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available through the courtesy of Dr. Owen Rhea, Professor Lewis Grant, and the Color.;~do State University Atmospheric Science Department. The accuracy of interpolated soundings was assessed by Rhea et ale (1979), who compared values interPOlated from the six National Weather Service sites to actual values obtained from ~oundings in Craig and Durango duri.r.g the winter of 1977-78. They concluded that mean errors in tempprature, wind speed and relative humidity were relatively small, but large errors occurred occasionally. Occasional large errors could affect the conclusions of this study, particularly if they are systematic (more likely to occur if winds are from a certain direction, for example) rather than random. Snowcourse water content data recorded by the Snow Survey Unit, Soil Conservation Service, u.S. Department of Agriculture, were used to identify dry, normal and wet winters and to validate our study approach. * Selection and Grouping of Basins by Sub-region Some of the variation in snowfall within and across basins is caused by shadowing by upwind mountina barriers (moisture is removed each time air passes over a barrier), and some by differences in wind direction. Variability in snowfall necessitates that this analysis be conducted basin by basin. Twelve basins in the mountainous area of Colorado were selected, be<::ause computer-determined information on their snowfall patterns (discussed later) was available and we felt they are representative of the snow catchment areas in the state. The selected basins are listed in Table 1 and outlined in Figure 2. * SnOWCOllrses are relatively flat sites in the mountains where monthly measurements of snow depth and water content are taken each winter. There are presemtly approximately 200 sites in Colorado. Records at some sites extend back to 1936. 10 I I I I I I I I I I I I I I I I I I I