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<br />." (>.~, ,j"'''' <br />,~);,., :,~ L) , S <br /> <br />. <br /> <br />Hydrology <br /> <br />Of the total annual basin surface water supply of 287,800 <br />acre-feet during the 1947-1965 period of study, 266,400 acre-feet <br />was diverted, 133,900 acre-feet was consumed, and 132,500 acre~feet <br />was returned to the river as unmeasured surface or subsurface inflow <br />, <br />resulting in a basin outflow of 153,900 acre-feet. <br /> <br />Basin Surplus Flows <br /> <br />. <br /> <br />Unless a basin water supply is completely regulated, it is <br />possible to have a water supply surplus and shortage within th~ same <br />year or even the same month. This condition occurs in the St.' Vrain <br />Creek basin because of untimely distribution of the native sno'wmelt <br />runoff in relation to water demands and the limited amount of basin <br />reservoir storage capacity. Almost one-half (46.7 percent) of the <br />basin 's average annual water runoff occurs in the months of May and <br />June, but only 19.3 percent of the annual water supply is available <br />in the late irrigation-season .high-demand months of August and <br />September. Diversions of natural runoff to storage averaged only <br />30,400 acre-feet annually during the study period, or a regulation <br />of only 11.9 percent of the natural basin runoff. <br /> <br />As a result of the high May and June snowmelt runoff and the <br />small amount of regulatory storage in the basin, significant amounts <br />of flow have passed through the basin unused in more than half of <br />the years of the study period. <br /> <br />The best measure of surplus flows available, without adetaileg. <br />field survey, is the miniJllUll\-flOw estimates made by the district wate* <br />commissioners. Data on monthly minimum flows of St. Vrain and Boulde# <br />Creeks were compiled from estimates recorded in the "Weekly Reports ' <br />of Ditches and Reservoirs" prepared by the water commissioners of <br />Districts Nos. 5 and 6. The average annual minimum flow of St. Vra:in' <br />Creek was 22,500 acre-feet. This surplus flow was highly erratic, <br />ranging from a high of 66,200 acre-feet in 1949 to a low of 1,200 acr~-" <br />feet reported for 4 years during the early and middle 1950's.Genera:LlYl~ <br />the minimum-flow point occurs between lifons and Longmont. ' '. <br /> <br />Minimum flows of Lefthand Creek occur just below the lowest <br />diversion on Lefthand Creek. According to the Lefthand Ditch <br />watermaster, esentially all of the remaining flow is diverted at <br />this point; therefore, no surplus flows exist on Lefthand Creek. <br /> <br />. <br /> <br />19 <br /> <br /> <br />" . <br /> <br />" <br />