TENDS towards zero).In TERMS of the systematic conception, this means that the instantaneous unit hydrograph represents the response of the catchment to an instantaneous impulse.It is useful as the IUH can be analytically computed and, on the basis of the "S" curve, a certain unit hydrograph of any reference duration may be derived from it.This ordinate of the instantaneous hydrograph is given by.\(u\left( t \right) = \;\frac{1}{i}\frac{{ds}}{{dt}}\;\)Where,I = Intensity of rainfall,S = ordinate of S-Curve.Uses:The development of flood hydrograph for extreme rainfall magnitude for use in the design of the hydraulic structure.Extension of flood flow records based on rainfall records.Development of flood record based on rainfall Development of flood forecasting and warning systems based on rainfall.Limitations:Unit hydrograph cannot be used for catchment are GREATER than 5000 km2Unit hydrograph cannot be used for catchment less than 2 km2Precipitation MUST be in the form of rainfall only IE not be in the form of snow.