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<br />I. I ntroducti on <br /> <br />, <br /> <br />Isohyetal maps of average annual precipitation have long been and <br />continue to be a backbone and starting point for many climatic, <br />hydrologic and basic water resource and land use studies. In Colorado <br />there have only been a few satisfactory attempts during the past several <br />decades to complete such a map. The most recent and most complete <br />attempt to date was the "Normal Annual (and Summer and Winter Season) <br />Precipitation Map of Colorado, 1931-1960" completed during the 1960s by <br />the U. S. Weather Bureau. This two map set has proven credible in <br />depicting, with local accuracy, the. great diversity of the precipitation <br />climate of Colorado. <br />The 1931-60 ~ap set, which was printed by the U. S. Geological <br />Survey and distributed by the Colorado Water Conservation Board, has <br />been out of print since the early 1970s. Although still considered <br />relatively accurate. the years have gradually taken a toll on. the <br />credibility of this product. Research results and computer simulations, <br />such as the orographic precipitation model of Rhea (1978), have <br />presented justification for challenging the accuracy of the original <br />analysis in portions of the Colorado Rockies. Also, considerably more <br />precipitation data have been collected since 1960 improving the data <br />base for the analysis. <br />In 1982, the Colorado Climate Center, with funding from the <br />National Oceanic and Atmospheric Administration National Climate Program <br />Office and the Colorado State University Agricultural Experiment <br />Station, initiated the effort to update the Colorado precipitation map. <br />The interagency Colorado Hydrometeorological Conmittee provided peer <br />review throughout the project. Orafting and printing services were <br />donated by the U. S. Geological Survey. <br /> <br />; <br />