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<br /> <br />water frequently are subject to contamination from the surface. <br />The data show that 17 supplies derived partly or wholly from <br />surface-water sources are not chlorinated. Of the 5~ well sup- <br />plies not chlorinated, several tap shallow aquifers that are <br />relatively unprotected from surface contamination. <br /> <br />Several small communities and a few having populations in <br />excess of 1,000, do not have community-operated water systems. <br />The chief reason appears to be the ease of obtaining individual <br />supplies from wells. por example, several communities in the <br />San Luis valley are underlain at shallow depths by water-bearing <br />materials that can be tapped by private wells at a cost of less <br />than $100. <br /> <br />A subsequent report will include additional conclusions <br />based on the data in this report. The conclusions will be aimed <br />chiefly toward showing prOblems related to the availability and <br />improved management of supplies. <br /> <br />SURFACE WATER <br /> <br /> <br />Surface water is the basic supply of all but one of Colorado's <br />cities having populations greater than 10,000 and of nearly all <br />towns in the mountain areas. Surface-water systems serve about <br />82 percent of the total population of the communities covered <br />by this investigation. Most available surface water in the State <br />has been appropriated for irrigation, and although urban needs <br />for water are small compared to the total resource, many commu- <br />nities face acute shortages. The few remaining unappropriated <br />rights are insufficient to alleviate the shortages, especially <br />along the eastern slope of the Rocky Mountains, where there is <br />a marked imbalance among available surface water, population, <br />and irrigable land. <br /> <br />Runoff <br /> <br />Runoff is the part of precipitation that appears in surface <br />streams. An average of about 16 inches of precipitation falls <br />yearly in Colorado: of which less than 3 inches appears as runoff. <br />The rest is consumed largely by evapotranspiration. <br /> <br />Figure 3A shows the distribution of annual runoff in Colorado. <br />Comparison of figures 3A, 3B, and 3C indicates that there is a <br />general relation among annual runoff, annual precipitation, and <br />altitude in the State. In general, runoff in Colorado originates <br />mainly above 7,000 feet, the amount per unit area increasing <br />rapidly with altitude which is the most important factor affecting <br />runoff in the State. <br /> <br />7 <br />