Colorado River Basin Water Management Evaluating and Adjusting to Hydroclimatic Variability (2007) / Chapter Skim
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3 Climate and Hydrology of the Colorado River Basin Region
3 Climate and Hydrology of the Colorado River Basin Region
Pages 73-111
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From page 73... ...
Scientific research on the Colorado River basin's climate and hydrologic systems has included measurements of the river's flow, long-term studies of climate and river hydrology, reviews of statistics associated with temperature and precipitation extremes, and studies of connections to regional and global climate systems. In the 20th century, long-term water management and planning in the region generally relied upon the gaged record of Colorado River flows; specifically, great reliance was placed on measurements made at Lees Ferry, supplemented by data recorded at other stations on the mainstem and on tributary streams.
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From page 74... ...
These new dendrochronological reconstructions have stimulated heightened interest in questions regarding the rarity and recurrence of drought conditions across the region. This chapter discusses fundamental features and dynamics of Colorado River basin climate (including climate trends and future climate scenarios)
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From page 75... ...
For these reasons, winter precipitation over the high-elevation portion of the upper basin plays an important large role in generating runoff and streamflow. Warm season precipitation plays a different role in the basin's hydrology.
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From page 76... ...
If a season of winter precipitation and water storage is "lost" because of drought conditions, there will be little opportunity to replenish supplies until the following winter. The Tropical Pacific and ENSO Ocean temperature patterns that have the greatest influence on Colorado River basin climate are in the tropical Pacific in a band that straddles the equator between Peru and the International Date Line.
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From page 77... ...
Jacob Bjerknes identified the equatorial Pacific as the source of climate variability associated with the Southern Oscillation. The winter storm track over the eastern Pacific Ocean shifts southward during El Niño episodes, often causing wet winters in the southwestern United States and dry winters in the Pacific Northwest and northern Rockies.
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From page 78... ...
. The main source regions of Colorado River basin precipitation and streamflow -- the mountains of Colorado, Wyoming, and northeastern Utah -- are not greatly impacted by ENSO events.
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From page 79... ...
Variations in SSTs in the tropical Pacific may herald changes in jet stream patterns, strength and track of Pacific winter storms, and future water supply conditions across the Colorado River basin. Different patterns may accentuate or counteract each other.
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From page 80... ...
variability. Figure 3-1 shows interannual precipitation variability across the upper Colorado River basin, spatially averaged over the basin upstream of Lees Ferry and aggregated to annual resolution (Kittel et al., 1997; updated data from ftp://ftp.ncdc.noaa.gov/
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From page 81... ...
19 18 17 16 15 14 13 12 11 10 9 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 Water Year ( End Year ) WRCC CEFA NOAA Westmap FIGURE 3-1 Annual precipitation for the Colorado River basin above Lees Ferry, 1895-2005.
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From page 82... ...
54 53 52 51 50 49 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 WRCC / CEFA Year NOAA Westmap FIGURE 3-2 Annual average surface air temperature for entire Colorado River basin, 1895-2005. NOTE: Red: annual values.
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From page 83... ...
Both in terms of absolute degrees and in terms of annual standard deviation, the Colorado River basin has warmed more than any region of the United States -- a fact that should be of great interest throughout the region. This trend continued through the first half of 2006.
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From page 84... ...
84 FIGURE 3-3 2000 to 2005 temperature departures from 1895-2000 average. Left: Shown in temperature units (°F)
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From page 85... ...
. Other studies of snowpack over the western United States find that declining trends in snow accumulation likely are not solely a manifestation of precipitation and snowfall variability, but rather reflect (at least in part)
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From page 86... ...
. To a first approximation, no appreciable trend in annual Colorado River basin precipitation has been detected (Figure 3-1)
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From page 87... ...
Taken as a whole, these future projections and past trends point to a strong likelihood of warmer future climate across the Colorado River basin. FIGURE 3-4 Nine-year moving average of observed annual air temperature averaged over the Colorado River basin (1950-2001)
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From page 88... ...
Concerns regarding the implications of future climate changes -- especially warming -- for Colorado River flows date back to at least the 1970s. Since then, the effects of the listed factors on Colorado River streamflow have been incorporated in different ways by several different hydrologic studies and papers.
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From page 89... ...
water resources considered the implications of changes in climate on runoff in the Colorado River basin (Gleick, 2000)
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From page 90... ...
from the National Center for Atmospheric Research forecasts little change in annual precipitation but shifts some winter precipitation to the summer months. In the Colorado River basin, summer precipitation is on average less efficient in generating runoff (because of higher evaporative losses)
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From page 91... ...
Collectively, the body of research on prospective future changes in Colorado River flows points to a future in which warmer conditions across the region are likely to contribute to reductions in snowpack, an earlier peak in spring snowmelt, higher rates of evapotranspiration, reduced late spring and summer flows, and reductions in annual runoff and streamflow. Earlier studies suggested substantial decreases in Colorado River annual flow volumes over the next century; more recent studies
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From page 92... ...
Results from these numerous hydroclimatic studies are not unanimous, and all projections of future conditions contain some degree of uncertainty. Nevertheless, the body of climate and hydrologic modeling exercises for the Colorado River basin points to a warmer future with reductions in streamflow and runoff.
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From page 93... ...
. To assess the accuracy of gage records, the USGS publishes accuracy information in annual Water Resources Data Reports rating the data records (part or whole)
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From page 94... ...
Discharge readings at Lees Ferry measure the combined runoff from the upper part of the Colorado River basin, which includes the upper Colorado, Green, and San Juan rivers (Topping et al., 2003)
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From page 95... ...
Climate and Hydrology of the Colorado River Basin Region 95 TABLE 3-1 Select Colorado River Gages Station ID Station Name Period 9011000 Colorado River near Grand Lake, CO 1904-1986 9019500 Colorado River near Granby, CO 1908-present 9034500 Colorado River at Hot Sulphur Springs, CO 1904-1995 9058000 Colorado River near Kremmling, CO 1904-present 9070500 Colorado River near Dotsero, CO 1940-present 9072500 Colorado River at Glenwood Springs, CO 1899-1966 9085100 Colorado River below Glenwood Springs, 1966-present CO 9095500 Colorado River near Cameo, CO 1933-present 9106000 Colorado River near Palisade, CO 1902-1933 9153000 Colorado River near Fruita, CO 1911-1923 9163500 Colorado River near Colorado-Utah state 1951-present line 9180500 Colorado River near Cisco, UT 1913-present 9188500 Green River at Warren Bridge, near 1931-present Daniel, WY 9191000 Green River near Daniel, WY 1912-1932 9216500 Green River at Green River, WY 1895-1939 9217000 Green River near Green River, WY 1951-present 9315000 Green River at Green River, UT 1894-present 9335000 Colorado River at Hite, UT 1947-1958 9379500 San Juan River near Bluff, UT 1914-present 9379910 Colorado River below Glen Canyon Dam, 1965-present AZ 9380000 Colorado River at Lees Ferry, AZ 1921-present 9402500 Colorado River near Grand Canyon, AZ 1937-present 9421500 Colorado River below Hoover Dam, AZ-NV 1934-present 9423000 Colorado River below Davis Dam, NV-AZ 1905-1907, 1949 present 9424000 Colorado River near Topock, AZ 1917-1982 9429490 Colorado River above Imperial Dam, CA- 1934-present AZ 9429500 Colorado River below Imperial Dam, CA- 1934-present AZ 9521000 Colorado River at Yuma, AZ 1904-1965, 1983 9521100 Colorado River below Yuma Main Canal 1963-present WW at Yuma, AZ 9522000 Colorado River at NIB AB Morelos Dam 1950-present near Andrade, CA SOURCE: Harding (2006)
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From page 96... ...
These depletions can be substantial: for exam ple, estimated annual average evaporation from Lake Powell is on the order of 0.5 million acre-feet, while at Lake Mead it is on the order of roughly 0.8 million acre-feet (USBR, 1977; 1981; 1991; 1998; 2002; 2004)
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From page 97... ...
When the Colorado River Compact was being negotiated, participants had only two to three decades of stream gage data, and only from a small number of stations. Over time, the Lees Ferry gaged record accumulated more and more years of flow data, and the
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From page 98... ...
. Although sophisticated techniques are being employed to help augment data gathered from stream flows, the network of gaging sta tions across the Colorado River basin (especially gages with long-term flow data)
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From page 99... ...
An important question that accompanies the use (exclusively) of the gaged record for river basin planning decisions is how representative the past record is of expected future conditions.
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From page 100... ...
It includes a comparison of reconstructions of upper Colorado River basin streamflow and discusses the features of the most recent Lees Ferry reconstructions, along with implications for sub-basin flow relationships. Scientific Basis of Streamflow Reconstructions Tree-ring reconstructions of past hydrologic conditions are based on the principles of dendrochronology, the science and study of dated tree rings (Fritts, 1976)
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From page 101... ...
The fact that trees are imperfect recorders of hydrologic variability is an inherent source of uncertainty and is reflected in the inability of tree ring-based models to account for 100 percent of the variance in the gaged record (e.g., Brockway and Bradley, 1995)
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From page 102... ...
Reconstructions of Colorado River Flows at Lees Ferry, Arizona As methods for tree-ring-based reconstructions have evolved, the set of streamflow data from the Lees Ferry gage has been a focus of reconstruction studies. Several reconstructions for Lees Ferry flow have been generated, first by Stockton and Jacoby (1976)
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From page 103... ...
These three points have important implications for water management decisions for the Colorado River basin and are revisited in the Commentary section at the end of this chapter.
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From page 104... ...
104 TABLE 3-2 Lees Ferry Reconstructions e Reconstruction Calibration Source of Gage Data Chronology Regression Variance Reconstruction Long-Term Mean c d Years Type Approach Explained Years (MAF)
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From page 105... ...
Some differences in the Lees Ferry reconstructions may be attrib uted to the tree-ring and gage data, including the length of the calibra tion period. Differences may also result from choices made in statis tical methods when processing tree-ring data, which can affect the characteristics of the chronology and, in turn, affect the reconstruction (see Meko et al.
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From page 106... ...
. In summary, differences among Lees Ferry reconstructions can likely be attributed to several factors.
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From page 107... ...
Questions relevant to drought and management in the upper Colorado River basin that can be addressed are: 1. How does the drought of the early 2000s compare to other past droughts of similar duration?
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From page 108... ...
Studies of extended periods of streamflow, however, considered along with other high-resolution climate reconstructions, have the potential to increase scientific understanding of the links between ocean/atmosphere circulation and Colorado River basin water supply. COMMENTARY A steady warming trend of about 2°F has been under way over the past three decades across the Colorado River basin.
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From page 109... ...
Reduced streamflow would also contribute to increasing severity, frequency, and duration of future droughts. In the context of multidecadal and multicentury hydroclimatic patterns across the Colorado River region, the Lees Ferry gaged record represents a chronologically limited sliver of information.
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From page 110... ...
Given both natural and human-induced climate changes, fluctuations in Colorado River mean flows over long-range time scales are likely to continue into the future. The paleoclimate record reveals several past periods in which Colorado River flows were considerably lower than flows reflected in the Lees Ferry gaged record, and that were considerably lower than flows assumed in the 1922 Colorado River Compact allocations.
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From page 111... ...
Nevertheless, the gage record of river flows will remain an important source of information for scientists and water resources planners. Measured values of streamflow of the Colorado River and its tributaries provide essential information for sound water management decisions.
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