negligible. Therefore, once they are depleted, they are gone. Connected aquifers are recharged at a faster rate. Those connected directly to large rivers may recharge very rapidly. Those connected to the surface in remote areas require the recharge water to travel sometimes long distances before being available for use in the aquifer. The reliable yield rate for such aquifers can be calculated and water withdrawals matched to recharge rates. Climate change that alters the rainfall patterns over recharge areas will affect the safe rate of water withdrawal. Unfortunately, populations and land use have been established based on previously available ground water. The shifts to surface water use could start a domino effect of consequences. Alternately, surface water may not be a viable alternative. Page 196  |  Top of ArticleThis could result in radical land use change or the depopulation of the region. The most consumptive use of water is irrigation. Changes in the availability of surface and/or groundwater could have a profound effect on agricultural practices.

In addition to changes in the average annual values of hydrometeorological variables, shifts in regional distribution, seasonality, extremes, variability, and recurrence frequency are also likely. Most global climate models (GCMs) predict distinct changes in the regional distribution and seasonality of key hydrologic variables ( Rind and Lebedeff 1984 ; MacCracken and Luther 1985 ; IPCC 2007 ). Although GCMs currently have a limited capability for predicting detailed regional hydrologic effects, some general patterns are apparent. Surface air is expected to warm faster over land than over oceans, with a minimum of warming predicted to occur around Antarctica and in the northern North Atlantic region. In other high northern latitudes, however, warming in the winter is projected to be 50% to 100% greater than the global mean. Average winter precipitation is also predicted to increase in the middle and high latitude continents, including areas over central North American and southern Europe. The predicted changes in summer precipitation are variable, with recent models showing decreases in central North America and southern Europe and increases over areas such as Australia and southeastern Asia ( IPCC 2011 ). Other investigators suggest that elevated temperatures and evaporation may be accompanied by decreased summer precipitation in the lower latitudes ( Shiklamanov 1987 ; World Meteorological Organization 1988 ).

In the northern and western United States, where runoff is largely derived from snowmelt, distinct shifts in the relative amount of rain and snow, as well as earlier snowmelt resulting from warmer winters, are likely. The resulting changes in runoff patterns will alter the magnitude, timing, and probability of flooding patterns. The availability of water during peak demand periods, such as the irrigation season, would also be impacted. In the southeastern United States, where runoff is largely precipitation driven, some GCMs also project shifts in current seasonal patterns ( Frederick and Gleick 1988 ; Smith and Tirpak 1988 ; Hains and Henry 1989 ), with attendant changes in vegetative land cover.

Changes in variability, or inter- and intra-annual deviations from mean conditions, can impact the adequacy and reliability of water resource projects. IPCC (2011) suggests that precipitation and seasonal runoff variability will increase in the future. For some parameters, the predictions are more uncertain and variability may stay relatively constant about a rising mean. For example, in the case of temperature, this latter case implies that changes in mean conditions would shift the entire temperature distribution upward, resulting in more days above some critical temperature on the high end and Page 197  |  Top of Articlefewer days with temperatures on the low end of the distribution; but the deviation from the mean would remain constant. Other investigators have also found that, despite considerable noise in GCM results, the standard deviations of surface temperatures appear to be more likely to decrease than increase under global warming scenarios ( Rind et al. 1989 ). These impacts on variability, however, are surmised from equilibrium-double CO₂ conditions after new mean conditions have been established. During the transition from current to new equilibrium conditions, variability (more higher highs and lower lows) may well increase.

Based on statistical reasoning, several investigators argue that small shifts in mean values can imply large changes in the frequencies of extreme events such as droughts and floods ( Mearns et al. 1984 ; Waggoner 1990 ). The analysis of tropical cyclones and their relationship to warm low latitude oceans suggests that severe flood frequency could increase in a greenhouse-enriched environment (Michaels 1989) . Some modeling evidence suggests that hurricane intensities would also increase with climatic warming ( Emanual 1987 ; IPCC 2007 ), but the climate models do not give a consistent indication of whether tropical storms will increase or decrease in either frequency or intensity as the climate changes. At present, tropical storms, such as typhoons and hurricanes, develop over seas that are warmer than approximately 26°C. Although the areal extent of seas exceeding this critical temperature will increase in a warming scenario, the critical temperature itself might change under warmer conditions ( IPCC 1990 ).

In the long term, alterations in the hydrologic cycle could be complicated by climate change-induced effects on plant growth and land use. Elevated CO₂ concentrations have been shown to increase photosynthesis and plant growth potential in laboratory studies. In these CO₂-enriched environments, plant transpiration is also generally reduced due to increased stomatal resistance ( Waggoner 1990 ). Under natural conditions, the net effect of these plant responses to increased CO₂ levels will vary, depending on plant type, relative changes in a range of climatic variables, and individual ecosystems. Complex plant-climate interactions may moderate or augment the predicted increases in evapotranspiration, with consequences for plant growth and development ( Rosenberg et al. 1989 ; Martin et al. 1989 ). These effects, coupled with changes in the hydrologic cycle, could ultimately alter vegetative patterns. Changes in the extent and type of vegetative cover could modify the water content of soil layers, thereby further influencing runoff and water availability ( Abramopoulos et al. 1988 ).

Land use could be further impacted by changes in agriculture, forestry, population distributions, and industrial activity, with associated feedback implications for the hydrologic cycle. Higher temperatures, modified rainfall patterns, and increased CO₂ concentrations would affect agricultural Page 198  |  Top of Articlepractices, the type and distribution of crops, and irrigation needs. Similarly, the distribution and composition of forests, wildlife, and other natural ecosystems could be altered. Smith and Tirpak (1988) project potential demographic shifts, as well as possible changes in the location of industrial and agricultural centers. Such conversions in land use would not only affect the relative balance between water supply and demand, but would also impact the soil and land surface characteristics, thereby influencing important components of the hydrologic cycle, such as infiltration, interception, evapotranspiration, and groundwater recharge.

It is important to recognize that many of these impacts represent the potential consequences of an equilibrium-double CO₂ scenario. Although abrupt changes and large transients are possible ( Schneider 1989 ), actual changes could occur more slowly over time or be moderated by other climatic factors. Changes in the hydrologic cycle may also work to the benefit or detriment of individual regions. For example, the IPCC (2011) predicts with high confidence that semi-arid areas of the western United States will experience a decrease in water availability.

Water resource projects are generally planned, designed, operated, and maintained to accommodate historical ranges and patterns of climatic variability, based on the assumption of a statistically stationary climate. Current, as well as reasonable projected (based on past experience), patterns of relative water supply and demand are also incorporated in system designs. The projected changes in the magnitude, timing, and distribution of hydrometeorologic parameters—particularly if coupled with demographic shifts and changes in industrial and agricultural activity—could impact the safety of hydraulic structures, as well as the ability of water resource systems to effectively balance available supplies against competing water uses. This section provides representative examples of the potential beneficial and adverse impacts of climatic changes on water resources.

Climate change is likely to impact both water supply and water demand. The predicted changes in the hydrologic cycle—changes in precipitation patterns, evapotranspiration rates, temporal and spatial distributions in magnitude of runoff, and frequency and intensity of severe storms—will affect the quantity and quality of water supplies ( Frederick and Gleick 1988 ). Long-term climatic impacts on demographics, industrial development, agricultural production, irrigation needs, natural systems, and energy use would ultimately impact water demand patterns ( Smith and Tirpak 1988 ).

To address basin vulnerability to climate change with regard to water supply, one must consider the relative demand, or the ratio of demand (consumptive depletions, including consumptive use, water transfers, evaporation, and groundwater overdraft) to annual mean renewable supply. Water is considered a critical factor in economic development when the relative demand Page 199  |  Top of Articleexceeds a value of 0.20 ( Szesztay 1970 ). High relative demand ratios indicate that existing supply is susceptible to stress from growing populations, increased industrial and commercial demand, and climatic fluctuations. The Alaskan basin, for example, has essentially no storage, but as relative demand is also extremely low, the basin is not susceptible to drought for domestic or industrial consumption. Conversely, the Lower Colorado River Basin has a high relative storage capacity, but as consumptive use is 96% of renewable supplies, the users of runoff are vulnerable to drought. Basins such as the Upper and Lower Colorado, Rio Grande, Great Basin, and Missouri are vulnerable to climate change-induced reductions in supply ( Frederick and Gleick 1988 ).

While relative storage and demand give some indication of the vulnerability of large river basins to climatic change, the range of potential impacts is dependent on other local factors, including legal regulations, compact, and treaties. The relative importance of in-stream water uses, such as hydropower production, recreation, navigation, wildlife, and aquatic habitat, can influence the flexibility a water resource system has to meet competitive water use demands and to reallocate water uses during times of scarcity. The climatic impacts on snowmelt and the timing of runoff would also influence the magnitude of impacts. For example, in the Sacramento-San Joaquin River Basin, Lettenmaier et al. (1989) found that the increased temperatures in four GCM scenarios produced major reductions in snow accumulation, resulting in increased winter runoff, but reduced spring and summer runoff. Although the total volume of water increased in these scenarios, Sheer and Randall (1989) found that the shift in seasonality resulted in an increased probability of spring flooding and substantially reduced water deliveries to consumers in the California Central Valley in the summer. Consequently, under current operating constraints and water allocation policies, water supplies during peak summer demand periods could become a problem.

Changes in temperature, runoff, snowmelt, evapotranspiration, and other hydrological and meteorological factors could have major effects on irrigated agriculture. Peterson and Keller (1990) examined the potential effects of global climate change on irrigation in both the Western and Eastern parts of the United States. They computed a potential Net Irrigation Requirement (NIR) for four scenarios: (1) present conditions; (2) climate change of +30°C; (3) climate change of +30°C and +10% precipitation; and (4) climate change of +30°C and -10% precipitation. The NIR increased under the latter three Page 200  |  Top of Articlescenarios compared to present conditions. The implication is that the percentage of cultivated land requiring irrigation will increase. Western states will find it increasingly difficult to maintain the present level of irrigation without developing new water sources through conservation and improved delivery system efficiencies. However, irrigation may well continue to increase in the east. Similar scenarios are likely in other parts of the world.

Flooding—water flowing beyond its normal confines, especially onto usually dry land—can result from a variety of hydrometeorologic events. Floods result from a combination of heavy precipitation from severe storms, snowmelt, combinations of rain and snowmelt, storm surge and/or wave effects; the physical characteristics of drainage basins and coastal reaches; and modifications to drainage basin characteristics. Both high intensity, short duration and low intensity, long duration precipitation events can exceed the storage capacity of a watershed, resulting in downstream flooding. The melting of either snow or ice (e.g., a glacier) in a watershed can also result in elevated downstream discharges, and warm rain on snow has often resulted in epic downstream flooding. In coastal areas, storm surges can result in flooding as the abnormal rise of water generated by a storm is over and above the predicted astronomical tide. In semi- and arid environments, the flood hazard is generally more related to the quickness and ferocity of the event than the magnitude. Flooding depends not only on the nature of the causative hydrometeorologic event, but also on the physical characteristics of the watershed, and the anthropogenic modifications that have been made in the watershed.

Engineered flood mitigation is typically based on estimating the magnitude of a design event with a specified frequency; for example, the 100-year event. The quantification of design events, whether from peak flow data or rainfall-runoff modeling, is based on statistical analysis that assumes a stationary time series. Therefore, climate change, by definition, will invalidate the theoretical basis of the analysis on which the design of all engineered flood mitigation structures is based. However, flood mitigation projects are designed with safety factors; therefore, climate change trends toward higher peak flows will, to a point, only reduce the margin of safety, not eliminate it. In addition, climate change may modify the time base of the design flood event hydrograph; for example, shorter or longer durations at or near peak flow. New, more flexible design and operation procedures will be needed. The innovative use of design concepts and policies that include overtopping embankments, emergency off-channel storage, fuse plugs, labyrinth spillway designs, and aggressive water harvest in urban and suburban areas are examples of flexible and robust designs and policies for adapting to the challenges of climate change.

Downstream flood protection can be maintained by incorporating expected climate change scenarios into reservoir operating rules, such as Page 201  |  Top of Articlekeeping reservoir water levels lower or higher. Changing reservoir operating procedures can have detrimental effects on multipurpose projects such as those incorporating hydropower production, recreation, water supply, and navigation. Conversely, reduced runoff and flood risk could facilitate the reallocation of flood storage space for other uses, such as raising reservoir levels to increase hydropower generation and recreational opportunities. These higher levels could have negative impacts on recreation as docking facilities and boat ramps are inundated.

This, as well as other discussions of the effects of climate change on flood hazard identification and mitigation, does not address three potential issues that could be important, but cannot be quantified at the present time. First, it is tacitly assumed that the fluid primarily responsible for flood hazard will remain Newtonian—that is, a water-sediment mixture—rather than non-Newtonian fluids associated with mud and debris floods. Second, it is assumed that climate change will result in a change of the average depth of precipitation; however, it will also likely result in a change in precipitation variance and temporal distribution. Any of these changes could have an effect on runoff, and hence, flood hazard and mitigation design. For example, under current climate conditions, Leopold (1951) and Bull (1964) demonstrated that a small change in the annual temporal distribution of precipitation caused the landscape to become unstable, with an increase in runoff and erosion. Therefore, the implications of changes in variance and temporal distribution must be considered. Third, flooding, which under current climate conditions is more an annoyance than a hazard, may become a serious concern. For example, there are major urban areas (Salt Lake City) and critical infrastructure (Edwards Air Force Base) located near terminal lakes that are currently subject to infrequent flooding, but could be a risk in the future with a changed climate.

The frequency and duration of extremes, if increased due to climate change, will adversely impact the navigation industry. Extreme high flows creates problems with bridge clearances, produce wave damage to waterside structures, increase project maintenance (dredging, bank stabilization, structures, etc.), potentially terminate navigation for short periods of time, create shoaling problems in channels due to increased sediment inflow, and may require a modification of dam capacity for lock and dam structures to handle higher-than-anticipated flows effectively, or will lead to more periods of suspended navigation. High flows also increase fuel consumption of vessels as extra power is required for travel both with and against an increased current speed.

Extreme low flows will result in possible long-term channel closures, with enormous negative economic impacts. More hazardous navigation conditions resulting from low flows cause delays, and subsequent increased travel Page 202  |  Top of Articletime from port to port. Increased maintenance of navigation aids will be required to insure safe operation within navigable waters. Shoaling patterns and shoaling rates can change, requiring an increase in maintenance dredging. Low flows in the winter increases the likelihood of an earlier ice cover and, depending on the temperature regime, a thicker ice sheet could be expected in cold regions. However, costs to the shipping industry for the Great Lakes due to a climate warming scenario has been investigated by Crissman (1988) , who found that the duration of the lake and connecting river ice covers would decrease, permitting a longer shipping season without the hindrance of the ice. This results in an economic gain for this particular transportation mode in this region because of lower operating costs.

Increased rainfall amounts or changes in storm intensities will affect design parameters at highway, pipeline, and rail crossings of water gaps. The designs typically provide minimum clearance of the bridge low chord above a 25-, 50-, or 100-year flood profile. Changes in rainfall patterns and intensities may result in some structures being over-designed. This is of no particular safety concern, but replacement structures should reflect changed conditions. Conversely, other structures may be under-designed and experience flows that encroach on the bridge. This will lead to higher flowlines upstream, increased scour at the bridge piers and abutments, and possible failure/overtopping, threatening traffic and endangering lives. Additionally, utility crossings such as water, gas, oil and telecommunications lines may be similarly adversely affected. Failure of these transportation and communication systems due to scour undermining them or removing protective overburden and allowing secondary breaks will lead to the disruption of services and potential environmental pollution. River and estuarine ports may experience increased periods of time where facility loading/unloading operations are shut down due to higher or lower water levels than were originally anticipated when they were designed. There may be a need to design bridges (highway and rail) to withstand the higher runoff and sediment loads to which they will be exposed. Better data sets of present scour and channel capacities will be needed to estimate future conditions properly.

Shifts in the magnitude and seasonality of streamflows would directly impact hydropower generation and capacity, as well as system flexibility and reliability. In a study of the TVA reservoir system ( Miller and Brock 1988 ), a warm and wet climate change scenario (4°C increase in average annual temperature and 31% increase in average annual runoff, with monthly variations in runoff ranging from +73% in March to −28% in November) increased average annual hydropower generation by 16%.

Miller and Brock (1988) also evaluated the impacts of a dry climate change scenario (31% reduction in average annual stream flow) on reservoir Page 203  |  Top of Articlesystem behavior and power production. Average annual hydrogenation was decreased by 24%. Reduced streamflows and operating heads also resulted in substantial losses in dependable hydrosystem capacity, particularly during the summer months when power demands for residential and commercial cooling are high under current conditions.

Crissman (1988) assessed the impact of climate change on the power production capability of the New York Power Authority system on the Niagara River. Decreased flows from the Great Lakes drainage area, coupled with increased lake evaporation, could have dramatic negative effects on the scheduling of power based on historical records.

Warmer temperatures and hydrologic changes could also impact the operation of fossil and nuclear plants. Elevated water temperatures adversely impact thermal efficiency and the power output of steam turbines. Higher air and water temperatures also reduce cooling tower effectiveness. Elevated water temperatures, which can be exacerbated by high air temperatures and reduced streamflows, can increase the potential for violating environmentally-based thermal discharge standards in fossil and nuclear plants. Nuclear power plants also have limits on the maximum intake water temperatures for auxiliary safety systems. When this limit is reached, the Nuclear Regulatory Commission requires plants to shut down for safety reasons. Increased water temperatures, therefore, can constrain nuclear power plant operations ( Miller et al. 1992 ).

In coastal areas, the prospects of rising sea levels, increased storm surges, and salt water intrusion could also impact power production. Power plant siting, the location of cooling water intakes, transmission efficiency, and system reliability become important issues ( EPRI 1989 ). In inland areas, water intakes may have to be relocated during extremely dry conditions.

See Chapters 7 and 8 for further discussion of energy.

Reduced flows with lowered lake levels and releases will diminish recreational use of lakes and their tailwaters. In most cases, an increase in water supply will increase recreational use of waterways ( Miller and Block 1988 ). However, the timing of these increases in water supply may conflict with recreational needs.

The warming of the earth's atmosphere will result in significant modifications in the hydrologic cycle and the quality of surface and ground waters. The water quality of surface water bodies, such as rivers and reservoirs, could be significantly stressed by temperature increases, reduced dissolved oxygen levels, and potentially higher nutrient loads. Presently stressed systems will be affected the most.

Aquatic and riparian ecosystems will change and may be dramatically altered in response to climate change. First, as noted elsewhere in this chapter, climate change will likely result in a change in reservoir operating rules, which may result in either enlarged or decreased riparian areas around and downstream of the reservoir. A fundamental concern would be the stability of the “new” riparian areas. If climate change includes long periods of drought followed by long wet periods, the reservoir riparian areas may never become fully developed, from an ecologic viewpoint, or hydraulically connected to the stream in the downstream reaches. Second, significant changes in reservoir operating rules and hydrologic conditions could also result in discharge temperatures and temperature ranges that may not sustain downstream fisheries. Third, under some climate change scenarios, some perennial rivers will become ephemeral and vice versa. A closely related fourth issue concerns the fate of terminal lakes, which provide critical habitats in many locations under various climate change scenarios. It should be mentioned that riparian and aquatic habitat associated with many terminal lakes (e.g., Walker and Pyramid Lakes in Nevada; the Salton Sea in California; and the Dead Sea in Jordan and Israel) are in danger, even under the current climate, because of changing water demand in their tributary areas. In some cases, climate change will result in ecosystem destruction, and in others, enhancement. Fifth, aquatic ecosystems associated with coastal areas and estuaries also will be affected by climate change. It is hypothesized that estuaries associated with rivers having continental watersheds (e.g., the Mississippi, Colorado, Nile, and Mekong rivers) covering multiple climate zones will be less affected than estuaries associated with rivers emanating from regional watersheds within a localized climate zone (e.g., Sacramento River).

Areas of increased rainfall will have increased flow downstream of water control structures such as dams. This may cause increased flooding and limit agriculture activities in low lying areas. In most areas, an increased water supply will increase agricultural production and/or decrease its cost. Other areas may experience decreased flows, directly impacting activities depending on surface water for irrigation, aquaculture, municipal, or industrial use. This could lead to ground water overdraft. In the case of long term navigation shutdown, in which goods are moved by alternate land transportation modes, a higher unit transportation cost will result.

Some regions will experience different or exaggerated climatic impacts because they exhibit extremes of natural climate. These include regions of deserts, permafrost, glaciers, and tropical rain forests. The IPCC (2011) Page 205  |  Top of Articleprojects that runoff will increase by mid-century at higher latitudes and in some wet tropical areas, but decrease over some dry regions at mid-latitudes and dry tropics, due to decreases in rainfall and higher rates of evapotranspiration. Areas affected by drought are projected to increase in areal extent.

Extreme shifts in rainfall and temperature could force changes in public works policies and planning. Project management may require re-analysis and alteration to reflect different future conditions. Projects currently in the planning stage may be adversely affected by conditions existing upon their completion in 10–15 years and throughout their economic life expectancy of an additional 50–100 years.

Public works activities tend to be reactive, in that public funds are often unavailable until a disaster strikes. For example, federal flood control activities had been sought for the lower Mississippi valley for almost a century before the great 1927 flood. Two years later, a comprehensive project was formulated and approved for the valley. Public policy should be active, anticipating potential climate change and working to adapt to climate changes during the early stages of project planning/design, rather than reacting to a disaster.

Where public works missions are divided among agencies and political units, fragmentation of responsibility can lead to less effective use of water resources. In reacting to potential climate change, comprehensive planning that crosses political and institutional boundaries—an aquascape approach—will mitigate the impact of adverse changes.

While predictions of regional climate change remain poorly defined and uncertain, the key climate change issue becomes how to prepare in the intervening years. The planning and implementation time horizon for major water resources projects is on the order of 10–30 years, and operational lifetimes often exceed 50 years. It is wiser and, ultimately, more cost-effective to consider the prospects of climate change than to ignore its possibility. Thus, in the near-term, response efforts should be directed at education, assessment, and research, accompanied by the development of longer-term adaptation and mitigation strategies. It is important to note that there will be significant benefits from these efforts even if global climate change does not occur.

The most pressing needs for research in water resources relating to climate change revolve around predicting future conditions and their variability, Page 206  |  Top of Articleincluding understanding the uncertainty of the predictions, determining how these changes will impact issues of importance to humans and the environment (e.g., floods, droughts, water quality), and determining how to mitigate or cope with the impacts.

One area of needed research relates to improvements to downscaling models that take climate data at scales of 2–5 degrees of latitude and longitude from GCMs and provide the information at regional scales. The North American Regional Climate Change Assessment Program provides the highest-resolution dynamically downscaled data in the United States. There are issues with the downscaling model for this program that need to be resolved, including model boundary conditions, internal variability, and physics consistency. More importantly, this data set has not shown convincing skill in reproducing past events—an indication of problems with GCMs or the downscaling model. Moreover, the data are only for the A2 scenario of the Intergovernmental Panel on Climate Change (IPCC)—the scenario with the highest growth of carbon dioxide to the year 2100. Data are needed for the range of IPCC scenarios. In addition, spatial resolution is only 50 km, and research is needed to further downscale this data to typical regional watershed basins, including addressing issues relating to bias corrections.

A second downscaling approach uses the statistically downscaled World Climate Research Programme's CMIP3 Climate Projections, which provide statistical information on about a 12 km² resolution. The projections have fundamental issues, including their assumption that statistical relationships—methods used to relate large scale circulation conditions and local surface conditions, spatial interpolations methods, and handling of GCM bias—are invariant in a changing climate. Different statistically downscaling methods have yielded different results, thereby reducing confidence in statistically downscaled data. Downscaling extreme events also is highly problematic, given the low predictability of the events and unknowns of how climate change will affect the frequency and magnitude of these events. Further research is needed to prove the efficacy of methods used to develop statistically downscaled data.

Research is needed to develop models of the physical environment that can take downscaled climate information and perform calculations over large regional basins, but at small enough scales to resolve important features of water resource projects (e.g., widths of levees, dam flood gates, and water diversion channels). For many important water resource problems, the models must be three-dimensional and dynamically couple surface and ground water, since the depletion and recharging of aquifers is an important issue for future water availability.

Research is needed to improve the physics of models that determine surface runoff through vegetation and urban areas, percolation of water to groundwater, groundwater flows through different porous media, sediment transport on watersheds and in rivers, turbulence modeling, and other phenomena. Research is needed to understand how climate change impacts Page 207  |  Top of Articlevariables such as evapotranspiration, groundwater recharge, land cover changes that affect watershed sediment yield, and moisture storage in soils and snow packs.

Research is needed to determine how climate change will affect the frequency, magnitude, and duration of extreme events such as floods and droughts. It is believed that climate change will increase climate variability and extreme events might be more frequent and longer lasting. Improved predictive capabilities—from determining the heights of levee systems to needed capacities of water supply projects to handle droughts—will be critical for engineering projects. However, it is not clear how to estimate hydrologic events in a changing climate.

Climate change and variability may significantly affect the environment, and research and development (R&D) is needed to determine how climate changes will affect chemical and biological processes that affect humans and plants and animals, including endangered and invasive species. R&D will be necessary to develop robust models that can determine the interaction of physical, chemical, and biological processes that are affected by climate change and variability.

Research is needed on how water quality characteristics that depend on climatic variables may evolve in a changing climate and how man can cope with or mitigate changes. For example, high water temperatures can reduce dissolved oxygen levels and impact aquatic life. Changes in the timing, intensity, and duration of precipitation can significantly affect water quality. Less precipitation can reduce streamflow and cause lakes and reservoir levels to fall, producing less dilution of pollutants. The increased frequency and intensity of rainfall can increase the movement of pollutants and sediment into these bodies of water.

To cope with or mitigate the effects of climate change and variability, models are needed that can determine how mitigation activities affect the environment from the scale of single species to ecosystem-level scales. For example, if water is released from water-control structures because of downstream low water levels, will it benefit an endangered species and how does this benefit compare with the risk of having low water levels in reservoirs that may impact the subsequent need for the water by urban areas?

There are socioeconomic areas needing further research. For example, there are instances of the successful reuse of waste water. For example, Orange County, California, uses waste water to recharge depleting aquifers. In Northern Virginia, 1.4 million people receive about 20% of their drinking water (up to 90% during droughts) from treated sewage water. However, a plan similar to the Orange County plan to recharge depleting aquifers was defeated in the San Gabriel Valley, with opponents citing calling the project “Toilet to Tap.” A plan to use waste water in San Diego County, California, was defeated partially because of perceptions on the safety of using waste-water. An Australian study concluded, “It is now generally accepted that social marketing or persuasion is ineffective in influencing people to use Page 208  |  Top of Articlerecycled water.” It is not understood what convinces some of the need and safety of recharging depleting aquifers or supplementing drinking supplies and generates emotional reactions in others.

Research is needed to understand the uncertainties in future scenario projections. There are uncertainties in climate science, simulations of global climate, emissions scenarios, bias-correction and spatial downscaling methods of GCM data, the physics, chemistry, and biology of natural systems, and relationships of variables in ecological systems. All of these uncertainties have to be understood to have confidence in projections.

An increased understanding of the potential impacts of climate change and of climate sensitive activities can be used to improve our flexibility to deal with future change. Sensitivity analyses should be used in the near-term to determine the critical thresholds for individual components of water resource systems, while scenario analyses can be effective in identifying the vulnerability of the integrated system to potential climatic changes and to climatic variability.

Increasing competition for a less predictable and more variable water supply will require improved data collection capabilities to manage water resource systems. In addition, improved data sets are needed to provide complete data records for impact analyses. Actions should include the increased gathering of climate, streamflow, and water quality data, improved instrumentation and data transmission equipment, better methods of managing and analyzing data, increased use of remote sensing technology, improved real-time operations and flood warning systems. To increase the quantity and quality of collected data, it is essential to find less costly ways of collecting and analyzing data.

Impact assessments, including sensitivity studies as well as scenario analyses, should be initiated quickly. Sensitivity studies can determine the critical thresholds for individual components of the water resource systems by evaluating the impacts of incremental changes in temperature, flow, and other pertinent meteorological variables, such as humidity or wind speed. Scenario analyses can be used to identify the vulnerability of integrated systems to changes in climatic variables. Changes in seasonality, the frequency and magnitude of extreme events, and the magnitude and distribution of key hydrologic variables, should be explicitly addressed in the formulation of scenarios. The impacts on multiple system uses and purposes, including those associated with environmental and recreational impacts, should be assessed.

Strategies should be developed to incorporate climate change uncertainty into water resources planning, with the ultimate aim of creating robust, Page 209  |  Top of Articleflexible water resource systems. Such strategies include improved hydrologic analyses procedures; monitoring of current trends; basin-wide integrated water management; flexible institutions and enhanced inter-agency and international cooperation; improved mediation procedures to resolve competitive water use issues; improved water conservation strategies; realistic water pricing policies; and more responsive legal and institutional frameworks to deal with future change.

The fragmentation of water resource missions and budgets among various agencies can lead to decisions and plans that are optimum for one agency's mission, but not for the nation.

Centralized planning and control should not be the goal, but centralized policies emphasizing a systems approach that transcends agency mission boundaries should be. Similarly, water resources planning and development by one nation can adversely impact other nations and, in cases of very large-scale projects, the global community. Integrated resource management across national boundaries should be encouraged.

Given the uncertainty of existing GCMs and regional models, their best near-term use may be in defining potential limits of the important factors. This information could be used by project personnel and designers in bracketing possible changes in the future. Current procedures can then be reviewed to determine if changes are desirable. While this process specifies neither the magnitude nor direction of the climate changes that occur, it does allow more formal consideration of the impacts of existing climate variability that now takes place.

Adaption strategies should be geared toward the development of techniques that incorporate climate change uncertainty into long-range planning with the aim of creating robust, flexible water resource systems. As historical records may no longer adequately predict future trends, hydrologic analyses procedures, such as flood forecasting and water supply determinations, need to account for the possibility of increased variability and changes in the frequency and intensity of extreme events.

As water supply variability is reduced with increased storage in larger basins, the need for basin-wide integrated water management supported by flexible institutions will increase in significance. Similarly, enhanced institutional arrangements for inter-agency cooperation, as well as improved mediation procedures among water use stakeholders, may be needed to resolve competitive use conflicts. Improved water conservation strategies, combined with realistic legal and water pricing policies, will become important in areas where dry conditions are likely to prevail.

Prospects of climate change offer the opportunity to increase the resiliency, efficiency, and productivity of existing water resource systems. Risk analysis should be included in the project design and operation to account for variability. Projects that are influenced by possible radical climatologic changes may require design lives of shorter duration. More responsive and flexible operating plans for existing and planned reservoirs will be required, and drought Page 210  |  Top of Articlecontingency plans should be developed. Measures to improve and encourage water and energy conservation will require development. The procedures that national or international water resource agencies must go through to permit modification to projects will most likely require streamlining.

While a variety of meaningful studies can be conducted using existing technology in conjunction with sensitivity and scenario analyses, a more definitive quantification of potential global climate change impacts on water resources systems/infrastructure will require improved GCM predictions, increased understanding of the various hydrometeorologic and ecologic processes impacted by potential climatic changes, and improved analytical tools. These long-term research and development requirements are discussed below.

Partial mitigation of greenhouse forcing may be effected through accomplishment of two basic objectives presented below.

Approaches to accomplish this objective include the development and use of innovative hydropower sources such as low-head (i.e., at navigation dams or irrigation projects) tidal power, and increased use of pumped storage capabilities or additional off-peak alternatives. Each of these are known to represent largely untapped sources of potential power that, heretofore, have not been deemed cost effective.

Within this same topic area, the heat exchange capabilities of various ground and surface water sources can be developed and used. As an example, the use of groundwater aquifer as heat sinks/sources can be explored. The potential of geothermal resources, including hot springs, geysers, and so on, can also be investigated.

The accomplishment of this objective can be promoted through improvement of hydropower generator and turbine efficiencies; the application of petroleum industry extraction techniques to extend groundwater development; and the examination of wastewater treatment methodologies that minimize methane and carbon dioxide production. Additional conservation measures and more efficient water use practices, including the lining of irrigation canals, best management practices on agricultural lands, the minimization of evaporation from arid-region transmission channels and reservoirs, the reduction of water use in individual households, the use of wetlands to trap runoff and assimilate pollutants, and increased recycling in industrial processes, appear promising. These and other measures and practices should receive increased emphasis in the near future.

The education of water resource managers and planners, as well as the public, to the long-term nature and seriousness of the climate change issue is essential if we are to be prepared for the future. The assumption of a stationary climate, or at least, the existence of a climate with predictable and limited variability, is ingrained in engineering training, design, and operation. Hydrologists and water resource planners may find it difficult to consider climatic shifts outside traditional expectations. Recent advances in the use of risk/reliability analysis should be endorsed and more widely taught. The utilization of multidisciplinary teams in problem solving, with all specialties involved adequately represented, will improve the transfer of knowledge among professionals from diverse disciplines and will improve problem solving capabilities.

Information and awareness programs should be instituted in both industrialized and developing countries to educate the public. The importance of water supply, wastewater treatment, water conservation, and the quality of the aquatic environment should also be taught in primary and secondary schools. Information on water and energy conservation measures imparted in such programs could result in substantial resource savings.

Climatic change has the potential to impact many interrelated water users, purposes of water resources systems, and the many technical disciplines associated with them. With regard to water resources systems/infrastructure, significant research and development investments will be required to quantify the impacts of climatic change and variability, and to crease methodologies/designs to mitigate those impacts. We believe that the two broad goals listed below must be accomplished if the impacts of climatic change and variability on water resources systems/infrastructures are to be dealt with effectively.

The amplification of these goals, and the basic components of the research and development required to accomplish them, are discussed below.

In the context of this discussion, the terms robust, viable, flexible, and resilient have specific meanings as they relate to the water resource system. Robustness refers to the ability of a system to perform in a predictable, controlled fashion when conditions approach and exceed its design limits. Viable systems are those whose design, construction, and operation have adequate flexibility and resiliency to meet performance, economic, social, Page 212  |  Top of Articlecultural, and environmental quality objectives in the face of climatic variability and potential climatic changes. Flexible systems can be used in ways not originally intended if conditions require a change. Resilient system maintain their flexibility and operating effectiveness without frequent repair and rehabilitation. Vulnerability refers to the degree to which a water resource system fails to provide robustness, flexibility, and resiliency.

Accomplishment of Goal 1, which would result in the design, construction/modification, and operation of water resources systems capable of adequately responding to climatic change and variability, will require the completion of four basic technical objectives associated with climatic variability and potential climatic changes as described below.

This will require the development of more refined global climate models and an increased use of mesoscale modeling in the development of regional climate change scenarios. To produce hydrologic and climatic data for use in water resources development and management, the data from these models would be provided at a spatial scale of 250 km² and a daily temporal scale. Improved data collection efforts, for the analysis of historical climatic trends and verification of models, should be initiated. The coupling of process-based ground and surface water models, with other models that focus on natural ecosystems, environmental quality, economics, climate, and land use, is required to provide the basic modeling framework to assess the impacts of potential climatic change and climate variability on water resources systems. However, several of the underlying descriptions of the physical, chemical, and biological processes within even the best of the current component models are known to require additional investigation. For example, many aspects of groundwater flow and transport, especially within the unsaturated zone, are poorly understood. Even more well-understood processes, such as watershed rainfall-runoff or turbulent three-dimensional hydrodynamic flow fields, have known inadequacies that will require significant research and development.

This will require the development of engineering tools that provide the capabilities to perform integrated systems analyses. These analyses are essential because they provide a mechanism for assessing the river basin-wide, or even inter-basin, responses to climatic inputs. Without these tools, the potential for current water resources infrastructures to flex, but not break, in the face of climatic change cannot be assessed. Use of these tools, however, requires the Page 213  |  Top of Articleestablishment of evaluation criteria and indices of economic value that appropriately reflect the environmental, economic, social/cultural, and performance requirements and public perceptions. The development of these criteria and indices—in forms ranging from reservoir release temperature objectives to hydropower efficiency targets to measures of the relative worth of meeting one of these objectives at the expense of the other (as examples)—should be coupled with integrated systems analysis tools. The development of such criteria will allow for the evaluation and identification of the resiliency and vulnerability of existing systems and infrastructure to climatic changes. This assessment of resiliency and vulnerability must then be documented along with the prioritization of efforts to ameliorate areas with significant levels of vulnerability.

The development of water resources planning and design procedures that meet multiple uses and purposes requires the incorporation of risk/reliability and uncertainty analyses in those procedures. Current planning and design procedures used by many water resources development agencies worldwide, provide for limited consideration of potential climatic changes beyond those previously observed and recorded. The incorporation of risk and uncertainty concepts will allow for a more straightforward consideration of climatic change and variability in system design and operation.

Methods should also be developed for identifying new water supplies. Weather modification, increased use of desalination, improved conservation, and the reuse of storm and wastewater are among those concepts meriting investigation. The implementation of institutional changes that make water a commodity responding to market forces, while providing for basic needs, should be explored within various international socioeconomic and cultural frameworks as a potential means of improving water conservation.

Having assessed the need for more robust water resources systems, and having developed the procedures required to plan and design these systems for the future, one would naturally implement these more “climate change-proof” systems. While construction activities are implicit in this statement, there are a number of additional themes that should be investigated. The modification of the legal and institutional constraints that limit water resources systems' flexibility should be considered. This may prove a monumental effort in that several decades of agency inertia and legislation Page 214  |  Top of Articlewill have to be overcome. In concert with this, the scope of participants who input decisions to water resources project operations should be broadened. Given the multiple, and often conflicting, uses these projects generally have, it is imperative that trust and cooperation between all interests be voiced openly and equally. This is extremely important in the design of a project, when changes in project features can often be easily made. Such coordination is equally important during the consideration of project operational changes, so that modifications to improve or enhance the accomplishment of certain project objectives are not done to the detriment of differing, and perhaps less obvious, concerns.

New construction materials and methods for rehabilitation/retrofitting of existing water resources infrastructure are also needed. Assuming that climatic changes and variability will require future modifications to water resources systems, the need for cost-effective rehabilitation measures will, undoubtedly, increase.

The development and use of knowledge-based tools for the implementation of flexible operations will improve the responses of water resources systems to climatic changes and variability. These tools, to be most effective, must be driven by real-time data.

The prospect of climatic change poses serious challenges to water resources managers concerning the availability and quality of future water supplies. Understanding the implications of these changes and preparing for an uncertain future raises several important issues regarding the development, maintenance, and management of water resources in the coming years. These issues are posed as a series of questions that must be adequately addressed to effectively cope with a changing world.

Reliable predictions:

1. What are the prospects for cyclic and non-cyclic climate change in the near and long-term?

2. Can the uncertainties regarding current climate change predictions be narrowed?

3. Can global climate change projections be translated into reliable regional climate scenarios on temporal and spatial scales useful for water resources analysis and management?

Impacts and vulnerabilities:

4. Can more reliable regional climate scenarios be used effectively to assess the sensitivity, range of potential impacts, and critical vulnerabilities of water resources systems?

5. Are the analysis tools and data sets currently available adequate to conduct reliable impact analyses?

6. Can integrated systems analysis procedures be used effectively to incorporate interrelated disciplines, such as forestry, agriculture, and environmental sciences, into water resources analysis?

Flexibility to cope with an uncertain future:

7. Are current water resources systems designed for current estimated levels of variability sufficient to accommodate predicted changes in climate?

8. Can information regarding future predictions and impact assessments be effectively incorporated into water resources planning, design, construction, and operation such that systems are rendered more robust and flexible to cope with an uncertain future?

9. Can water resources planners and agencies move to a more proactive role in the recognition and resolution of potential problems?

Institutional and legal considerations:

10. Are current institutional and legal frameworks capable of effectively dealing with increased stresses on available water supplies?

11. Are procedures available to effectively address issues of equity, water allocation, and competitive uses if the relative balance between available supplies and demand is significantly changed?

12. How can mechanisms be developed to increase inter-agency and intergovernmental cooperation in the sustainable development of water resources?

13. Should water be treated as a commodity subject to market forces after basic needs have been provided for?

Mitigation:

14. How can water resource systems be used to help mitigate the production of greenhouse gases?