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<br /> <br />001512 <br /> <br />this point is suggested by data in table 3; wherein in general, although <br />not always, the larger drainage basins have the lowest unit area runoff <br />(Culler, 1961). <br /> <br />Geology <br /> <br />Geologic rock type and the character of individual fprmatio,tls can <br />cause considerabl.e variation in runoff within short distances. Large. <br />expanses of alluvium of varying porosity within a drainage basin can have <br />a notable effect on the runoff. With consolidated rocks the extent of <br />weathering, and, the amount of fracturing and faulting, the porosity of <br />the rock will be reflected in the runoff. In the upper Cheyenne River <br />Basin for example (Culler, 1961, table 8), the measured runoff from <br />areas underlain by shales was 2 1/2 times that from areas underlain by <br />Tertiary gravels, with approximately equal precipitation. Local informa." <br />tion of a similar nature may be available for relating runoff or infiltratiotl <br />capacity to geologic rock type, all of which will permit better estimation <br />of water yield from proposed stock pond areas. <br /> <br />Methods of correlating <br /> <br />Correlation analyses are coming into use as a tool in relating the <br />runoff from areas to var iables of rainfall amounts and intens ity, soil, <br />geology, vegetation cover, channel characteristics, and drainage area. <br />When data of this,\type are available for such an analysis, it is possible <br />to give proper welght to all factors affetting the runoff available for stotk- <br />pond use. These analyses will permit making better adjustments of direct <br />measured runoff to obtain estimates for new area or better estimates of <br />the frequencies of runoff from a measured drainage area. <br /> <br />A nomograph for estimating annual runoff in inches using the vari- <br />abIes of drainage area, p'recipitation over temperature, and mean annual <br />precipitation is given in figure 1. This procedure gives reasonably accur- <br />ate results for areas of the Southwest when the greater part of the annual <br />yield comes as surface runoff derived from rainstorms. It does not give <br />acceptable results in areas w\1ere runoff is derived from snowmelt. <br /> <br />Extrapolation beyond the area limits of the nomograph should not <br />be made as these relationships were derived from small watershed areas. <br />The graph on the right-hand side of the figure relating annual yield to per~ <br />cent chance of occurence is independent of the remainder of the nomograph <br />and can be used to estimate the probability of obtaining a given supply a <br />certain percent of the years. For example, if the average annual supply <br />is estimated to be 1 inch of runoff, in? years out of 10 at least, 0.26 of <br />an inch of runoff can be expected and in 9 years out of 10 at least 0.1 of <br />an inch of runoff can be expected. <br /> <br />21 <br /> <br />-}" <br /> <br />,~ <br />