Managing Water Resources in the West Under Conditions of Climate Uncertainty A Proceedings (1991) / Chapter Skim
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10. Climate Uncertainty: Implications for Operations of Water Control Systems
10. Climate Uncertainty: Implications for Operations of Water Control Systems
Pages 177-216
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From page 177... ...
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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From page 178... ...
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~.
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From page 179... ...
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.
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From page 180... ...
CROCK ~ '~'_-/ ~ ! ~:- - ~ :> 'I FIGURE 10.1 Colorado River basin upstream of the inflow to Mexico.
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From page 181... ...
. 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.
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From page 182... ...
a Lake Powell minimum objective release of 8.2 mar per year; · additional releases from Lake Powell to equalize end-ofyear 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~.
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From page 183... ...
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.
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From page 184... ...
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.
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From page 185... ...
\ _1^~~~ APR I L HAY JUNE JUL ~ 3.5 o 2 o ~n 2.5 c: G 2.0 r o z o 1 . ~ .5 o FIGURE 10.3 Relationship among Lake Powell inflows, Lake Powell outflows, and Hoover Dam releases from April through July 1983.
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From page 186... ...
Approximately 140 days would be required to discharge that quantity at a rate of 50,000 cfs (1,415m3s~~. 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.
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From page 187... ...
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 sib. With the completion of Glen Canyon Dam in 1962, streamflow variability was sharply narrowed.
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From page 188... ...
Climate Variability Perceptions of climate stability in the Colorado River basin are not borne out by historical data. In fact, the third factor that contributed to the 1983 lower basin flooding was the variability of the climate in the arid American Southwest.
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From page 189... ...
Thus, one can argue that it is not the streamflow variability in itself that caused the 1983 spring flooding and the associated damages. Flood plain encroachment and a full reservoir system, in conjunction with streamflow variability, converged to create appropriate conditions for the events of 1983.
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From page 190... ...
However, since the precipitation and runoff of 1983 were so abnormal compared with recent years, proposals for increasing storage space will not necessarily be received warmly (Broadbent, 1983~. The response to the 1977 to 1978 drought indicated that there is substantial support for maintaining full reservoirs upstream and that the flood control for lower basin residents and other interests is not given high priority by the other beneficiaries of the river.
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From page 191... ...
For this reason, this drought analysis focuses on annual operation of the two major reservoirs in the upper and lower basins Lakes Powell and Mead and related effects on water availability relative to aggregate demand. Unimpaired measured streamflow at Lee's Ferry was taken to be the "base case" hydrology and compared against reconstructed sequences of equal length from tree-ring data dating back to 1520 (D.
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From page 192... ...
The model simulates annual regulation of Lake Powell and Lake Mead. Seasonal operation is not critical in this study because of the large amount of available storage capacity compared to annual runoff.
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From page 193... ...
Drought Parameters for the Colorado River A study of the Colorado River using streamflow indicates that there is a tendency for low flows to follow low flows and for high flows to follow high flows. One possible reason for this behavior
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From page 194... ...
It is apparent that a severe and sustained drought in the Colorado River basin is not necessarily produced by a single continuous drought event but may arise from a series of events separated by 1 or 2 years. For the period of record, a series of droughts, which account for the critical period, began in 1954 and continued through 1971.
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From page 195... ...
2.954 0.333 0.769 1.103 2.290 1.216 6.609 3.304 4.692 1.617 2.753 2.339 5.023 5.089 5.211 2.294 1.486 1.945 5.117 7.410 CURRENT THRESHOLD LEVEL n , i__ Lou ~T. 1 520 1 540 1~ 1 580 1 580 1 600 YEAR FIGURE 10.7 Streamflow sequence at Lee's Ferry, 1520 to 1599.
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From page 196... ...
Index-Sequential Hydrologic Sequences In order to develop probability estimates of water supply, many U.S. agencies use the index-sequential method to create a set of realistically probable streamflows (Kendall and Dracup, 1991; U.S.
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From page 197... ...
1523 6 ODD 6.000 2 2 1528 2.300 2.300 3 5 1532 27.20 5.440 4 2 1538 8.000 4.000 5 7 1542 33.10 4.729 6 1 1552 2.100 2.100 7 1 1556 0.600 0.600 8 2 1559 6.500 3.250 9 3 1562 7.400 2.467 . 10 5 1567 9.400 1.880 11 5 1573 16.30 3.260 l 12 3 1580 13.90 4.633 13 5 1584 29.90 5.980 14 7 1590 34.60 4.943 1 15 3 1598 6.600 2.200 , TABLE 10.3 Hydrologic droughts on the Colorado River at Lee's Ferry, Arizona, from 1600 through 1679, as reconstructed from tree-ring analysis.
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From page 198... ...
Coupled with this are year-to-year water demands. Lower basin demands include all uses from Lake Mead and downstream.
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From page 199... ...
Operations of Water Control Systems TABLE 10.4 Upper Colorado River Basin Water Demands (mar)
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From page 200... ...
200 Managing Water Resources TABLE 10.5 Normal Lower Colorado River Basin Water Demands (mar)
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From page 201... ...
Conversely, there is a 10 percent probability that the reservoir storage will be equal to or less than the indicated amount. Furthermore, Lake Powell is at minimum power pool level at about the 90 percent exceedance level, as shown in Figure 10.9.
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From page 202... ...
~ 520-99 " ~ __. O ~' -- ~ -- I I -- I -'1 ~1 - I 0 0.2 0.4 0.6 0.8 EXCEEDANCE PROBABI; ITY FIGURE 10.9 The amount of storage Lake Powell would contain in the year 2000 under conditions like those that have occurred in three historic hydrologic periods: 1520 to 1599, 1600 to 1679, and 1906 to 1985.
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From page 203... ...
Lake Powell average storage levels were calculated by taking the average from 80 wrapped sequences for each simulation year. Results are shown in Figure 10.12.
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From page 204... ...
, 0.4 0.6 0.8 1520-99 ~ ; . ; a_ EXCEEDANCE PROBABILITY FIGURE 10.13 The amount of storage Lake Mead would contain in the year 2000 under conditions like those from the periods 1520 to 1599, 1600 to 1679, and 1906 to 1985.
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From page 205... ...
The minimum power pool exceedance probability predicted by the base case is greater than 95 percent. Average storage levels for Lake Mead are shown in Figure 10.16; the figure indicates a maximum between the base case and severe and sustained drought scenarios.
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From page 206... ...
. MEASURED 1906-85 1520-99 _ I I I 1 1 ~ I I I I o 0.2 0.4 0.6 0.8 EXCEEDANCE PROBABILITY FIGURE 10.15 The amount of storage Lake Mead would contain in 2020 under hydrologic conditions like those from three past periods.
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From page 207... ...
The 1600 to 1679 sequence indicates upper basin shortages of about 300,000 acre-feet (0.37 x 109 me) at about the 35 percent exceedance level.
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From page 208... ...
- , -, - , - - - . -- O 0 ~0 ~0 ~0 ~1 EXCEEDANCE PROBAB I L I TY FIGURE 10.18 Possible upper basin water shortages in 1hc year 2010 under two historic hydrologic scenarios.
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From page 209... ...
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 mar (1.8 x 109 m3)
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From page 210... ...
J 1 CD ~ 0.9 ~ o.a I 0 7 con 0.6 O.5 0.4 0.3 0.2 0.1 o MEASURED 1906-85 . , 0 0.2 1520-99 1600-79 1 1 1 1 1 1 1 0.4 0.6 EXCEEDANCE PROBABILITY 0.e 1 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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From page 211... ...
6 0.5 0.4 0.3 0.2 0.1 o T 0 0.2 1520-99 . MEASURED 1906-85 1600-79 , 0.4 0.6 0.8 1 EXCEEDANCE PROBABILITY 211 FIGURE 10.23 Lower basin water shortages for the year 2020 under hydrologic conditions like those from three historic periods.
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From page 212... ...
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 mar (1.9 x 109 m3)
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From page 213... ...
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.
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From page 214... ...
1977. Impact of the Colorado River basin and Southwest water supply.
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From page 215... ...
1975. Lake Powell Effect on the Colorado River Basin Water Supply and Environment.
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From page 216... ...
216 Managing Water Resources U.S. Bureau of Reclamation.
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