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

1

simulate the hydrologic processes important for river forecasting, including soil moisture, snowfall, and
snowmelt.

The NWSRFS is comprised of two linked models: a soil-moisture accounting model that

calculates gains and losses of water in the soil through various processes (e.g. evaporation, transpiration,

infiltration); and a snow accumulation and ablation model that calculates the accumulation of snow and the

contribution of snowmelt to soil moisture and runoff. The soil-moisture accounting model is a modified

version of the Sacramento Model described in Burnash et al. (1973). The Sacramento Model is widely used

and generally accepted as one of the most reliable in varied climatic conditions on several continents,

including both arid and humid regions (Nemec and Schaake, 1982). The model distributes soil moisture into

an upper and lower zone. Movement between zones is controlled by a physically based percolation

equation whose parameters are controlled by the free water in the upper zone and the soil-moisture

deficiency in the lower zone. The snowmelt model uses air temperature as the sole index to energy

exchange at the snow-air interface and is described in detail in Anderson (1976). The inputs to the model

are areal temperature and precipitation data; the output is streamflow (runoff) on a 6-hourly basis.

The NWSRFS models the Upper Colorado River Basin as a series of approximately 50 small

sub-basins that are linked together. For forecasting purposes, all of the sub-basins are modeled

simultaneously. For calibration purposes, however, each of these sub-basins is modeled separately. In this

study, we modeled three sub-basins which were selected based upon: (1) the existence of an adequate

historical streamflow record (at least 35 years), (2) a relatively high volume of streamflow, (3) streamflow

records classified as "good" or better by the U.S. Geological Survey (USGS), and (4) the presence of only

limited withdrawals and upstream regulation. These three basins are the White River at Meeker, the East

River at Almont, and the Animas River at Durango. In addition, the NWS has developed a composite model

(referred to here as the ''Two-elevation model") that divides the entire Upper Colorado River Basin into two

elevation zones and uses a limited number of data stations to predict inflow into Lake Powell. Given the

12