Contents preface (VII) introduction 1—37

2.7.1. Hydrograph Analysis
Hydrograph represents the temporal variation of total runoff at a gauging point in a stream. The total runoff includes both direct and ground water runoff. In hydrologic studies, one needs to establish a suitable relationship between surface flow hydrograph and the effective rainfall. Surface flow (or direct runoff) hydrograph is obtained by subtracting base flow (i.e., ground water runoff) from the total storm (or runoff) hydrograph. Division of a total runoff hydrograph into direct and ground water runoffs for subsequent analysis to analyse hydrologic problems is termed hydrograph separation or hydrograph analysis. There is no ready basis for differentiating between direct and ground water runoffs which have been defined rather arbitrarily. Therefore, the method of hydrograph separation too is arbitrary only.
For the purpose of unit hydrograph (Art. 2.7.2) theory, the hydrograph separation should be such that the time base of the direct runoff remains almost the same for different storms of the catchment basin. This can be attained by terminating the direct runoff at a fixed time after the time of occurrence of the peak of the hydrograph. The time interval N (in days) from the instant of occurrence of the peak to the time marking the end of the direct runoff (point C in Fig. 2.20) is empirically expressed as (9)

N = bA^0.2

(2.21)

Here, A is the drainage area in km² and b is a coefficient ranging from 0.8 to 0.85. The point A in Fig. 2.20 marks the beginning of the direct runoff and is identified easily as the point at the beginning of the rising limb where there is sharp increase in the runoff rate. Line AC provides the simplest method of base flow separation. The ordinates of hydrograph with respect to line AC, therefore, give the magnitudes of the direct runoff at the relevant time.
The most widely used method for hydrograph separation cosists of extending the recession (or base flow) curve existing before the commencement of the direct runoff (due to the storm under consideration) till it intersects the ordinate passing through peak of the hydrograph (at point B). Line segments AB and BC demarcate the separation between the surface runoff and base flow. The method is based on the reasoning that as the stream level rises there is flow from the stream into the banks of the stream. Therefore, the base flow (into the stream) should continuously decrease until the stream level starts falling and bank storage begins to return into the stream. It is, however, assumed that the decrease in base flow (i.e., AB) conforms to the usual recession existing prior to the storm.
In yet another method of base flow separation, the base flow recession curve (after the depletion of flood water as at E) is extended backward till it intersects the ordinate through the point of inflection on the recession limb at D. Points A and D are joined arbitrarily by a smooth curve. This method is preferred when the ground water contribution is expected to be significant and likely to reach the stream quickly.
After hydrograph separation, one can obtain surface (or direct) runoff hydrograph (DRH).