The shape of the flood hydrograph from a catchment area is a function of the hydrologic input to that region and of the catchment characteristics, such as area, shape, channel, and overland
slopes, soil types and their distribution, type and extent of vegetative cover, and other geological and geomorphological watershed features. One of the primary measures of the relative timing of hydrologic events is basin lag t1. Basin lag is defined as the time between the center of mass of the rainfall excess producing surface runoff and the peak of the hydrograph produced. The lag time is influenced by such parameters as the shape and average slopes of the drainage area, the slope of the main channel, channel geometry, and the storm event pattern. Various investigators have proposed relationships predictive of basin lag, but Snyder's equation , based on the data from large natural watersheds, is the most widely used and adapted by others t1=Ct(LcaL)P3
where t^basin lag (hour), Ct=coefficient depending on basin properties, Lca=distance (miles) along the main stream from the base gage to a point opposite the basin centroid, and L=maximum travel distance (miles) along the main stream (1 mile=1609 m). The Soil Conservation Service  defines t1 as i1=0.64
where tc (hour) is the time of concentration, another primary measure of the relative timing of hydrologic events. The time of concentration is usually defined as the sum of the overland travel time from the furthest basin point and the channel travel time to the outlet of concern.
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