PUB00083

• Year: 1997
• Title: Historical Dry and Wet Periods in Colorado ‎(Draft Copy)‎
• CWCB Section: Water Conservation & Drought Planning
• Author: McKee, Doesken, Kleist
• Description: Analysis tool used to describe both drought and wet events for periods in Colorado.
• Publications - Doc Type: Tech Report

localized. The SSI in Figure 11 also indicates the dry conditions in 1966, 1977 and 1981. Years

with large streamflow (ST) are not necessarily associated with large snowpack (SN) since wet

springs after April 1 can lead to large streamflow (ST). The very high streamflow (ST) of 1984

was associated with such a wet spring. The regional SSI have been used to depict the timing of

wet and dry periods similar to precipitation in Table 5 but for snow in Table 12. Table 12

indicates the snow wet and dry periods do occur in the same basic time periods of the

precipitation but they are often of different durations and have more variation within the periods.

The "M's indicate a mixed response during the period and the "--" indicate a different response.

The correlation of the snowpack for all basins is given in Table 13. These values remain above

0.50 with the primary exception the southwest areas (San Juan, Animas and Dolores, SA, and Rio

Grande, RG) to the northeast (South Platte, SP). The high correlations here is a result of the

snow at the higher elevations comes from storms with westerly wind components. The

correlation of winter precipitation to snowpack is given in Table 14. The number circled are the

comparison of precipitation snowpack in the same basins. The higher correlation of the top

portions of the table across to the eastern drainages is same as in the previous table. This shows

also that the high elevation snowpack has only small correlation to the winter precipitation on the

eastern plains.

Although the winter precipitation and April I snowpack (SN) have reasonable correlations

(Table 14), the time series of SSI do not translate very well into the same dry and wet periods as

precipitation (Table 12). The reason for this lies in two aspects of precipitation. The first reason

is that the SSI is only winter while the 12 month to 24 month SPI is dominated by warm season

precipitation. The second reason is that in winter the snow is a combination of active storms that

have well defined upward vertical motion fields with area affected by the topography and periods
24