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10
Climate Uncertainty: Implications for Operations of Water Control Systems

John A. Dracup and Donald R. Kendall

University of California, Los Angeles

Loyola Marymount University, Los Angeles

INTRODUCTION

The key question we are addressing here is how water resource systems will be operated under conditions of uncertain future climate variability and change. This question in turn raises the question of whether or not a current water resource system is adequately designed for climate variability and change. We attempt to answer these two questions using the method of climate variability analogies as suggested by Glantz (1988; 1989). Traditionally, water resources design, such as reservoir sizing, has been predicated on an assumption that the statistical parameters of historic streamflow series are stationary: that is, the parameters that characterize the historical streamflow series do not change with time. This is not to say that traditional water resource system designs assume that climate is actually stationary; traditional designs account for changing climates by considering the variance of streamflow (σ2) about its mean (Q). Stationarity assumes, however, that future variations in climate, as expressed in streamflows, will be similar to those observed in the past.

Future scenarios of climate variability of interest to water resource managers are those of long-term extreme high or low streamflows. The scenarios developed here will be based on historical analogies. We will use the high flow event that occurred on the Colorado River during the spring of 1983 as an analogy of climate variability or change causing significant increased streamflow in the Colorado River. We will use low flow events evident from tree-ring studies in the Colorado River basin as analogies of climate variability or change causing significant decreased streamflow in the Colorado River.



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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona 10 Climate Uncertainty: Implications for Operations of Water Control Systems John A. Dracup and Donald R. Kendall University of California, Los Angeles Loyola Marymount University, Los Angeles INTRODUCTION The key question we are addressing here is how water resource systems will be operated under conditions of uncertain future climate variability and change. This question in turn raises the question of whether or not a current water resource system is adequately designed for climate variability and change. We attempt to answer these two questions using the method of climate variability analogies as suggested by Glantz (1988; 1989). Traditionally, water resources design, such as reservoir sizing, has been predicated on an assumption that the statistical parameters of historic streamflow series are stationary: that is, the parameters that characterize the historical streamflow series do not change with time. This is not to say that traditional water resource system designs assume that climate is actually stationary; traditional designs account for changing climates by considering the variance of streamflow (σ2) about its mean (Q). Stationarity assumes, however, that future variations in climate, as expressed in streamflows, will be similar to those observed in the past. Future scenarios of climate variability of interest to water resource managers are those of long-term extreme high or low streamflows. The scenarios developed here will be based on historical analogies. We will use the high flow event that occurred on the Colorado River during the spring of 1983 as an analogy of climate variability or change causing significant increased streamflow in the Colorado River. We will use low flow events evident from tree-ring studies in the Colorado River basin as analogies of climate variability or change causing significant decreased streamflow in the Colorado River.

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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona The idea of forecasting by the use of historical climate analogies has been used by Glantz (1988; 1989), who poses four caveats in their use. First, the use of the climate analogy should be made clear so that it is not misleading when viewed in a different context. Second, the climate analogy should not be overextended to support unjustified conclusions. Third, one should recognize that the analogy may be inappropriate for cultural or historical reasons. Fourth, one should note that analogies can lead to the development of possible but inconsistent scenarios. The use of historical climate analogies provides a different look into the future than that provided by large-scale computer models. Furthermore, climate analogies provide an insight into how the responsible agencies actually dealt with the climate variability as it occurred The topic of the operation of water resource systems under climatic stress addressed here is, of course, not new. One of the best discussions of this can be found in the National Research Council's Climate, Climate Change, and Water Supply (1977) study. This study emphasizes that ''unless the exact sequence of future flows can be predicted with certainty there may be little benefit to hydrologic system design.'' The study proposes designing water resource systems with robustness (the ability to perform reasonably well under a variety of possible climates) and resilience (the ability of a system designed for one climate and set of conditions to be modified in response to persistent new climates or conditions) (Matalas and Fiering, 1977). The question to be considered here, then, is whether climate analogies can be used to determine whether or not an existing water resource system exhibits the properties of robustness and resilience. Forecasting by analogy using the Colorado River as an example also has been studied by Brown (1988). However, she focused on the Colorado River Compact rather than on floods and droughts along the river. THE COLORADO RIVER BASIN If the streamflow in the Colorado River somehow could be equated with the number of words written about it, the river would constantly flow as a torrent (Dracup, 1977; Dracup et al., 1985; Hundley, 1975; Rhodes et al., 1984). The Colorado River dominates water resource development in the seven states of the southwestern United States (see Figure 10.1).

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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona It also is one of the most carefully managed river systems in the world. Its multipurpose uses of water supply, hydroelectric power, and water-based recreation compete with its management priority of flood control. With the exception of the deserts of the Great Basin, this 243,000-square-mile basin has the greatest water deficiency (average precipitation less potential evapotranspiration) of any basin in the contiguous United States. Yet, more water is exported from the Colorado River basin than from any other basin in the United States. The basin has been divided by the Colorado River Compact into the upper Colorado basin and lower Colorado basin for purposes of interstate administration. The upper basin drainage includes the areas of Arizona, Colorado, New Mexico, Wyoming, and Utah that drain into the Colorado River above Lee's Ferry, Arizona. It is bounded on the east and north by mountains forming the Continental Divide, and on the south it opens to the lower Colorado region. The lower basin drainage includes most of Arizona, parts of southeastern Nevada, southeastern Utah, southeastern California, and western New Mexico. WATER AVAILABILITY ESTIMATES A wide range of climates occur in the Colorado River basin because of differences in altitude, latitude, and topographic features. In the north, summers are short and warm and winters are long and cold. In the south, the summers are longer and the winters are moderate at low altitude, but colder temperatures occur in the mountains. About 83 percent of the water that flows in the Colorado River basin comes from the upper basin. The average annual precipitation throughout the entire upper basin is about 16 inches (40.6 cm), which amounts to 93,440,000 acre-feet per year (115 × 109 m3). Approximately 15 percent of the precipitation runs off, and most is lost to evapotranspiration. One of the most famous and controversial hydrologic records in the United States is that of the virgin flow of the Colorado River at Lee's Ferry, Arizona. Lee's Ferry is defined as a point on the Colorado River one mile below the mouth of the Paria River. Estimates of virgin flow have been made there for the upper basin since 1896; however, runoff has been measured and recorded only since the first gaging station was established at Lee's Ferry during the summer of 1921. (The Bureau of Reclamation now uses natural

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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona FIGURE 10.1 Colorado River basin upstream of the inflow to Mexico.

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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona flow, not virgin flow, for its operation studies.) The importance of this flow is accentuated by the Colorado River Compact, which requires that the upper basin deliver 75 million acre-feet (maf) (92.5 × 109 m3) at Lee's Ferry each 10 years. Estimates of the long-term annual average flow vary from 11.8 to 16.8 maf (14.5 to 20.7 × 109 m3) depending on the time period selected (Colorado River Board of California, 1969). Recent tree-ring analysis dating back to 1512 has indicated the long-term mean to be approximately 13.5 maf (16.6 × 109 m3) (Stockton, 1977). The current estimates of available surface-water supply within the upper basin are less than those at the time the Colorado River Compact was negotiated. This is because of the abnormally wet period that occurred during the early part of this century. The range of annual natural flow at Lee's Ferry has varied from a low of 5.0 maf (6.2 × 109 m3) in 1977 to a high of 24.0 maf (29.6 × 109 m3) in 1917. The average natural flow from 1931 through 1989 of 14.2 maf (17.5 × 109 m3) may be closer to the long-term mean. The laws governing the Colorado River have been presented in detail by Meyers (1966) and Hundley (1975; 1983). Only a brief summary of the major treaties, laws, and compacts will be presented here. The allocation of Colorado River water is based on the concept of beneficial consumptive use. The allocation system operates at four levels: international, interregional, interstate, and intrastate (Weatherford and Jacoby, 1975). The international allocation was accomplished by the Mexican Treaty of 1944. Mexico was guaranteed an annual amount of 1.5 maf (1.8 × 109 m3) except in times of extreme shortage. However, this treaty contained no provision for water quality. Thus, joint agreements in 1965 and 1973 called for a temporary agricultural drainage water bypass and eventually a desalting plant to improve the quality of water crossing the border. The interregional allocation was achieved when Congress approved the Colorado River Compact, which became effective in June 1929. Sectional rivalry has caused the states included in the drainage basin to agree to an equal apportionment in the use of the Colorado River system waters between the states of the upper basin and the states of the lower basin (an agreement set forth in Articles III (b) and III (d) of the Colorado River Compact). Traditionally, the fertile lowland valleys in the lower basin states have developed economically more rapidly than have the mountain headwater "areas of origin" in the upper basin states. The upper basin states insisted that an equitable apportionment of

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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona the river be made to them prior to the expenditure of large sums of federal money, which might result in a modification of equities adverse to the upper basin states. This is in essence what was achieved in the Colorado River Compact. The intent of this landmark document was to give each basin the perpetual right to the "exclusive beneficial use of 7.5 maf (9.25 × 109 m3) of water per annum." However, the lower basin was assured that depletion in the upper basin would allow at least 75 maf (92.5 × 109 m3) of flow to the lower basin at Lee's Ferry in each successive 10-year period. Thus, the lower basin received a guaranteed 10-year, not annual, minimum flow, and the upper basin assumed the burden of any deficiency caused by a hydrologic dry cycle. It is important to note that the division of the use of water between the upper and lower basins is a fixed amount rather than a proportional amount (such as one-half of a 10-year moving average). CURRENT COLORADO RIVER MANAGEMENT TECHNIQUES The joint operation of Lakes Powell and Mead is subject to the following criteria, according to the Law of the River (Nathanson, 1978): a Lake Powell minimum objective release of 8.2 maf per year; additional releases from Lake Powell to equalize end-of-year active storages in Lakes Powell and Mead if Lake Powell would otherwise contain more water in storage; and sufficient storage in the upper basin reservoirs to assure future deliveries to the lower basin without impairing annual consumptive use in the upper basin (called 602(a) storage under the Colorado River Basin Project Act of 1968). As a result of the Law of the River, each basin currently possesses many storage facilities, including a large linchpin reservoir: Lake Powell for the upper basin and Lake Mead for the lower basin. The storage in these two reservoirs totals 51.0 maf (62.9 × 109 m3), or 85 percent of the total storage in the entire Colorado River basin. The reservoir system now stores about four times the annual flow of the river. This volume of water in storage reflects the determination of the basin states to conserve as much water as possible, providing a margin of safety in the event that a run of

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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona dry years occurs. The upper basin states prefer that releases to the lower basin be the absolute minimum required by law. Furthermore, current operational regulations require that the river's manager, the U.S. Bureau of Reclamation, maintain Lakes Mead and Powell at or near equal volumes of water in storage at the end of each operating year (which occurs on September 30) (Nathanson, 1978). Each basin is thus assured "equal ownership" of the river. Flood control protection is provided to residents, farms, and businesses below Hoover Dam. Flood control operations rest on two central elements: scheduled dedicated water storage space made available to catch the spring runoff in Lake Mead; and a forecast of how much water will enter Lake Powell from April 1 through July 31, produced by the National Weather Service (NWS) Colorado Basin River Forecast Center in Salt Lake City, Utah. The objective of the flood control procedure, in effect, is to create enough storage space, through reservoir releases from August through January, to catch the predicted April through July runoff. The plan, in action since 1968 and slightly modified in 1982, uses a monthly streamflow forecast generated for the period January through July that predicts the spring inflow to Lake Powell. Adjustments in the forecasted storage space for the inflow can then be made to keep downstream releases below damaging levels and at the same time conserve as much water as possible. The NWS Colorado Basin River Forecast Center uses monthly estimates to arrive at forecasts of the maximum probable and minimum probable April through July runoff. To meet these runoff estimates, the Bureau of Reclamation increases flood control space in Lake Mead starting on August 1 of each year to have 5.35 maf (6.6 × 109 m3) available by January 1. According to the flood control plan for the Colorado River, Lake Mead is the only major basin reservoir with an explicit flood control space schedule. Prior to the construction of Glen Canyon Dam, the standard flood control procedure was to have 5.8 maf (7.1 × 109 m3) of storage available on January 1, as recommended by Debler (1930). This storage requirement was increased each month until a maximum requirement of 9.5 maf (11.7 × 109 m3) was reached on April 1. These procedures were formalized by the U.S. of Army Corps of Engineers in 1955 and were continued until 1968. The Bureau of Reclamation's scheduled outflow release rates through the dams and the storage space availability based on the

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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona NWS inflow forecasts have worked well in recent decades in minimizing water lost through unneeded anticipatory releases and potential flooding below Hoover Dam. This has helped to maximize hydroelectric generation, water conservation, storage, and flood control. However, the conditions in the Colorado basin in the early 1980s were radically different from those of the 1960s and the 1970s. The changes in those conditions were significant contributors to the flooding that occurred in the spring and summer of 1983. OPERATION DURING HIGH STREAMFLOWS: THE COLORADO RIVER SPRING FLOODS OF 1983 On January 1, 1983, there were 6.6 maf (8.1 × 109 m3) of storage space available in Lake Mead and upstream—more than the required January target of 5.35 maf (6.6 × 109 m3). Yet even with the surplus storage space available, the reservoir system was overwhelmed by the magnitude of the spring inflow to Lake Powell. Because of late precipitation and cool weather throughout the upper basin, snowpack continued to increase during April and May. Figure 10.2 shows the rapid and unusual changes in the forecasted inflow to Lake Powell from January through June 1983. Figure 10.3 illustrates the relationship between Lake Powell inflows, Lake Powell outflows, and Hoover Dam releases from April through July 1983. Because of the massive influx of water into Lake Powell, the Bureau had to increase outflows from Glen Canyon Dam. This, in turn, obliged the Bureau to raise outflows from Hoover Dam. The releases at Hoover Dam, which historically had been held to approximately 25,000 cubic-feet per second (cfs) (708 m3 s-1), were elevated to over 40,000 cfs (1,132 m3 s-1) in July. This is a critical point, for 40,000 cfs (1,132 m3 s-1) was the targeted maximum outflow rate from Lake Mead under the 1968 revised flood control procedures. The Bureau of Reclamation operators successfully limited Hoover release rates to 40,000 cfs (1,132 m3 s-1) except for the month of July, during which Hoover releases averaged 41,854 cfs (1,184 m3 s-1). However, flooding downstream of Hoover Dam begins when the flow exceeds 19,900 cfs (538 m3 s-1). The rapid sequence of meteorological events occurring late in the spring, coupled with the problem of attempting to move massive amounts of water through Lakes Powell and Mead in a short period of time, resulted in streamflows greater than those

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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona FIGURE 10.2 Forecasted inflow to Lake Powell from January through June 1983. Source: Dozier and Brown, 1983. FIGURE 10.3 Relationship among Lake Powell inflows, Lake Powell outflows,  and Hoover Dam releases from April through July 1983.

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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona experienced during the previous two decades by lower basin residents and businesses. The unusual and unexpected flooding along the Colorado River during 1983 was the result of three converging factors: the sudden required operation of a full river system, an encroachment into the downstream flood plain, and climate variability in the basin. Each of these is discussed below. The Full River System The filling of Lake Powell behind Glen Canyon Dam began in 1963 and was completed in 1980. During this 17-year period, there were virtually no required flood control operations on the Colorado River. Runoff in excess of downstream water supply and hydropower generation was easily stored. However, in 1980 Lake Powell became full, which required that the river now be operated in a careful, prudent manner; there became little room for forecast error. The forecasted inflow to Lake Powell had to be not only accurate, but also carefully monitored on a real-time basis. Monitoring inflows allows corrective management responses if conditions permit. However, in 1983, Lake Powell inflows rose so rapidly that there was no time for mitigating responses. For example, on May 24, 1983, the unregulated inflow to Lake Powell was approximately 37,000 cfs (1,047 m3 s-1). Eight days later, on June 1, the unregulated inflow was 102,000 cfs (2,887 m3 s-1) (U.S. Bureau of Reclamation, 1983). Even the availability of real-time data may not have been sufficient to manage a wet year such as 1983, since it takes substantial time to move water through dams with structurally limited release rates. The 1983 April through July inflow into Lake Powell was more than 14 maf (17.3 × 109 m3). Approximately 140 days would be required to discharge that quantity at a rate of 50,000 cfs (1,415 m3 s-1). Physical Encroachment into the Flood Plain Physical encroachment into the lower basin flood plain is a function of the defined flood plain boundaries, the relative stability in the annual streamflows, and societal decisions. Thus, encroachment into the lower basin flood plain, which would not have been possible in the absence of the upstream storages, was encouraged by a combination of technological fixes and lax zoning

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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona practices in counties bordering on the Colorado River (Arizona Republic, June 22, 1983; July 1, 1983). The two major dams on the Colorado River have performed as planned in controlling the variability of streamflow rates (Figure 10.4). Glen Canyon and Hoover dams have consequently provided substantial protection to the lower Colorado flood plain in terms of their ability to reduce the river's meanderings and the flooding associated with high spring streamflows. Even in 1983, releases at Hoover Dam did not significantly exceed 50,000 cfs (1,415 m3 s-1). The history of development in the flood plain roughly began with the construction of earthen levees in the area around Yuma, Arizona. The levee system was constructed to protect agricultural land (fertile flood plain soil) from the annual rush of spring snowmelt (U.S. Army Corps of Engineers, 1982). With the completion of Hoover Dam and the subsequent decrease in spring streamflow variability, more flood plain acreage became available for development. This resulted in the construction of residential and commercial structures in these areas. As the 1982 review of flood control operating procedures notes (U.S. Army Corps of Engineers, 1982): Few, if any, structures were located in the 40,000 cfs (1,132 m3 s-1) flood plain in the lower Colorado River at the time of the closure of Hoover Dam (1935) and for some years thereafter. For many years the flood control operation plan for Hoover Dam has incorporated a "target maximum" flood control release of 40,000 cfs (1,132 m3 s-1). With the completion of Glen Canyon Dam in 1962, streamflow variability was sharply narrowed. This coincided with the period of the greatest physical encroachment into the flood plain, including construction and development within the streamflow profile of less than 28,000 cfs (792 m3 s-1). The period when the Colorado reservoir system was filling with water constituted a time during which true exposure to climatic impacts, such as precipitation variability, did not exist. It was not representative of a new climatic regime in the basin, but only of anthropogenic interference with the flow of the river. The encroachment into the flood plain was possible because water was in storage upstream and also because the filling of Lake Powell was drawn out for almost two decades. Two decades are more than sufficient to affect societal perceptions of climate stability.

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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona FIGURE 10.15 The amount of storage Lake Mead would contain in 2020 under  hydrologic conditions like those from three past periods. FIGURE 10.16 Lake Mead average storage comparison: the average storage levels  the lake would contain if history repeats itself.

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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona shortages. Year 2000 shortage probabilities are shown in Figure 10.17. The base case hydrologic sequence (1906 to 1985) indicates no shortages. The sequence 1520 to 1599 indicates shortages of about 300,000 acre-feet (0.37 × 109 m3) at the 75 percent exceedance level. The 1600 to 1679 sequence indicates upper basin shortages of about 300,000 acre-feet (0.37 × 109 m3) at about the 35 percent exceedance level. Shortages of about 300,000 acre-feet (0.37 × 109 m3) at the 95 percent and 85 percent exceedance levels are indicated by year 2010 for the two drought sequences (see Figure 10.18). The base case sequence indicates no shortages. Similar results are shown for simulation year 2020, except that exceedance probabilities have increased in all cases, as shown in Figure 10.19. That is, shortages are present at the 98 percent and 88 percent exceedance probability levels for the two drought scenarios, while base case shortages are indicated at 50 percent exceedance probability. Average shortages for the 32-year simulation period are shown in Figure 10.20. Upper basin shortages are not shown as exceeding 300,000 acre-feet (0.37 × 109 m3), which is consistent with the prevailing Bureau of Reclamation operation policies. FIGURE 10.17 Possible upper basin water shortages in the year 2000 under hydrologic conditions like those from 1520 to 1599 and 1600 to 1679.

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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona FIGURE 10.18 Possible upper basin water shortages in the year 2010  under two historic hydrologic scenarios. FIGURE 10.19 Possible upper basin water shortages in the year 2020  under three historic hydrologic scenarios.

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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona FIGURE 10.20 The average water shortage that would occur in the upper basin if history repeats itself. Lower Basin Shortages Calculated lower basin shortages are much more severe than those for the upper basin. For simulation year 2000, no shortages are predicted by the base case hydrology, although 1.45 maf (1.8 × 109 m3) shortages are calculated at the 70 and 30 percent exceedance levels for the drought sequences 1520 to 1599 and 1600 to 1679, respectively, (see Figure 10.21). For the year 2010, the base case hydrology yielded 1.45 maf (1.8 × 109 m3) shortages at about the 50 percent exceedance level, while the 1520 to 1599 drought sequence shows the same shortages at the 95 percent exceedance level. The drought sequence 1600 to 1679 indicates 1.45 maf (1.8 × 109 m3) shortages at 85 percent exceedance, as shown in Figure 10.22. For simulation year 2020, the base case exceedance levels increase to about 60 percent, as shown in Figure 10.23. Note that for the hydrologic periods 1520 to 1599 and 1600 to 1679, the exceedance probabilities stay approximately the same. Lower basin average shortages are shown in Figure 10.24.

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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona FIGURE 10.21 Lower basin water shortages in the year 2000 under hydrologic conditions like those during the periods 1520 to 1599 and 1600 to 1679. FIGURE 10.22 Lower basin water shortages in the year 2010 under hydrologic conditions like those during the periods 1520 to 1599, 1600 to 1679, and 1906 to 1985.

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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona FIGURE 10.23 Lower basin water shortages for the year 2020 under hydrologic  conditions like those from three historic periods. FIGURE 10.24 Lower basin average water shortages that would occur  if history repeats itself.

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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona Conclusions of Drought Analysis A repeat of the hydrologic periods 1520 to 1599 or 1600 to 1679 on the Colorado River would have significant impacts on water availability and shortage allocations in the upper and lower basins. It should be noted that results presented here are not speculative what-if climate scenarios. They represent outcomes of hydrologic sequences that have been reconstructed from the historical past. Observation of Figure 10.6 indicates that the river has had above-average flow of 15.06 maf (18.6 × 109 m3) over the past eighty years as opposed to its previous average of about 13.5 maf (16.7 × 109 m3) per year. The potential for anthropogenic climatic change effects could exacerbate a situation in which the equilibrium level of the Colorado River over the long term (its mean flow) is lower than that measured over the past century. Shortages on the order of 300,000 acre-feet (0.37 × 109 m3) (under current system operation regimes) could occur in the upper basin as a result of hydrologic conditions similar to those revealed in the study of tree-ring data. Lower basin shortages on the order of 1.5 maf (1.9 × 109 m3) have probabilities of occurrence of over 90 percent and about 85 percent, respectively, for the 1520 to 1599 and 1600 to 1679 droughts. Conclusions and Implications for Climate Change Flooding of the Colorado River in the lower Colorado River basin caused substantial damage to homes and businesses in the spring and summer of 1983. Abnormal meteorological events—the greater than normal precipitation—contributed to the flooding but were not solely responsible for it. Two other factors that contributed to the flooding were: (1) the practice of maintaining a system of full reservoirs to satisfy demands from consumptive users, hydroelectric generators, and recreational interests; and (2) physical encroachment into the flood plain, made possible by the dams along the river. Although these factors could be managed physically, thereby averting the flood risks seen in 1983, there are many contrary interests that may interfere with steps to mitigate or prevent flood damage in the future. Although priorities for Colorado River management are mandated by U.S. law, such management has historically been the product of political and economic constraints created by the river's many beneficiaries. If the business-as-usual approach has been an optimistic one, then areas of future study need to focus on activities that can help

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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona mitigate negative impacts in the upper and lower Colorado River basins. These include conservation practices, the conjunctive management of surface and ground water, and systems for orderly and mutually agreeable movement of water to higher-valued uses, including temporary water transfers under drought conditions. The merits of the drought study lie in the use of data drawn from actual climatic events. While there is a degree of bias in the tree-ring reconstructed sequences, estimates of the streamflow mean are considered reliable. It is doubtful that Lake Powell and Lake Mead would be allowed to drop to minimum power pool levels before other types of shortage procedures were invoked, since in a real-time operation there is no way to forecast whether a drought has ended. This analysis distributes shortages in the upper and lower basins in a manner that would be consistent with existing laws and operating criteria. This analysis is not intended to be a statement of how shortages would actually be allocated. An area for future research is a critical assessment of potential shortage allocations and strategies that might be invoked by the Secretary of the Interior in the event of a severe, sustained drought. It is in the political arena that decisions will be made about ways of responding to, mitigating, and avoiding drought effects. The extraordinary amount of storage along the Colorado River gives the region robustness and resilience. The lesson of importance to policymakers is to develop flexibility in the operating procedures that matches the variability of the resource being controlled: namely, stremflow. For example, rigid operating procedures may need to be set aside and new procedures developed that allow for changing the current power pool and base flow requirements. REFERENCES Arizona Republic. June 22, 1983. The flood facts. P. A2. Arizona Republic. July 1, 1983. Residents who build near river should have expected trouble, watt says. P. A2. Arizona Republic. Sept. 3, 1983. $80 million flood was ''unavoidable.'' P. B1. Benjamin, J. R., and C. A. Cornell. 1970. Probability, Statistics and Decision for Civil Engineers. New York: McGraw Hill. Broadbent, R. (Commissioner, U.S. Bureau of Reclamation). 1983. Prepared statement for the U.S. House of Representatives, Com-

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