7997

• UCREFRP Catalog Number: 7997
• Author: Nash, L. L. and P. H. Gleick.
• Title: The Colorado River Basin and Climatic Change, The Sensitivity of Streamflow and Water Supply to Variations in Temperature and Precipitation.
• USFW Year: 1993.
• USFW - Doc Type: EPA 230-R-93-009,
• Copyright Material: NO

change scenarios result in distributions of annual streamflow that are roughly log-normal (Figures 4-5).

Temperature increases cause annual flows to decrease and to consolidate, i.e. the distribution narrows, and

low-flow years become more frequent. Precipitation increases of 20% spread the distribution at the upper

end. This result is also evident in the coefficient of variation, which increases in most of the scenarios that

incorporate a 20% precipitation increase (Tables 5-8). The implication is that increased flows are likely to

increase variability on an annual basis.

The statistical significance of these results was estimated following the method used by K1emes

(1985: App. B). For each scenario, the mean and standard deviation ~,(1) of the annual streamflow series

were treated as perfect estimates of the true mean and standard deviation for the distribution of annual

flows. Subsequently, 125 series of 35-year flows were randomly generated from a log-normal distribution

defined by J.' and u. The mean and standard deviation of each 35-year series were then plotted (u versus

J.'), and the 90% confidence region was defined to be the ellipse that contained 90% of these points. These

confidence regions are illustrated for the White River model in Figure 6.

Using the above method, only three scenarios were significant for all basins at the 90%
confidence level: T + 4' C. P-20%; T + 4' C, P-10%; and T + 2' C, P-20%. For the White River, one additional
scenario, T + 2' C and P + 20%, was also significant. None of the GCM scenarios were significant at the 90%
level. The statistically significant scenarios correspond to a minimum change in mean annual streamflow
of 18% on the White River, 25% on the East River and 22% on the Animas River (Nash and Gleick, 1991).

Seasonal Runoff

Temperature increases cause peak runoff to occur earlier in the year. A temperature increase

of 2' C shifts peak runoff from June to May for the White and Animas rivers. For the East River, peak runoff

still occurs in June, although it is not nearly so exaggerated. For all three basins, the 2' C rise creates a

double peak, with high runoff occurring in both May and June. When temperature is increased by 4' C, the

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