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<br />6.0 Summary and Conclusions <br />A study to document the drought (dry) and wet periods in Colorado during this century <br />has been conducted using precipitation, snowpack and streamflow observations. Drought and <br />wet periods have been defined using the Standardized Precipitation Index (SPI) for a variety of <br />time scales including 3, 6, 12, 24 and 48 months. A range of time scales are used since the time <br /> <br />for precipitation to be transformed to water sources of soil moisture, snowpack, streamflow, <br /> <br />reservoir water, and ground water vary. The 3-6 month periods are more related to snowpack <br /> <br />and soil moisture. Time scale of6-12 months have more relationship to streamflow and reservoir <br /> <br />water which the 24-48 month time scale is related to reservoir water, streamflow and <br /> <br /> <br />groundwater. A fundamental character of precipitation is that the longer the averaging period the <br /> <br /> <br />less impact a short time period has on accumulated precipitation. Consequently, the longer time <br /> <br /> <br />scales (averaging periods) have fewer dry and wet periods but the periods last longer. In the U.S. <br /> <br /> <br />the 3 month time scale leads to approximately 65 drought periods in 100 years with an average <br /> <br /> <br />duration of 6 months. A time scale of 24 months is needed to reduce the number of droughts to <br /> <br /> <br />about 10 per 100 years with a duration of nearly 13 months. Thus the frequency of drought (wet) <br /> <br />periods, the duration, and the magnitude of precipitation deficit are all a function of the time scale. <br /> <br />Severity of drought and wet periods are not affected by time scale. <br /> <br />An analysis of the fraction ofpreciptiation observing sites in dry or wet conditions <br /> <br />indicates that approximately 60% of the state can be dry or wet at the 24 month time scale. Using <br /> <br />the percentage of sites being dry or wet, the dry and wet periods this century are: <br /> <br />26 <br />