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

GISS 2 grid point, GCM runoff increases by 30%. Hydrologic modeling using temperature and precipitation
inputs from the same grid point indicate that runoff would increase only between 10 and 12%. In general,
GCMs underestimate decreases in runoff and overestimate increases when compared to corresponding
outputs from the NWSRFS hydrologic model.

Discussion of Hvdrologic Modeling Results

In the first study to analyze the impacts of climatic change on the Colorado River, Stockton and

Boggess (1979) used Langbein's relationships (Langbein and others, 1949) to estimate the effects of a 'Z C

temperature rise and a 10% decrease in precipitation. Their results suggested that streamflow in the upper

basin would decline by about 44%. Following up on that work, Revelle and Waggoner (1983) developed

a linear regression model of runoff, using precipitation and temperature as independent variables. Their

results indicated that a 'Z C temperature Increase would decrease mean annual streamflow by 29%, while

a 10% decrease in precipitation would decrease runoff by about 11%. In combination, these changes would

result in a 40% decrease in runoff, in close agreement with Stockton and Boggess's earlier result.

In contrast, our studies with a conceptual model suggest less severe impacts on runoff and a

relatively greater sensitivity of annual runoff to precipitation rather than temperature changes. A 'Z C

temperature rise decreases mean annual runoff by less than 10% in the three sub-basins studied. When

combined with a 10% decrease in precipitation, runoff decreases are on the order of 20%. These results

are comparable to other studies of arid and semi-arid basins that have used conceptual hydrologic models

(e.g. Gleick, 1987b; Flaschka, et aI., 1987), supporting Karl and Riebsame's (1989) conclusion that the

Langbein relationships overstate the role of evaporation.

In a recent study, Schaake (1990) modeled the Animas River altering temperature, precipitation,
and potential evapotranspiration independently. (In contrast, in this study, changes in PET were linked to
changes in temperature.) Schaake found that a 'Z C temperature rise and a 10% increase in PET resulted

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