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

,

not accounted for in a model calibrated on current climatic conditions. Nevertheless, the short time-step

used (6-hourty) implies that the model's storage behavior beyond calibrated conditions is only for limited

periods and should have a relatively minimal impact on average annual runoff outputs. And, to the extent

that studies focus on relatively short-term and "moderate" changes in climate, significant changes in model

parameters would not be expected (Nemec and 5chaake, 1982).

Another assumption of the model is that water withdrawals are not significantly affecting runoff.

Because withdrawals are not accounted for in the model directly, they are implicit in the values chosen for

other parameters. Thus, as withdrawals increase in a particular basin, the calibration of all parameters for

that basin change to account for the decrease in streamflow. So long as withdrawals remain a relatively

small factor in basin streamflow, this omission should not be critical to the model's ability to simulate

different climate scenarios. To minimize this problem, sub-basins were selected in which withdrawals were
known to be relatively minor.s

A further weakness of the Two-elevation model is that model parameters have been averaged

spatially. In general, the strength of the NWSRFS model is its use of physically based parameters to

describe hydrologic processes. Thus, while the exact value of parameter may not be known, a reasonable

range of values can be determined from existing data. This becomes increasingly difficult as the scale of

the model is Increased. For example, it is much more problematic to choose infiltration parameters for the

entire Upper Colorado River Basin than for a small (and presumably more homogenous) sub-basin. Thus,

while the Two-elevation model may "fit" the data as well as any sub-basin model, these results should be

treated more skeptically. Nonetheless, because of the time and resources required to study the more than

50 sub-basins, the Two-elevation model was included in this study because it provides the only means of

assessing the potential impacts of climate change on the entire Upper Colorado River Basin.

8rhe inability to account for withdrawals explicitly is of greater concern for the Two-elevation model
because substantial withdrawals are occurring.

16