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

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precipitation, and present streamflow information are used as inputs into the model, and future streamflow

forecasts are produced as outputs. For the purposes of this study, however, the model was run in

calibration (or simulation) rather than forecasting mode. To calibrate the model, past records of temperature

and precipitation are correlated with concurrent streamflows. Independent parameters (associated with soil

moisture accounting, snow ablation and snowmelt, and streamflow routing) are subsequently modified to

improve the fit of simulated to observed data. By altering historic temperature and precipitation data, future

climate scenarios and their resulting streamflows can also be simulated. The comparison of simulations

obtained from actual historic data and altered data provides information about the changes in streamflow

that might be expected from changes in climatic conditions.

Model Calibration

The standard test for credibility of a given hydrologic simulation model is verification with data

not used in model calibration. In many cases, however, the data set is too limited to permit this type of

testing. Because the model used in this study is a forecasting model used daily for operational purposes,

all model testing and calibration has been done by the National Weather Service in Salt Lake City. The

entire 35-year record (1949 to 1983, inclusive) was used to calibrate each of the sub-basins.

The World Meteorological Organization model intercomparison program suggests that various

criteria be used to test general purpose streamflow models, including differences between simulated and

observed flows, mean flow, characteristics of maximum and minimum flows, and seasonal characteristics

(WMO, 1985; WMO, 1987). A set of these criteria are evaluated for the NWSRFS model calibration runs.

The results are summarized in Table 3 and are presented in detail in Appendix A. In all cases, the model

has a fairly good fit. The analysis of daily streamflow data for all models shows a consistent bias of

overpredicting low flows and underpredicting high flows. In general, however, the model appears to perform

satisfactorily so long as predicted flows are within about 20% to 25% of the mean.

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