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<br />accumulation (defined here as time scale) of 3 months, 6 months, 12 months, 24 months, and 48 <br /> <br />months. The dramatic character of climate is that as the time scale is increased the dry periods <br /> <br />with SPI below zero becomes less frequent but they last longer and the wet periods behave in the <br /> <br />same manner. At the 3 month time scale, there are approximately 90 droughts per 100 years and <br /> <br />the number decreases to about 10 between 24 months and 48 months. Time scale becomes an <br /> <br />important part of any discussion of drought. Notice for Fort Collins at 48 month, there has been <br /> <br /> <br />no drought since 1980. The real problem of drought in Colorado is that the accumulation period <br /> <br /> <br />of precipitation (time scale) for water in the form of soil moisture, snowpack, streamflow, <br /> <br /> <br />reservoir storage, and ground water are all different. <br /> <br />The definition of drought used in this analysis is based on the SPI and a time scale must be <br /> <br /> <br />identified. Dry or drought is defined as a period in which the SPI equals or is below -1.0. The <br /> <br /> <br />drought is defined as starting when the SPI first becomes less than zero and it ends when the SPI <br /> <br /> <br />again becomes greater than zero but it must get below -1 at some time. This definition is arbitrary <br /> <br />and better ones may evolve with experience. Thus, each drought has a beginning date and an <br /> <br />ending date which defines a duration. Wet periods have similar definition with positive values of <br /> <br /> <br />SPI. A current intensity of the dry and wet is also defined in terms of the SPI in Table 3. Three <br /> <br /> <br />measures of drought are the time scale, intensity and duration. The notion of time scale also <br /> <br /> <br />makes it possible to be in or out of a short-term drought when in or out of a long-term drought. <br /> <br /> <br />In Figure 5 for Fort Collins all of the periods below an SPI of -lor above 1 are shaded to allow a <br /> <br />visual identification of drought periods and wet periods. The 3 month droughts come and go <br /> <br /> <br />quickly as anyone or two month period can start or end a drought. Short term droughts can be , <br /> <br />very significant for dryland agriculture and forest fires. Longer term drought are more significant <br />for domestic and urban water supply and multiple year concerns for agriculture and natural <br />14 <br />