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not for floodplain delineation information and land use studies. Policv Statements: The minimum number of years (length of record) required for obtaining acceptable! reasonable values from discharge-frequency analysis is 25 years. A length of record of 10 years is acceptable only when compared with a similar watershed and estimates are made using hydrologic models and precipitation records. Expected Probability is considered only for the design of structural work, not for floodplain delineation information allld land use studies. Any negative skew coefficient must be substantiated for a basin study. Where a later floodflow-frequency analysis pJrOvides discharges that differ from those established previously in studies on the same stream, the latest discharges shall be adopted if the previously established discharge do not fall within the 95 and 5 percent confidence limits (90 percent confidence interval) of the most recent estimates. The previously established discharges shall be adopted of they fall within 75 and 25 per~ent confidence limits (50 percent confidence interval) of the most recent estimates. Where the previously established discharges fall between 50 and 90 percent confidence intervals of the most recent estimates, the situation shall be presented to the Board for resolution (2 CCR 408-1 Rnle 5.B.6). 5.6.2 Regional Regression EQuations/Curve2 The use of regional flood hydrology methods are generally acceptable for basins where only minor or moderate development is exptX:ted over the next ten years, The most commonly used procedure is the computation of regression equations/curves which is based on real-time data which is available from neighboring drainage basins, These data may be derived from statistical methods or previously published hydrologic values, ~ .. Policv statement: A minimum of five data points are required for an acceptable regional regression analysis. 5,6,3 Rainfall Runoff Models 7