adapting water resources management to global climate change

A D A P T I N G WATER R E S O U R C E S M A N A G E M E N T TO G L O B A L

C L I M A T E C H A N G E

P H I L I P W I L L I A M S

Principal, Philip Williams &Associates, Consultants in Hydrology, Pier 35, The Embarcadero, San Francisco, CA 94133, U.S.A.

Abstract. This paper provides an overview of the impact of global climate change on water resources management. Changes in precipitation and temperature of the scale predicted by General Circulation Models for a doubled CO2 level will significantly affect annual runoff, runoff variability, and seasonal runoff. These in turn will affect water supply, flood protection, hydropower generation, and environmental resources. In addition, climate change will significantly affect the geomorphic response of the watershed, increasing soil erosion and altering the hydrologic response of the watershed. These geomorphic changes will in turn affect water supply, flood hazard, and riparian ecosystems.

Possible water resources management responses are identified. This includes reallocation of water supply from less valuable irrigated agriculture to municipal uses; changes in agricultural methods; increasing incentives for integrated flood management; increasing incentives for watershed management; integration of ecosystem needs in water resources planning; and the need to redesign the operation of existing water projects.

Introduction

For thousands of years, societies have adapted their cultures and economies to their climatic and hydrologic geography. Many human activities are intricately linked to the natural flow regime and character of streams and rivers. Since the Industrial Revolution, and especially in the last 50 years, mankind's accelerating demands for energy, water supply and developable land have transformed the nature and character of water resources in many parts of the globe. 'Water development', through dams, diversions and levees, has altered the natural hydrologic regime in response to economic demands.

In planning and designing water development projects, engineers invariably use historic hydrologic and climatic data. Implicit in all these designs is an assumption of 'stationarity', or constancy of key hydrologic variables such as average annual flow, annual variability of flow and seasonal distribution of flow (Changnon, 1987).

With the projected changes in global climate due to the greenhouse effect, such an assumption is no longer valid. This can cause substantial dislocation and ineffi- ciencies in water resources management in different climatic regions of the globe.

Climatic Change 15: 83-93, 1989. �9 1989 Kluwer Academic Publishers. Printed in the Netherlands.

84 Philip Williams

Changes in Hydrologic Variables

The effect of increasing concentrations of greenhouse gases on global climate has been simulated using General Circulation Models (GCMs) (NASA, 1984). Although these models give only rough approximations, they indicate increases in average annual runoff of up to 60% in latitudes between 40 and 60 deg. for doubled CO 2 concentrations. While GCMs can project seasonal distribution and annual variability, their resolution, on the order of 100,000 km 2, is too large to assess the impacts on key hydrologic parameters of a particular watershed. For this a more specific water balance model must be used that simulates the relevant hydrologic processes. Many water balance models may be applied to assess the impact of climate change, and these have been discussed by Gleick, 1986, and Beran, 1986.

For water resources management, the three most important hydrologic variables are:

1. Average Annual Runoff

This is determined by the manner in which the seasonal precipitation and temperature affect the annual water balance of the watershed. Annual runoff responds non- linearly to changes in annual precipitation (see Fleischke et al., 1987). The effect of annual changes in precipitation can be overridden by seasonal changes and by the effects of temperature on soil moisture conditions (Gleick, 1987). In water-short semi-arid areas, the problems can be most severe. For example, in the Colorado River Basin, annual precipitation changes of plus or minus 10%, combined with temperature increases of 2 *C have been predicted to decrease annual runoff about 30% (Stockton and Boggess, 1979).

3. Runoff Variability

The frequency of wet and dry years is an important parameter in planning for floods and droughts. Using ground moisture condition as an indicator, GCM modeling for the U.S. indicates that major shifts may occur in the frequency distribution (NASA, 1984). In a higher latitude area, such as the Pacific Northwest, the GCM model predicts that runoff will increase, due to large increases in precipitation. There will also be an increase in the frequency of wet years from 26 to 47%. In a semi-arid area, such as the Colorado Basin, runoff is predicted to decrease, with the number of dry years increasing from 24 to 36%.

3. Seasonal Runoff

The distribution of runoff within the year can also be greatly affected by climate change. For example, GCM modeling indicates that most of the increase in runoff

Adapting Water Resources Management to Global Climate Change 85

in the Pacific Northwest will occur in the wettest period, the winter, thereby increasing the seasonal variability; most of the reduction in runoff in the semi-arid Colorado Basin will occur in the spring and summer.

In watersheds dependent on snowmelt runoff, large seasonal runoff changes can occur, even if seasonal precipitation remains unchanged. This happens where average temperatures increase, causing precipitation to occur as rain rather than snow (Gleick, 1987). An area particularly vulnerable to such changes is California's Cen- tral Valley, where about 40% of the runoff occurs as snowmelt.

Geomorphic Effects of Changes in Runoff

Changes in precipitation and runoff can greatly affect watershed erosion rates, fluvial sediment transport, and the stability of stream channels. These factors must be considered in water resource planning, as the geomorphic changes in turn affect runoff amounts and seasonal distribution.

Figure 1 is indicative of the gross effect of climate on average annual watershed erosion rates. In arid climates, a 10% increase in average annual precipitation can increase sediment delivery by 100%. Precipitation increase in wet areas on de- forested or unforested hillslopes initiates landsliding and significant increases in sedimentation in river channels downstream. In semi-arid areas increases in runoff can initiate massive gullying and riverbed downcutting (Dunne and Leopold, 1978). With loss of soils and vegetation cover, runoff becomes 'flashier', flood peaks higher, and seasonal flow variability increases (Chorley et al., 1984). In addition, groundwater recharge can decrease. These geomorphic changes in themselves can cause substantial reduction in water availability.

Impacts on Water Resources Management

The impacts of climate change on water resource management are complex and far-reaching. To illustrate just the first order effects, Tables I and II summarize likely impacts on water resources for a worst-case scenario for a semi-arid and humid area on the following four major water resources management considerations.

1. Water Supply

The impacts of even small changes in runoff in arid and semi-arid zones will greatly affect water supply. In such regions water resources are usually fully allocated with a heavy reliance on carryover storage in reservoirs or aquifers - to provide water during drought years. Reduction in total runoff and increases in drought frequency can significantly reduce the effectiveness and reliability of storage reservoirs. Nemec and Schaake (1982) analyzed the impact of changes in precipitation on reservoir yield and showed that a 1% reduction in average annual precipitation caused a 2% reduction in yield. They determined that, to compensate for a 25%

86 Philip Williams

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Fig. 1. Central Valley Watershed.


TABLE I" Examples of first order impacts of climate change on water resources of semi-arid areas

Annual runoff Runoff frequen- Runoff seasonal- Sediment pro- reduced cy changed - ity changed - duction in-

more droughts less even dis- creased tribution

1. Water supply reservoirs

Proportion- Reduced yield Reduced yield Loss of storage ately larger and reliability and reliability reduced yield reduced yield

2. Flood protec- n.a.

3. Hydropower Reduced genera- reservoirs tion

4. Environ- Water quality mental instream and resources downstream

uses impaired.

Fewer wet years More wet season Loss of reservoir more flood runoff. Could storage. River protection, increase flood channel aggrades

hazard, increased flood hazard

Reduced firm Reduced genera- Loss of storage load tion and firm reduced genera-

load tion

Water quality Dry season Water quality impaired water quality impaired by instream uses and instream sediment, affected uses impaired, instream uses significantly, by aggradation.

TABLE II: Examples of first order impacts of climate change of humid areas on water resources

Annual runoff Runoff frequen- Runoff seasonal- Sediment pro- reduced cy changed - ity changed - duction in-

more droughts less even dis- creased tribution

1. Water supply Increased yield Increased yield Reduced yield reservoirs ~ and reliability and reliability

2. Flood n.a. Flood frequency Flood frequency protection increase, fiver increase, sig-

channel changes, nificant increase significant in flood hazard increase in flood hazard

3. Hydropower Increased Increased Reduced genera- reservoirs generation generation and tion and firm

firm load load

4. Environ- Water quality Water quality Minor adverse mental instream and instream and impacts resource downstream downstream

uses uses improved improved

Loss of storage reduced yield

Loss of reservoir storage, increase in flood hazard

Loss of storage reduced generation

Water quality and instream uses impaired by sediment

88 Philip Williams

reduction in precipitation in a semiarid climate would require a 400% increase in size of reservoir, if the same yield and reliability were to be maintained.

In addition, reservoirs in semiarid areas have a relatively short life span due to sedimentation. Where climate change causes increases in sediment delivery, existing storage will be lost more rapidly.

In arid and semiarid climates, aquifers are only recharged during infrequent wet years. Climate change that reduces the frequency of wet years can greatly reduce the long-term yield of the aquifer.

In semiarid areas, seasonal shifts in runoff can cause uneven distribution in runoff, thereby reducing the proportion of runoff that may be captured by a reservoir or recharged to an aquifer.

2. Flood Protection

Much of the high-value agricultural land and many urban centers are located on low-lying flood plains. These areas are usually protected by levees and flood stor age reservoirs. Increases in flood frequency can make these structural flood control measures significantly less effective in protecting against large floods. At present in the U.S., flood damages are about S 2 billion/year (FEMA, 1981). There would be a substantial increase where developed floodplains are inundated. In addition, larger floods can extend the floodplain, causing damage to development formerly not at risk. The problem is compounded by geomorphic changes in the river system due to changing watershed erosion. Sedimentation in the river bed raises flood levels. Erosion of the riverbed can undermine levees.

3. Hydropower Production

The value of hydropower production is dependent on the runoff and available reservoir storage. Reductions in runoff produce a proportionately greater reduction in power production than increases in runoff increase it. In most hydro dams, not all runoff can be stored or passed through the powerplant; therefore, in the wet season, some runoff is 'spilled' and does not increase power production. In addition, reliable firm load power, available at the time of peak demand, is more valuable than run-of-the-river power. Firm load power generation is reduced by change in flow frequency and loss of reservoir storage.

4. Environmental Resources Management

River water quality is greatly affected by flow rates and volumes, and by turbidity. The most vulnerable rivers are those in drier areas with low flows and highly seasonal runoff. Water quality in these rivers can be greatly impaired by reduction in flow and changes in seasonality.

In-stream uses by fish and riparian ecosystems are greatly affected by changes in


flow, sediment level, and river morphology. Downstream uses - the ecosystems of estuaries or lakes dependent on freshwater inflow - will also deteriorate. Instream and downstream ecosystems in semiarid areas are particularly susceptible to increased drought frequency, changing perennial streams to intermittent or ephemeral ones.

A Case Study -- The Impact on California's Central Valley

Possible first-order impacts of climate change have been discussed individually above. However, water resources management requires integrating many competing uses and objectives; a change in any one of these affects all the others. To illustrate the combined effects of climate change on a water management system it is useful to look at a specific example.

California's Central Valley has one of the most intensively managed water resource systems in the world (see Figure 1). The 153,000 km 2 watershed has an average annual natural runoff of 35 km 3 that is highly seasonal, with peaks occur- ring in the winter from rainfall and the spring from snowmelt. About 50 percent of the runoff is diverted and consumed for water supply. About 85% of this is used to irrigate 30,000 km 2 of agriculture primarily in the arid San Joaquin Valley. The remaining 15% provides municipal and industrial water primarily to the San Fran- cisco area, but about 0.6 km 3 yr -~ is exported to Los Angeles.

Water development has been accomplished by the construction of a number of large multipurpose reservoirs such as Shasta, Oroville, and Folsom Dam in the Sierra foothills in the wetter northern part of the Valley. These large reservoirs store water primarily from the spring snowmelt, and then release it into the Sacramento River during the summer, so that it can be pumped into two large canal systems feeding the arid San Joaquin Valley. The reservoirs provide carryover storage to ensure a reliable water supply in the event of an extended drought.

The reservoirs are multipurpose as they provide water supply, power generation and flood control. Power generation is approximately 8000 Gwh per year, much of which is used for pumping irrigation water (State of California, 1979). Flood control is provided by lowering the reservoirs during the winter rainflood season, and by an extensive levee system downstream.

The operation of the irrigation system has caused reduction in river flows, deterioration in water quality, and drastic reductions in river and estuarine fisheries.

Even small climate changes can have major adverse effects on a complex water system like California's. If shifts such as those observed by Bradley et al. (1987) represent a long-term climate change, they will require a major reevaluation of the benefits and operation of the system. Expected changes projected by GCM simula- tions will have an even greater effect (EPA, 1988). Most of California's water development was planned and designed in the 1930s, '40s, and '50s, assuming stationarity of the climate. This period was when water allocation decisions were made,

90 Philip Williams

reservoir operational criteria defined, and benefits calculated. Since then, in line with Bradley et al.'s analysis, snowmelt runoff in the last 30 years appears to have decreased. At the same time in the last decade there have been a succession of wet winters, including record floods in 1986. The operational dilemma faced by Cali- fornia's water resource managers provides a foretaste of the increasing problems in reconciling competing water resource management goals as climate shifts occur.

The experience of California shows the following trends developing that are con- sistent with global climate change: - Decreasing spring runoff reducing water supply and increasing competition for

available reservoir storage (California Department of Water Resources, 1988). - Increasing rainflood frequency, requiring redesign and upgrading of levees and

flood control dams (e.g., Corps of Engineers, 1987). - Increasing demands for instream and downstream flow releases to protect eco-

systems from further degradation due to reduction of spring flows (Williams, 1988).

Water Resources Management Response to Climate Change

There is a high degree of uncertaintly concerning the magnitude and direction of the effects of climate change on the hydrologic cycle. The following are possible management responses that will be made to climate changes that cause increased competition for water resources:

1. Re-allocation of Water Resources

Water resource exploitation has usually been carried out to further specific social goals rather than purely economic ones. Examples are increasing the population of the western U.S., or industrializing Third World countries such as Ghana or Sri Lanka. Social goals have been reflected in political water allocation decisions or water rights procedures. With increasing competition for a scarcer resource these decisions must be re-evaluated. The outcome of such a re-evaluation would prob- ably give higher priority to urban and industrial water supply than irrigated agriculture. Thus, it is likely that there will be reductions in the amount, and significant reductions in reliability, of irrigation supply. Examples of this may be giving priority to the demands of the City of Cairo from the Aswan Dam, or the City of Los Angeles from the Central Valley.

2. Stricter Accounting of Water Resource Economics

Increasing competition for river flows and reservoir storage increases their vallue. Large-scale irrigation schemes have been almost always based on political rather than economic rationale, and are usually operated at a loss to benefit a specific economic sector such as irrigated agriculture. With the increasing value of water in


semi-arid areas, large irrigation schemes will become increasingly less attractive to build and maintain. For example, the actual marginal cost of new reservoirs in the U.S. is of the order of S 1/cubic meter of storage; the average water delivery cost is of the order of S 0.1/cubic meter, but typically, the irrigation delivery charge is of the order of S 0.01 m3 (EDF, 1983). Only urban and industrial users can now afford the true costs of water delivery in many semi-arid areas. The increasing dis- parity between costs and revenues would likely lead to reconsideration of earlier political decisions and increased use of water marketing as a means of allocation.

3. Increased lncentives for Alternative Agricultural Production

At present there is little incentive to explore alternatives to large-scale cash crop irrigation. However, other productive alternatives exist, such as dryland farming, less water-intensive food crops, and water conservation measures, such as drip irrigation. At the same time there will be an increased incentive to maximize production in areas where rainfall is plentiful.

4. Increased Incentives for Energy Conservation

Decreasing power production and decreasing reliability will increase the effective cost of hydropower, providing an additional incentive for energy conservation measures.

5. Increased Incentives to Institute Flood Management rather than Flood Control

Increased flood frequency and decreased flood control reservoir effectiveness will increase the flood hazard in existing flood plain areas. In many instances additional costs of levee reconstruction or dam spillway modification will not be as economic as floodproofing and flood plain zoning in controlling flood damages.

6. Increased Incentives for Watershed Management

The value of protecting soils and vegetation in the watershed will increase as the rate of flood damages increases and as the value of dry season flows increases. In many instances, it would be more cost-effective to protect a vegetated watershed than construct additional reservoir storage.

7. Integration of Ecosystem Needs in Water Resource Planning

In many instances fisheries, estuarine ecosystems and riparian woodlands have direct benefits, supporting the livelihood of many people. Traditionally these flows have been ignored by water resources developers. Now they must be considered. If flows are reduced below certain limits, the ecosystem can be severely degraded.

92 Philip Williams

8. Redesign of Water Engineering Facilities

Large-scale investment in hydraulic engineering infrastructures may become ob- solete with changing climate, requiring redesign and reconstruction. Examples are increasing the size of canals, pipelines and pumping plants to accommodate increasing variability in runoff, relocation of intake structures due to river channel changes, and replacing spillways on dams to accommodate larger floods.

In order for policymakers to respond effectively to climate change, the first requisite is to understand the challenge as a resource management question, not as a hydraulics engineering or plumbing problem. The efficient, sustainable use of water resources requires the resolution of many competing goals. Approaching this question with a broader perspective will require major rethinking of existing poli- cies.

References

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Bradley, R. S. et al.: 1987, 'Precipitation Fluctuations over Northern Hemisphere Land Areas since the Mid-19th Century', Science 237, 171-175.

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Chorley, R. J., Shumm, S. A., and Sugden, D. E.: 1984, Geomorphology. London, Methuen. Dunne, T. and Leopold L.: 1978, Water in Environmental Planning, San Francisco, W.H. Freeman and

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Revelle, R. and Waggoner, P. E.: 1984, 'Effects of Carbon Dioxide-induced Climatic Change on Water Supplies in the Western United States', in Changing Climate, Report of the Carbon Dioxide Assess- ment Committee, National Research Council, Washington, D.C.: National Academy Press, pp. 419- 432.


State of California, Department of Water Resources: 1988, Testimony presented to Congressional Subcommittee on Water and Power Resources, October 17, 1988, by Robert Potter, Deputy Direc- tor.

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Stockton, C. W. and Boggess, W. R.: 1979, Geohydrologieal Implications of Climate Change on Water Resource Development. U.S. Army Coastal Engineering Research Center, Fort Belvoir, Virginia.

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Williams, P. B.: 1988, The Impacts of Climate Change on the Salinity of San Francisco Bay. Report prepared for ERL Corvallis, OR Office of Research and Development, U.S. EPA, 39 pp.

(Received 3 March, 1988; in revised Form 27 December, 1988).

adapting water resources management to global climate change

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