Stream: Tabeguache Creek (Upper Segement) Executive Summary

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The summer seasonal flow recommendation, which meets 3 of 3 criteria and is within the accuracy range of the R2CROSS model is 4.75 cfs. The flow recommendations of 1.9 cfs (July 1 — November 30) and 1.6 cfs (December 1 — March 31) were lowered due to water availability constraints. It should be noted that the timing of the recommended flows was adjusted as a result of stakeholder input. Hydrologic Data and Analysis After receiving the cooperating agency's biologic recommendation, the CWCB staff conducted an evaluation of the stream hydrology to determine if water was physically available for an instream flow appropriation. This evaluation was done through a computation that is, in essence, 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 Tabeguache Creek — Upper no such gage is available at the LT; although flows have been gaged on Tabeguache Creek. A gage was located in the upstream reaches (Tabeguache Creek near Nucla). However, the data from this gage was of limited value being both old and a short Period of Record (POR). While the use of some of this data might be possible, it made sense to examine nearby drainages for gages with better data records that could serve as "representative" gage stations. SAN MIGUEL RIVER AT NATURITA, CO (USGS 09175500) was selected as the "representative" gage for the flow regime of Tabeguache Creek — Upper at LT. This gage has a longer and more current data record than the Tabeguache Cr. gage and the San Miguel River gage is near and morphologically similar to Tabeguache Cr. Specifically, the San Miguel gage -4-