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-5- • volume of approximately 698.2 acre-feet (0.32 acre-feet per acre). The runoff hydrograph for Williams Draw was due primarily to snowmelt. Snowmelt hydrographs typically have a diurnal effect, with flows higher during the day than during the nighttime. It is believed that the runoff volume indicated by the area under the hydrograph shown on Figure 2 is greater than the actual volume of the runoff in Williams Draw, because of the above mentioned effect and because of the relatively few measurements. A greater number of measurements would better define a smaller runoff hydrograph. The computed runoff volume for Williams Draw for the 1980 runoff season is considerably greater than the regional analysis would indicate. The line of best fit for the data (Figure 1) indicates little to no runoff for the Williams Draw Basin. For purposes of this analysis the unit average annual runoff rate for these four draws was considered to be • 0.25 acre-feet per acre which is considered more representa- tive. The runoff occurs mostly during the month of May. To support flood irrigation of a cash crop, water must be available throughout the growing season. Since the major portion of the surface streamflow occurs during the early spring, whereas the irrigation demand will extend through September, it is necessary to provide reservoir storage to capture the spring runoff for delivery throughout the season. Irrigation Water Demand Irrigated farming in the North Park area is essentially limited to pasture grass to provide grazing and hay for cattle and horses. The consumptive use of pasture grass in North Park was computed using the Blaney-Griddle technique and weather data from the Walden weather station. Crop coefficients for pasture grass were derived from a lysimeter • Leonard Rice Consulting Water Engineers, Inc.