TAKES place all the time over the surface as overland flow and through the channels as open CHANNEL flow is surface runoff.Runoff = Direct runoff + Base flowDirect runoff : This is the part of runoff which immediately reaches the stream after precipitation. This includes surface runoff, prompt interflow and precipitation on the stream surface.Base flow = Total runoff - Direct runoffDirect Runoff hydrograph:(i) The hydrograph obtained after deducting the base flow from the flood hydrograph is known as DRH.Unit Hydrograph:(i) It is a DRH RESULTING from one unit depth of rainfall excess (Direct runoff) occurring uniformly over the basin at a uniform rate for a specified DURATION of ‘D’ hours.The theory of unit hydrograph was developed by Sherman in the year 1932 and assumptions are given below-1. Time invariance-According to this assumption the DRH for a given effective rainfall is always the same in the catchment IRRESPECTIVE of the time, when the rainfall or storm takes place.2. Linear response-This is the single most important assumption in the theory of unit hydrograph. According to this assumption any change in the input value is proportionately reflected in the output value.