2014-08-13_PERMIT FILE - C1981018A (2)

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• Because flows at the higher percent duration points generally represent groundwater discharge (baseflow) conditions (lorns et al., 1965a), the differences in flow between the two stations are presumably the near-total result of groundwater inflow between the two stations. Note also that the curves again converge during extreme low flow conditions. This indicates that low flows are also controlled primarily by upper basin hydrologic conditions and implies that the inflow of groundwater between the two stations is not as reliable as that originating above Meeker. Searcy (1959) indicates that the average annual flow of a stream can be found by determining the area under the flow duration curve. This determination is presented in Table II.C-2 for the White River above Rangely, Colorado. The indicated average annual flow of 672.0 cubic feet per second (485,990 acre-feet per year) converts to an average annual yield of 3.27 inches over the 2790 square mile watershed. By contrast, the average annual flow of the White River near Meeker, Colorado was found to equal 621.3 cubic feet per second (449,320 acre-feet per year) by the same method, converting to an average annual yield of 11.07 inches aver the 762 square mile watershed. This again indicates that the upper basin supplies the majority of the water in the White River. Lorns et al. (1965) developed a method for estimating the amount of groundwater contributed to streams using flow duration curves. They found that the method applies primarily to headwater streams whose flows come largely from snowmelt and are affected little by the activities of man. Thus, the methodology • applies to the White River. According to this methodology, the average time that flow in a snowmelt stream is affected by groundwater discharge can be approximated by drawing a line tangent to the steep part of the flow duration curve of the stream and noting the departure point from the lower portion of the curve. The flow represented by the part of the curve below this departure point consists primarily of water discharged from groundwater storage. Arithmetic integration between the departure point and the 100 percent duration point is used to determine the groundwater discharge rate during baseflow conditions. The average rate of groundwater discharge during the time that streams are controlled by direct runoff (i.e., during the snowmelt season) can be approximated by the flow duration curve discharge that is equaled or exceeded 70 percent of the time. By adding this value to that obtained by integrating the flow duration curve below the departure point, the average annual rate of groundwater discharge contributed to the stream is obtained. An estimate of the long-term average groundwater discharge of the White River above Rangely, Colorado was made using the above outlined methodology and the previously developed long-term flow duration curve for the station (see Figure II.C-8). Figure II.C-9 presents the pertinent groundwater discharge variables. Computation of long-term average annual groundwater discharge is presented in Table II.C-3. • Permit Renewal #3 (Rev. 8/99) II.C-16