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The recommended types of synthetic unit hydrographs for use in estimating inflow design floods in Colorado are shown in Table 1. The Clark unit hydrograph is recommended for urban watersheds and smaller (less than 50-square mile) predominantly agricultural watersheds. DimE~nsionless unit hydrographs and S-graphs are recommended for Rocky Mountain, Great Plains and Colorado Plateau watersheds. The Clark unit hydrograph can be an acceptable alternative for small (less than 10-square mile) watersheds of those types and when verification studies indicate that the Clark unit hydrograph yields more reasonable results than the dimensionless or S-graphs. TabVe 1 Selection of synthetic unit hydm~ographs for use in estimating inflow design €loods in Colorado VA/atershed Dimensionless Unit Hydr~graph ~lark Unit Type or S-graph I~ydrograph RocN,y Mountain Preferred Acceptable Alternative Gre~~t Plains Preferred Acceptable Alternative Colorado Plateau Preferred Acceptable Alternative Agric~ultural Not Recommended except for agricultural Preferred watersheds larger than 50 square miles Urb~~n Not Recommended Preferred CVark Unit Hvdro4raah Parameter Estimation The ~~lark unit hydrograph is a three parameter method; time of concentration (T~), storage coefficient (R), and the time-area relation. Equations for estimating T~ and R were estimated from an analysis of all relevant watershed data (Sabol, 1987 and 1993), and synthetic time-area relations are available. Time of Concentration: Time of concentration is the travel time, during the corresponding peric~d of most intense rainfall excess, for a floodwave to travel from the hydraulically most distant point in the watershed to the point of interest (concentration point). Three time of cona~ntration (T~) equations are recommended depending on the type of watershed: Rocky Mountain, Great Plains and Colorado Plateau T~ =2.4A~L2sLcs S-.z 19 March 2007 7