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Sidney (southwestern Nebraska) (Gale research Company, 1985). This suggests that the infiltration rates in these three regions may be similar. The Kansas Water Resources Research Institute (KWRRI) conducted a water yield study for the Solomon River Basin, approximately 90 miles southeast of the Frenchman Creek basin. The average annual precipitation and land use in the two basins are generally similar. In this study, back calculations from the area under the surface runoff hydro graph were used to estimate average CN for AMC-II conditions. The CN for terraced crop land for 2-, 5-, and 10-yr storms was found to be 60, The same CN is reported to have been used by the state SCS office for wheat-fallow land with storage terraces (KWRRI, 1981). In the hydrologic study for the Beaver Creek watershed in Kansas, which is about 50 miles southeast of the Frenchman Creek watershed, a sensitivity analysis for CN values was conducted and a CN value of 60 was found to be reasonably conservative (Dames & Moore/ URS, 1999). Reported CN values are 39, 61, and 74 for good pastures or ranges; 62, 71, and 78 for good terraced row crops; and 67, 78, and 85 for good straight row crops for hydrologic soil groups A, Band C, respectively (USDA, 1972). Somewhat similar or lower values are reported for irrigated pastures, straight row crops, and contoured crops in Contra Costa County, California (USDA, 1972). In view of the above, CN values of 60 and 64 are adopted for the sub-watersheds of Frenchman Creek. Additional computations have been made using CN values of 64 and 68 with different sets of lag times. 2.4 LAG TIMES OF SUB-WATERSHEDS In the literature, several methods and empirical equations have been proposed to estimate the times of concentration and lag times for sub-watersheds. These methods and equations have been found to result in widely different values of lag times for one and the same sub-watershed (Prakash, 1986), The USACE used the Snyder's method with the aforementioned values of CNs in their hydrologic analysis of Frenchman Creek watershed. This method resulted in the estimated peak flows shown in Table 3, which are 2 to 3 times those reported in previous studies. Table 3 Estimated Peak Flows Using Snyder's Method and eN Values of 72,81, and 85 Return Period 10-year 25-year 50-year 100-year 500-year Estimated Peak Flow (cfs) 14,005 19,295 26,006 28,363 41,011 Draft Letter Report Independent Review of Hydrologic Analysis for Frenchman Creek Contract No. DACW41-00-D-0026-000I Flood Plain Management Services Special Study Holyoke, Colorado June 18,2001 Page 4