UH):The unit hydrograph is the unit pulse response function of a linear hydrologic system.The unit hydrograph is a simple linear model that can be used to DERIVE the hydrograph of any duration resulting from any amount of excess rainfall for the GIVEN area.Assumptions in the Unit Hydrograph Theory:1. Time Invariance: This implies that the direct runoff hydrograph for a given effective rainfall (ER) in a catchment is time-invariant i.e. it is ALWAYS the same irrespective of when it occurs.2. Linear Response: The direct runoff response to rainfall excess is ASSUMED to be linear i.e. any change in the input value is linearly reflected in the output value.