Stream: Cochetopa Creek (Lower Segement) Executive Summary

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described through the following description of the steps used to complete the evaluation for this particular stream. The first step required in determining water availability is a determination of the hydrologic regime at the Lower Terminus (LT) of the recommended ISF reach. In the best case this means looking at the data from a gage at the LT. Further, this data, in the best case, has been collected for a long period of time (the longer the better) including wet and dry periods. In the case of Cochetopa Creek — Lower there is a USGS gage record of discharge on the stream. However, the gage station is upstream from the LT. The USGS gage is COCHETOPA CREEK BELOW ROCK CREEK NR PARLIN, CO. (USGS 09118450); it has a period of record (POR) of 28 years collected between 1981 and 2009. The gage is at an elevation of 8,470 ft above mean sea level (amsl) and has a drainage area of 334 mil. The hydrograph (plot of discharge over time) produced from this gage includes the consumptive uses and out -of -basin transfers of several diversions. However, the existence of these diversions does not preclude use of the data from the gage. To make the measured data transferable to Cochetopa Creek — Lower 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 Cochetopa Creek — Lower above the LT by multiplying the "adjusted" gage discharge values by an area ratio; specifically, the area of Cochetopa Creek — Lower above the LT (373.48 mi) to Cochetopa Creek near Parlin, CO (334 mi2). The resulting proportioned hydrograph was itself "adjusted" (decreased) to reflect the consumptive irrigation depletions and out -of -basin diversions 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 Cochetopa Creek — Lower is to compute the Geometric Mean of the area- prorated "adjusted" data values from the Cochetopa Creek near Parlin, 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 1. -5-