Stream: Willow Creek Executive Summary

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"representative" gage station. The gage station selected for this purpose was CURECANTI CREEK NEAR SAPINERO, CO. (USGS 09125000); it has a period of record (POR) of 27 years collected between 1945 and 1972. The gage is at an elevation of 7,867.43 ft above mean sea level (amsl) and has a drainage area of 35 mil. The hydrograph (plot of discharge over time) produced from this gage includes the consumptive uses of some diversions. However, the existence of these diversions does not preclude use of the data from the gage. To make the measured data transferable to Willow Creek above the LT, the consumptive portions of these diversions were added back to the measured hydrograph. The resulting "adjusted" hydrograph could then be used on Willow Creek above the LT by multiplying the "adjusted" gage discharge values by an area ratio; specifically, the area of Willow Creek above the LT (73.03 mil) to Curecanti Creek near Sapinero, CO (35 mi2). The resulting proportioned hydrograph was itself "adjusted" (decreased) to reflect consumptive irrigation depletions upstream of the LT. The final hydrograph thus represents a distribution of flow over time that has been reduced to reflect existing human uses. (The Following discussion is based upon the US Geological Survey's Techniques of Water - Resources Investigations Series, Book 4: Hydrologic Analysis and Interpretation, Chapter A3: Statistical Methods in Water Resources (Chapter 3: Describing Uncertainty) by D.R. Helsel and R. M. Hirsch. This technical reference provides the scientific background and guidance important to the systematic interpretation of hydrologic data. The document is available online and is a valuable aid to understanding and interpreting the analyses described here.) The next step in producing a representation of the discharge at Willow Creek is to compute the Geometric Mean of the area - prorated "adjusted" data values from the Curecanti Creek near Sapinero, CO hydrograph. This step is of value because of the inherent statistical weaknesses found in any collection of data intended to measure natural stream discharge. Without getting into the details of statistical theory, it is worth noting that a set of discharge measurements is inherently inaccurate, no matter how well collected, due to the difficulties attendant to data collection, especially hydrologic data. To give deference to this fact and to increase the value of the hydrograph product of this analysis, the Geometric Means of the data were computed and plotted along with the 95% Confidence Intervals about the data. The resultant hydrograph, including recommended Instream Flow values, is displayed in Figure 1 with the data displayed in Table 2. -5-