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<br /> <br />February 2, 2007 <br /> <br />Veronica A. Sperling, Esq. <br />Moses, Wittemyer, Harrison and Woodruff, P.C. <br />P.O. Box 1440 <br />Boulder, CO 80306-1440 <br /> <br />Re: #1704 - City of Durango <br />Boating Park - Case No. 06CW09 <br /> <br />Dear Ronni: <br />The pUipose of this letter is to provide a summary of my opinions in rebutt.al to <br />the opinions disclosed by objectors in the City of Durango RICO water rights application <br />in Case No. 06CW09. <br /> <br />CWCS REPORT <br /> <br />Hydrologic Calculations - The January 4, 2007 CWCB Expert Witness Report <br />for Ted Kowalski states there are times when the c~ajmed RICO flows exceed the 401h <br />percentile flow rate. Calculations to support this opinion were not included in the report, <br />but a CWC8 staff memorandum dated June 26, 2006 offered a similar opinion that the <br />proposed RICD flows do not satisfy the 401h percentile guidelines. The CWCS staff <br />memorandum includes tables that provide data concerning the exceedance probability <br />and comparisons between modeled and requested f1O\\IS. The ewes staff also <br />prepared graphs of the exceedance probability for the proposed RICO flows. The <br />CWCB staff calculations are incorrect and oversta~e the percentile for two reasons: <br /> <br />1. The CWCS staffs calculations of the percentile flow fates were apparently made <br />tr.rough use of the Log-Pearson Type III statistical method. Although Log- <br />Pearson is an appropriate method for calculat!ng the exceedance probability, I <br />have not been able to reprocluce the CWCB staffs results. In previous boating <br />park ap;ll;cations, Randy Seaholm (eWCB staff) told me that the statistical <br />calcula~~ons used by the eWeB staff were based on a procedure described in the <br />USGS report entitled "Guidelines for Determining Flood Flow Frequency." This <br />USGS proced~re includes ~skew factors" that are designed for calculating peak <br />flood flows. These skew factors are inapplicable for calculating 40% recurrence <br />intervals, :-esulting in incorrect results. <br /> <br />3700 So~th !n~o Stree;: <br /> <br />Englewcod. ColorcGO aOll 0 <br /> <br />(303) 761-4130 <br /> <br />Ff\.x (303) 761-2802 <br />