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<br />. <br /> <br />~ 'I ~ I <br />n () ~_ :.' u 11 <br /> <br />. <br /> <br />5. <br /> <br />Incremental Flow at State Line - The incremental flow is the total flow <br /> <br /> <br />less the base flow. In other words, the incremental flow is equal to the <br /> <br /> <br />volume of water at the gage resulting from the short-term runoff event. <br /> <br />. <br /> <br />. <br /> <br />The calculated short-term runoff at the State Line gaging station exceeds <br /> <br /> <br />the short-term runoff that would occur under virgin streamflow conditions. Under <br /> <br /> <br />historical conditions in the McElmo Creek basin, ilTigation practices have <br /> <br /> <br />maintained soil moisture levels that are generally higher than in non-ilTigated <br /> <br />areas. lf rainfall occurs after a field has been fully ilTigated, a significantly <br /> <br /> <br />larger amount of runoff may be contributed than if the field is non-ilTigated. <br /> <br /> <br />Also, the existence of ilTigation drainage facilities may facilitate runoff from <br /> <br /> <br />rainfall events. Estimates were made of short-term runoff originating in the <br /> <br /> <br />lower reaches of the Mancos River basin, as described in the 1986 report by <br /> <br /> <br />Wheeler entitled, "Documentation for Mancos River Hydrology Analysis." <br /> <br /> <br />Short-term runoff per square mile in the Mancos basin was calculated to be <br /> <br /> <br />less than half of the historical runoff in the McElmo basin. The volume of <br /> <br /> <br />short-term runoff in the Mancos River basin was affected by ilTigation to a <br /> <br /> <br />much smaller extent than in the McElmo Creek basin, for the following reasons: <br /> <br /> <br />(1) ilTigation in the Mancos basin is less extensive than in the McElmo basin, <br /> <br /> <br />and (2) there are no significant transbasin diversions into the Mancos basin that <br /> <br /> <br />contribute to increased streamflows. Therefore, the estimate of short-term <br /> <br /> <br />runoff under virgin flows in the McElmo Creek basin was estimated to be equal <br /> <br /> <br />to one-half of the historic runoff set forth in Table 4. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />A monthly summary of the estimated short-term runoff at the gaging <br /> <br /> <br />station near the Colorado-Utah state line is presented in Table 5. The average <br /> <br /> <br />annual volume of short-term runoff is estimated to be about 3,100 acre-feet, <br /> <br /> <br />but annual flows may vary from about 400 to 9,600 acre-feet. It is assumed <br /> <br /> <br />that the average volume of short-term runoff at sites on McElmo Creek upstream <br /> <br /> <br />of the state line is related to the volume at the state line gage in proportion <br /> <br /> <br />to the size of the drainage basin and the average annual precipitation above <br /> <br />. <br /> <br />-17- <br /> <br />. <br /> <br />;j\,j;&: <br />