TIME TO EQUILIBRIUM FOR SPATIALLY-VARIABLE WATERSHEDS

An algorithm for calculating the time to equilibrium of complex waters heds is developed for spatially varied watershed characteristics and e xcess rainfall intensity. The wave travel time through the flow path o riginating from the hydraulically most remote point in the basin essen tially determines the equilibrium time of the system. The wave travel time is computed for two distinct phases: overland flow and channel fl ow. The hydraulically most remote point is stationary for all spatiall y uniform rainstorms but may vary for spatially distributed storms. Th e time to equilibrium varies inversely with rainfall intensity raised to the power 0.4. Using a raster-based approach, algorithms were devel oped to determine the distance-drainage area relationship and to perfo rm the numerical integration of the time to equilibrium. Although appr oximate, the algorithm provides maps showing the isochromes of surface runoff and travel time to the outlet, along with time-area histograms for uniform or spatially distributed rainstorms. Isochrones determine the response time required for water, dilute suspensions and pollutan ts to reach the watershed outlet under complete equilibrium. The effec ts of infiltration in delaying the time to equilibrium are significant only when delta/K is large.