Base flow separation methods

Base flow separation divides the storm hydrograph into direct runoff and base flow so that the direct runoff hydrograph (DRH) can be analysed. The common methods are:

 Q |        peak
   |       /\
   |  A   /  \  N days
   | ____/    \____B___C
   |      \ ___/  ..
   |       (1)(2)(3)
   +------------------------ t

1. Straight-line method (Method 1): join the point A where the rising limb begins to the point B on the recession limb where direct runoff ends (a horizontal or sloping straight line). It is simple and is used for small catchments and perennial streams.

2. Fixed-base length method (Method 2): the end of direct runoff is taken $N$ days after the peak,

$$N = 0.83\,A^{0.2}$$

($A$ in km$^2$, $N$ in days). Extend the pre-storm base flow recession curve forward from A to a point below the peak, and join this point to the point B at $N$ days after the peak by a straight line.

3. Variable-slope method (Method 3): extend the pre-storm base flow recession curve forward to the time of the peak (A to D). Extend the post-storm base flow recession curve backward to the point of inflection C on the recession limb. Join D to C by a straight line.

4. Master recession curve / semi-log method: plot the recession limb on semi-log paper ($\log Q$ against $t$). The point where the straight (base flow) recession line begins to depart from the plotted curve is the end of direct runoff.

All methods are arbitrary, so the same method should be used for all storms of the catchment. Volume of direct runoff = area between the hydrograph and the base flow line.