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Table 6. Palmer Drought Severity Index Classifications <br />PDSI Value Classification <br />4.00 or more Extremely wet <br />3.00 to 3.99 Very wet <br />2.00 to 2.99 Moderately wet <br />1.00 to 1.99 Slightly wet <br />0.50 to 0.99 Incipient wet spell <br />0.49 to -0.49 Near normal <br />-0.50 to -0.99 Incipient dry spell <br />-1.00 to -1.99 Mild drought <br />-2.00 to -2.99 Moderate drought <br />-3.00 to -3.99 Severe drought <br />-4.00 or less Extreme drought <br />The National Climatic Data Center (NCDC) has developed an archive of monthly PDSI values <br />from 1895 to the present for every climate division of the U.S. Within Colorado, there are five <br />NCDC climate divisions, with Division 5 corresponding exactly to the portion of the Rio Grande <br />basin that lies in Colorado. Monthly PDSI values for the Rio Grande basin, over the last 100 <br />years, are shown in Figure 3. <br />Palmer Drought Severity Index <br />NCDC Colorado Division 5 <br />Rio Grande Basin <br />10 <br />8 <br />6 <br />N 4 <br />D <br />a z <br />~' o <br />~ -z <br />~ -4 <br />~ 10-yr Average <br />-6 <br />-8 <br />-10 <br />~n o ~n o ~n o ~n o ~n o ~n o ~n o ~n o ~n o ~n o ~n <br />O O O N N M M V V l.() L() CO CO I~ r W W O O <br />W O O O O O O O O O O O O O O O O O O O O <br />~ <br />Calendar Year <br />Figure 3. Monthly PDSI Values for the Rio Grande basin in Colorado <br />As can be seen in Figure 3, extreme droughts (PDSI -4.0 or less) have occurred three times in the <br />basin since the turn of the century. The most recent extreme drought ended in 1957. The <br />drought of longest duration and depth took place in the 1930s. The ideal model study period <br />would be one that captures both wet and dry periods. <br />C:Acdss\Task2-3.doc Identify Study Period April 16, 1999 Page 11 of 18 <br />