Managing Water Resources in the West Under Conditions of Climate Uncertainty A Proceedings (1991) / Chapter Skim
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6. Climate Change and Climate Variability: The Paleo Record
6. Climate Change and Climate Variability: The Paleo Record
Pages 71-100
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From page 71... ...
Comparison of these expected conditions with those actually revealed by the geological record has helped advance knowledge of the mechanisms driving climate change. Climate variations on time scales of decades to centuries are particularly important to consider for water resources management.
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From page 72... ...
Changes in the relative fluxes between the biological and geological components of the carbon cycle have also played a part by modifying the carbon dioxide concentration of the atmosphere. Such changes in the processes regulating global climate may be thought of as changes in the boundary conditions of the climate system.
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From page 73... ...
Hence, we can calculate global ice volume from the record of the ratio of the oxygen isotopes (~8o/~6O) in foraminiferan shells from dated ocean sediments.
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From page 74... ...
of the axis. The fit between the global ice volume record and these variations in solar receipts calculated from celestial mechanics is not perfect.
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From page 75... ...
Some possible explanations for the importance of the 100,000-year periodicity have to do with the growth, inertia, and effects of the major continental ice sheets associated with the ice ages. Polar Ice Remaining polar ice provides the most remarkable record of past climate, particularly for the last glacial cycle, which started more than 110,000 years ago.
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From page 76... ...
Third curve: Ice volume, derived from the oxygen isotope ration in formaniferan shells of ocean sediments. Lower line: local insolation at 78 degrees south.
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From page 77... ...
Middle curve: Temperature deviation from modern temperature calculated from ice D/H ratios. Top curve: Reconstructed atmospheric carbon dioxide concentration.
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From page 78... ...
The most recent ice age ended suddenly, having reached its most severe depths around 18,000 to 20,000 years ago. There were extensive continental ice sheets in both the Northern and Southern hemispheres that started to shrink approximately 14,000 years ago (Broecker and Denton, 1990~.
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From page 79... ...
and Colorado pinyon (Pinus edulis) are major features of the present vegetation of the Colorado Plateau, but both were absent in the Late Glacial period.
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From page 80... ...
80 L1J Cry LO ICY Q a)
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From page 81... ...
GCM results using the known boundary conditions (orbital position, carbon dioxide content of air, global ice volume, and sea surface temperatures) at these times produce similar patterns of regional climate.
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From page 82... ...
repeated ice ages or glaciations, there have been shorter lurches of widespread, if not global, extent. The most studied of these is an event known as "the Younger Dryas." During the rapid emergence from the last glaciation about 11,000 years ago, northern Europe and northeastern North America plunged back from interglacial conditions broadly similar to today's to the chill of an ice age (a temperature decline of about 6°C)
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From page 83... ...
In dry regions, the width or thickness of the annual ring is often controlled by the availability of moisture, whereas in cool, moist regions ring width or maximum wood density records summer temperatures. An extensive literature documents the techniques used to extract climate information from tree rings (Fritts, 1976; Hughes et al., 1982; Cook and Kairiukstis, 1990~.
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From page 84... ...
The physical basis for using tree rings as hydrologic indicators in semiarid regions is well understood. The most frequently used treering variable in drought and hydrologic studies has been the ringwidth index, a measure of departures from normal of annual diameter growth of the tree.
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From page 85... ...
and tree ring sites. Cells containing no suitable tree ring chronologies are hatched.
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From page 86... ...
For example, the Sierra Nevada of northern California appear to have been normal or wetter than normal in 1669 and 1670, when severe drought appears to have occurred in the Colorado Rockies and the central Arizona highlands. In the 1840s drought, however, the year 1847 stands out as an exception to this generalization and points to the possibility of synchronous severe drought over all major watersheds of the Southwest.
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From page 87... ...
87 so 33 ID ~ o · ..
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From page 89... ...
Basin-Specific Streamflow Reconstructions Where the water resources of a particular watershed or river system are in question, it is often useful to go beyond the mapping of tree-ring variations, as described above, to the quantitative reconstruction of specific hydrologic time series. Tree-ring reconstructions of streamflow have been conducted by the Laboratory of Tree-Ring Research at the University of Arizona for three major river-basin systems in the western United States: the upper Colorado River basin, with parts in Colorado, Wyoming, Utah, and New Mexico; the Salt and Verde rivers in central and eastern Arizona; and the Four Rivers group (Yuba, Sacramento, American, and Feather)
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From page 90... ...
90 Managing Water Resources Em, ,: ~J Four Rfwers~ it/' ~ \ t__~W |Sall-Verdo | | Rlver Basin | Colorado Basin FIGURE 6.9 Map showing locations of three areas for which regional or basin-specific tree-ring reconstructions of streamflow have been conducted by the Laboratory of Tree-Ring Research. BASIN PROPERTIES TREE-RING SI TES ~ CLIMA TE DA TA (DESIGN PROJECT i, / STREAMFLO W DA TA COLLECT SAM PLES PREPARE SAMPLES DATE SAM P LES M EAS U R E SAM P LES CALIBRATE MODEL VERIFY MODEL RECONSTRUCT FIGURE 6.10 The major steps in a tree-ring reconstruction of streamflow.
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From page 91... ...
For this reason, 10-year moving averages of the Lee's Ferry reconstruction are of particular interest. Nonoverlapping 10-year means of the reconstruction beginning in 1521, 1531, and so forth until 1951 are graphed in Figure 6.11 along with similar averages for the actual natural flow series as provided by the U.S.
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From page 92... ...
was used to begin each 10-year period in Figure 6.11 because the minimum flow in the actual natural-flow series happened to occur during the period 1931 to 1940. The minimum nonoverlapping 10-year-average reconstructed flow by this grouping is 11.0 mar for 1581 to 1590.
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From page 93... ...
An example of an area depending on several widely separate runoff sources is metropolitan southern California. The Los Angeles area draws much of its supply from two source regions: the upper Colorado River and northern California.
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From page 94... ...
The most severe sustained droughts inferred from lowest 20-year moving average reconstructed flows were as follows for the two series: . For the Colorado River at Leets Ferry9 flow dropped to 10.95 mar for the years 1579 through 1598.
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From page 95... ...
On the other hand, the aforementioned 1579 to 1598 drought on the Colorado River coincided with the third lowest nonoverlapping 20-year mean flow on the Four Rivers index. The synchrony in droughts and wet periods between the two regions can perhaps be judged more clearly from a scatter plot of the 20-year moving averages (Figure 6.13~.
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From page 96... ...
On this time scale, moisture anomalies in the two regions are apparently neither consistently synchronous nor compensating. Climate change, whether due to increasing levels of atmospheric carbon dioxide or other influences, could conceivably produce monotonic trends in decadal rainfall totals over larger regions and on longer time scales than discernible from tree-ring data or gaged streamflow records.
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From page 97... ...
Reconstructions of streamflow from tree-ring data indicate that such variations are of a magnitude that cannot be ignored in planning for management of water resources in the West. Reconstructions for the upper Colorado River basin and the northern Sierra Nevada of California both emphasize that hyro
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From page 98... ...
The modern gaged streamflow record may therefore be an unrepresentative sample for estimating water availability. The large range of departures of reconstructed flows averaged over 20year periods also suggested that hydrologic models for annual flow simulation incorporate nonstationarity in the mean.
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From page 99... ...
for Longmire, Washington, derived from tree rings.
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From page 100... ...
1975. Long-term Streamflow Records Reconstructed from Tree Rings.
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