Stream: Middle Creek (Lower Segment) Executive Summary

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a "water balance ". In concept a "water balance" computation can be viewed as an accounting exercise. When done in its most rigorous form, the water balance parses precipitation into all the avenues water pursues after it is deposited as rain, snow, or ice. In other words, given a specified amount of water deposition (input), the balance tries to account for all water depletions (losses) until a selected end point is reached. Water losses include depletions due to evaporation and transpiration, deliveries into ground water storage, temporary surface storage, incorporations into plant and animal tissue and so forth. These losses are individually or collectively subtracted from the input to reveal the net amount of stream runoff as represented by the discharge measured by stream gages. Of course, the measured stream flow need not be the end point of interest; indeed, when looking at issues of water use to extinction stream flow measurements may only describe intermediate steps in the complex accounting process that is a water balance carried out to a net value of zero. In its analysis, CWCB staff has attempted to use this idea of balancing inputs and losses to determine if water is available for the recommended Instream Flow Appropriation. Of course, this analysis must be a practical exercise rather than a lengthy, and costly, scientific investigation. As a result, staff has simplified the process by lumping together some variables and employing certain rational and scientifically supportable assumptions. The process may be 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 Middle Creek - Lower no such gage is available at the LT. In fact, there is no gage on Middle Creek. It is thus necessary to describe the normal flow regime at Middle Creek - Lower above the LT through a "representative" gage station. The gage station selected for this purpose was GREENHORN CREEK NEAR RYE, CO. (USGS 07107900), a gage with a 9 year period of record (POR) collected between 1973 and 2001. The gage is at an elevation of 7,220 ft above mean sea level (amsl) and has a drainage area of 9.56 mil. The hydrograph (plot of discharge over time) produced from this gage includes the consumptive use of some upstream diversions. However, the existence of these diversions is not a major limitation upon the use of the data from the gage. To make the measured data transferable to Middle 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 Middle Creek - Lower above the LT by multiplying the "adjusted" gage discharge values by an area ratio; specifically, the area of Middle Creek - Lower above the LT (26.58 mil) to Greenhorn Creek near Rye, CO (9.56 mi2). In this instance, due to the absence of existing significant upstream consumptive irrigation uses or transbasin diversions on Middle Creek - Lower above the LT, the resulting proportioned "adjusted" hydrograph was not further "adjusted" (decreased). Nevertheless, the final hydrograph represents the existing distribution of flow over time. {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 -4-