10 ‎(2)‎

• Board Meeting Date: 11/18/2007
• Description: ISF Section - Uncontested 2007 Instream Flow Appropriations
• Board Meetings - Doc Type: Memo

Biologic Flow Recommendation
The summer flow recommendation, which meets 3 of 3 criteria and is within the accuracy range
of the R2CROSS model is 2.6 cfs. The winter flow recommendation, which meets 2 or 3 criteria
and is within the accuracy range of the R2Cross model is 2.2 cfs. These recommendations were
derived by averaging the results of the two data sets. It is our belief that recommendations that
fall outside of the accuracy range of 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.
Hydrologic Data
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. The hydrograph below was derived from data collected by the
USGS stream gage for Willow Creek Near Dixon, WY (ID #09258000), which has a drainage
area of 24 square miles (See Gage Summary in Appendix). The total drainage area of the
Willow Creek drainage is approximately 49 square miles. The period of record for this gage was
1954-1993, the period of record used by staff in their analysis was 1954-1993, or 41 years of
record. In addition, since the Willow Creek Ditch is the calling right on the stream and the
downstream terminus for the proposed instream flow water right, the mean monthly diversion
records for the Willow Creek Ditch were examined (Table 3). The mean monthly diversions
provide an additional source of data and affirmation that the summer flow recommendation of
2.6 cfs is available for appropriation on an average basis.
Table 2 and Figure 1 show that the summer flow recommendation of 2.6 cfs is available at least
50% of the time during the time period of April 1 through August 31. The winter flow
recommendation of 2.2 cfs was reduced to 2.0 cfs during the time period of September 1 to
March 31 based on limited water availability.
Table 2: Estimated flow on Willow Creek
Exceedences Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
1 % 9 7.04 40.6 106 118.2 90 58.6 11 11 11 7.5 6
5% 5.1 5.5 17 63 80 73 35 8.4 6.4 7.8 5.8 4.6
10% 3.6 4 10 46 65 62 25 6.7 4.4 6 4.5 3.9
20% 3 3.3 6.7 30 49 51 15 4.8 3.1 4 3.4 3
50% 2.2 2.5 3.5 13 26 33 6 2.6 2 2.5 2.3 2.2
80% 1.6 1.7 2.5 6 14 15 2.8 1.3 1.2 1.7 1.5 1.6
90% 1.3 1.4 2 4.4 9.5 9.4 2 1 0.8 1.3 1.2 1.2
95% 0.8 1.3 1.8 3.6 7.2 5 1.3 0.34 0.34 1 1 1
99% 0.3 0.3 1.4 2.7 4 2.3 0.424 0 0 0.5 0.75 0.8