Stream: Willow Creek Executive Summary

• Description: Stream: Willow Creek Executive Summary
• State: CO
• Water Division: 4
• Title: Stream: Willow Creek Executive Summary
• Water Supply Pro - Doc Type: Report/Study

the model, over 250% of the measured discharge or under 40% of the measured discharge may
not give an accurate estimate of the necessary instream flow required.
Table 1: Data
Party
Date
Q
250 % 40%
Summer 3/3
Winter 2/3
BLM
06/04/2007
0.49
0.2 -1.2
Out of range
0.74
BLM
06/04/2007
0.26
0.1 -0.7
Out of range
0.47
BLM
06/04/2007
0.38
0.2 -0.9
Out of range
0.33
BLM
06/04/2008
5.09
2.0 -12.7
2.34
Out of range
The summer flow recommendation, which met 3 of 3 criteria and is within the accuracy range of
the R2CROSS model is 2.3 cfs. The winter flow amount, which meets 2 of 3 criteria, is 0.5 cfs.
The winter flow recommendation was derived by averaging the results of the data sets.
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
Willow Creek no such gage is available at the LT. In fact, there is no gage on Willow Creek. It
is thus necessary to describe the normal flow regime at Willow Creek above the LT through a
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