A theoretical hydrograph representing the direct runoff response of a watershed to one unit of effective rainfall applied uniformly over a specified duration. It is a fundamental tool in rainfall-runoff modeling.
The unit hydrograph (UH) is a conceptual model that represents the direct runoff hydrograph resulting from one unit depth (typically 1 cm or 1 inch) of effective rainfall (excess rainfall after losses) generated uniformly over a catchment during a specified duration. Introduced by L.K. Sherman in 1932, it is based on the principles of proportionality and superposition: the direct runoff from any amount of effective rainfall can be obtained by multiplying the unit hydrograph ordinates by the rainfall depth, and the total runoff from a complex storm can be computed by summing the contributions from individual time increments. Unit hydrographs are derived from observed rainfall-runoff data by deconvolving the excess rainfall from the measured direct runoff hydrograph. When observed data are not available, synthetic unit hydrographs can be constructed using empirical methods such as the Snyder, SCS (NRCS), or Clark methods based on watershed characteristics. The instantaneous unit hydrograph (IUH) represents the theoretical response to an instantaneous pulse of effective rainfall and forms the basis for more advanced linear systems approaches to rainfall-runoff modeling. Despite being based on simplifying assumptions of linearity and time-invariance, the unit hydrograph remains one of the most widely used tools in engineering hydrology for flood estimation and watershed analysis.