Contents preface (VII) introduction 1—37

2.7.2. Unit Hydrograph
A unit hydrograph (or unit-Graph) is the direct runoff hydrograph resulting from one centimeter (or one millimeter or one inch) of excess rainfall generated uniformly over a catchment area at a constant rate for an effective duration (1). The unit hydrograph for a catchment basin is the direct runoff hydrograph produced by a unit (usually 1 cm) rainfall excess from a storm of

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D-hour duration and, therefore, is the lumped repsonse of the basin to the storm. The unit hydrograph is a simple linear model that is most widely used for obtaining the surface runoff hydrograph resulting from any amount of excess rainfall. The physical characteristics of a catchment basin (shape, size, slope etc.) remain invariant to a large extent. Therefore, one may expect considerable similarity in the hydrographs of different storms of similar rainfall characteristics. This forms the basis of the unit hydrograph first proposed by Sherman (10). The unit hydrograph is a typical hydrograph for a catchment basin and is so called because the runoff volume under the hydrograph is adjusted to 1 cm (or 1 mm or 1 inch) equivalent depth over the basin. It should, however, be noted that the variable characteristics of storms (such as rainfall duration, time-intensity pattern, areal distribution, magnitude of rainfall) do cause variations in the shape of the resulting hydrographs. Therefore, it would be incorrect to imply that only one typical hydrograph would suffice for any catchment basin. The following basic assumptions are inherent in the unit hydrograph theory (1) :

1. 
   The excess rainfall has a constant intensity (1/D cm/hr) within effective storm duration of D hours.
   

2. 
   The excess rainfall (giving rise to 1 cm depth of runoff) is uniformly distributed throughout the entire catchment basin.
   

3. 
   The base time of direct runoff hydrograph (i.e., the duration of the direct runoff) resulting from an excess rainfall of given duration is constant.
   

4. 
   The ordinates of all direct runoff hydrographs of a common base time are directly proportional to the total amount of direct runoff represented by each hydrograph. This means that a rainfall excess of r cm due to a storm of duration D hours in a catchment basin will produce a direct runoff hydrograph whose ordinates would be r times the corresponding ordinates of a D-hour unit hydrograph of the basin.
   

5. 
   For a given catchment basin, the hydrograph, resulting from a given excess rainfall, reflects the unchanging characteristics of the catchment basin.
   

3.