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<br />34 <br /> <br />Values in Fig. 8 are designated as factor (M). Average precipitation <br />values, 1951-1980, will be designated by PA. Thus, the 0.50 probability <br />precipitation, P(0.50) is: <br />P(0.50) = M x PA . <br />The values in Fig. 8 are all less than 1.00. They range from a minimum <br />of 0.95 at Burlington to a maximum of 0.99 at many locations. No <br />isolines of M have been drawn on the map since the range of values is so <br />small. Data points have been pl aced on the map and it is rather easy to <br />estimate M within ~ 0.01 for any location in the entire state. The <br />characteri sti c that the medi an is 1 ess than the a ve ra ge is typ i ca 1 for <br />precipitation in most parts of the world, especially dry climates. A <br />few wet years increase the average value but are offset by a greater <br />number of below average years. <br />2) Precipitation in dry years. One definition of a dry year for <br />any ,location in Colorado is a year when the precipitation total is in <br />the lowest 20% of all yearly totals. The threshold precipitation value <br />that separates a dry year (by this definition) from a near normal or wet <br />year is the precipitation total which is not exceeded 20% of the time. <br />This is known as the 0.20 nonexceedance probability. The ratio of the <br />0.20 probability precipitation value to the median (0.50) value <br />i ndi cates the magnitude difference between a dry year and a "normal" <br />year. The ratio of the 0.20 probability precipitation to the 0.50 <br />probability precipitation is designated as factor (D) and is shown in <br />Fig. 9. This factor may be used with the preceeding factors to <br />determine the 0.20 probabil ity precipitation from the average annual <br />precipitation map, P(0.20), from the following relation: <br />P(0.20) = D x M x PA . <br /> <br />. <br />