Unit Hydrograph

A unit hydrograph is the hydrograph of direct water surface runoff that results from a total depth of one unit of excess rainfall (e.g., 1 in, 1 mm, etc.) uniformly distributed over the basin and occurring within a specified duration of time. Unit hydrograph method is useful in representing the effects of variable rainfall patterns on a particular basin, since the procedure developed can make use of the linear theory practiced in various branches of engineering. Two characteristics of linear systems are that they are linearly scalable (proportionality) and can be added together (superposition). Scaling the unit hydrograph by the amount of excess rainfall illustrates the concept of proportionality, while adding multiple direct runoff hydrographs illustrates the concept of superposition.  The unit hydrograph theory is based on several assumptions. The storm used in deriving the unit hydrograph should be restricted to constant rainfall intensity within the time duration, implying a short duration storm event. The time of concentration for the basin is considered to remain constant for any rainfall intensity and duration. The rainfall intensity should also have uniform spatial distribution over the watershed area, implying that the watershed is not too large. If the watershed area is too large, then it can be divided into subareas of which each has to be analyzed separately. The unit hydrograph reflects the unchanging characteristics of the watershed when channel conditions remain unchanged and the catchment does not have appreciable storage. An example for a violated condition is when the watershed contains many reservoirs or when the water flood overflows into the floodplain producing considerable storage. The total duration of a unit hydrograph, referred as the base time, depends only on rainfall duration, not the excess rainfall intensity. Here, the unit hydrograph notation to be used is UHd, where “d” denotes the rainfall duration.