USED when rainfall duration and intensity are VARIABLE because in that case, we need to divide the rainfall into VARIOUS small durations and then find its response.The advantage of IUH is that it is independent of the duration of EFFECTIVE rainfall hyetograph and has one parameter less than the D-hr unit hydrograph. IUH is independent of rainfall characteristics.It is indicative of catchment storage characteristics.An instantaneous unit hydrograph is also obtained from S- curve. But that is an approximate method.Also called Clark's method Uses and LIMITATIONS of unit hydrograph: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 500 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.