What is a unit hydrograph and how is it used?

A unit hydrograph (UH) shows the direct runoff response of a catchment to one unit (usually 1 cm) of excess rainfall occurring uniformly over a specified duration. Assumptions: linear system (proportional response), time invariance, excess rainfall uniform in space and time. To get runoff hydrograph: apply UH to each rainfall increment and sum (superposition). Synthetic UHs (SCS, Snyder) can be developed from catchment characteristics when data unavailable. UH is fundamental tool for flood hydrograph prediction.