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COCHETOPA CR. BELOW ROCK CR. NEAR PARLIN, CO (USGS 09118450) was selected <br />as the "representative" gage for the flow regime of Cebolla Creek — Lower at LT. This gage has <br />a longer and more current data record than the Cebolla Cr. gages and Cochetopa Creek is near <br />and morphologically similar to Cebolla Cr. Specifically, the Cochetopa gage has a 28 year POR <br />collected between 1981 and 2009. The gage is at an elevation of 8,470 ft above mean sea level <br />(amsl) and has a drainage area of 334 mil. While the hydrograph (plot of discharge over time) <br />produced from this gage includes the consumptive use of several upstream diversions, the <br />diversions are not a major limitation upon the use of the data from the gage. To make the <br />measured data transferable to Cebolla Creek above the LT, the consumptive portions of <br />diversions were added back to the measured hydrograph. The "adjusted" hydrograph that <br />resulted could be used on Cebolla Creek - Lower above the LT by multiplying the "adjusted" <br />gage discharge values by an area ratio; specifically, the area of Cebolla Creek — Lower above the <br />LT (224.7 mil) to Cochetopa Cr. below Rock Cr. nr Parlin, CO (334 mi2). The resulting <br />proportioned hydrograph was itself "adjusted" to reflect the consumptive irrigation depletions <br />(decreased) and transbasin diversions into (increased) Cebolla Creek upstream of the LT. The <br />final hydrograph thus represents a distribution of flow over time that has been reduced (and <br />increased) to reflect existing human uses. <br />{The Following discussion is based upon the US Geological Survey's Techniques of Water - <br />Resources Investigations Series, Book 4: Hydrologic Analysis and Interpretation, Chapter A3: <br />Statistical Methods in Water Resources (Chapter 3: Describing Uncertainty) by D.R. Helsel and <br />R. M. Hirsch. This technical reference provides the scientific background and guidance <br />important to the systematic interpretation of hydrologic data. The document is available online <br />and is a valuable aid to understanding and interpreting the analyses described here.) <br />The next step in producing a representation of the discharge at Cebolla Creek — Lower above the <br />LT was to compute the Geometric Mean of the area- prorated data values from the Cochetopa <br />Creek near Parlin, CO Hydrograph. This step is of value because of the inherent statistical <br />weaknesses found in any collection of data intended to measure natural stream discharge. <br />Without getting into the details of statistical theory, it is worth noting that a set of discharge <br />measurements is inherently inaccurate, no matter how well collected, due to the difficulties <br />attendant to data collection, especially hydrologic data. In this particular case, the short period of <br />record lends even greater merit to the use of this statistical tool. To give deference to this fact <br />and to increase the value of the hydrograph product of this analysis, the Geometric Means of the <br />data were computed and plotted along with the 95% Confidence Intervals about the data. The <br />resultant hydrograph, including recommended Instream Flow values, is displayed in Figure 1 <br />with the data displayed in Table 2. <br />-5- <br />