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The final coefficient is a weighting factor termed the "climatic characteristic'. The <br />climatic characteristic transforms the hydrologic accounting procedures into an index that <br />can be compared across varying water balances. The index ranges from about -6 in <br />extreme drought to +6 for extremely wet periods. This is what is known as the Palmer <br />Index. The Palmer Index values and corresponding conditions are shown in Table 3. <br />Table 3: PDSI values with correspondinv conditions. <br />Palmer Classifications <br />4.0 or more <br />extremely wet <br />3.0 to 3.99 <br />very wet <br />2.0 to 2.99 <br />moderately wet <br />1.0 to 1.99 <br />slightly wet <br />0.5 to 0.99 <br />incipient wets ell <br />0.49 to -0.49 <br />near normal <br />-0.5 to -0.99 <br />incipient dry spell <br />-1.0 to -1.99 <br />mild drought <br />-2.0 to -2.99 <br />moderate drou ht <br />-3.0 to -3.99 <br />severe drought <br />-4.0 or less <br />lextreme drought <br />Limitations of the Palmer Index have been identified. Many studies have identified the <br />problems with spatial comparability of the PDSI Karl (1983, 1986), Alley (1984), <br />Heddinghaus and Sabol (1991), Guttman et al. (1992) and Wells, et al. (2004). Alley <br />(1984) noted that the PDSI addresses two critical components of drought, the intensity <br />and duration, but the index uses arbitrary rules in quantifying these properties. Guttman <br />(1998) suggests the SPI to be a better index than the Palmer because it is simple, spatially <br />consistent in interpretation, probabilistic for use in risk and decision analysis, and can be <br />tailored to a time period of interest. He notes alternatively that the PDSI is complex, <br />spatially variant, difficult to interpret and temporally fixed. <br />Originally, the National Climatic Data Center began calculating the PDSI for the entire <br />country. However, Colorado was only broken into 5 areas: Platte Drainage, Kansas <br />Drainage, Arkansas Drainage, Rio Grande Drainage, and the Colorado Drainage. Given <br />the diverse climatic characteristics of Colorado due to extreme terrain variations, these <br />regions are not particularly useful for Colorado. <br />In an effort to improve the utility of the Palmer index in Colorado, Doesken et al., <br />1983, created the Colorado Modified Palmer Drought Severity Index ( CMPDSI). The <br />CMPDSI creates 25 geographical subregions of the state that are more climatically <br />similar than the original 5 regions calculated on the national scale. The CMPDSI regions <br />are shown on the map in Figure 2 and regional descriptions are given in Table 4. <br />