An isolated 3-h rainfall event on a small catchment produces a hydrograph peak and point of inflection on the falling limb of the hydrograph at 7 hours and 8.5 hours respectively, after the start of the rainfall. Assuming, no losses and no base flow contribution, the time of concentration (in hours) for this catchment is approximately

Concept:Time of concentration: It is the time required for surface runoff to travel from hydraulically most distant point of the basin to the outlet.Time of concentration depends upon the SLOPE, the CATCHMENT characteristics and the flow path.For a hydrograph analysis, time of concentration is defined as the time duration from the end of excess rainfall to the point of INFLECTION.For small catchment areas, lag time (tp) is equal to time of concentration (tc)tc = Hydrograph peak time - Half of rainfall durationwhere,tp = Lag time, TB = Base timeQp = Peak ordinate, T = Storm durationOA = Approach SEGMENTAB = Rising limbBC = Crest segmentDE = Recession limbCalculation:T hr rainfall = 3hrHydrograph peak at 7 hoursPoint of inflection at 8.5 hoursTime of concentration is given by,∴ tc = Hydrograph peak time - Half of rainfall duration∴ tc = 7 - (3/2) = 5.5 hrsHence, the time of concentration (in hours) for this catchment is 5.5 hrs.