In hydrology, unit hydrograph theory holds that the direct-runoff response of a catchment to a unit depth of effective rainfall of a given duration is a fixed, linearly scalable and additive characteristic of that catchment, enabling derivation of unit hydrographs of any other duration from a known one via the S-curve method (continuous superposition of shifted unit hydrographs) or by decomposition into an instantaneous unit hydrograph (IUH), the limiting case as rainfall duration tends to zero. For catchments lacking observed rainfall-runoff records, synthetic unit hydrograph methods (e.g., Snyder's method) relate hydrograph characteristics — lag time, peak discharge, time base — to a catchment's geomorphological parameters, transferring calibration coefficients from a similar gauged catchment.
In hydrology, unit hydrograph theory holds that the direct-runoff response of a catchment to a unit depth of effective rainfall of a given duration is a fixed, linearly scalable and additive characte…