Flood hydrograph, DRH, UH, baseflow separation

Flood hydrograph

A flood (storm) hydrograph is the graph of discharge against time at a river section during a flood caused by a storm. It has a rising limb, a crest (peak) and a recession (falling) limb, and it includes both direct runoff and base flow.

 Q |       peak
   |       /\
   | rising/  \ recession
   |   ___/    \___
   |__/  base flow  \____
   +--------------------- t

Direct runoff hydrograph (DRH)

The DRH is the hydrograph of the surface runoff (and interflow) only. It is obtained by subtracting the base flow from the storm hydrograph. The area under the DRH equals the volume of direct runoff, which is the effective rainfall depth multiplied by the basin area.

Unit hydrograph

The unit hydrograph is the DRH due to 1 cm (1 mm) of effective rainfall, uniformly distributed over the catchment, at a constant rate during a specified duration (the $D$-hour UH).

Methods of base-flow separation

1. Straight-line method: a horizontal or sloping straight line from the start of the rise to the point on the recession limb where direct runoff ends.
2. Fixed-base length method: the end of direct runoff is taken $N = 0.83A^{0.2}$ days after the peak ($A$ in km$^2$). The pre-storm recession is extended forward to the time of the peak, and this point is joined to the end point by a straight line.
3. Variable-slope (three-point) method: the pre-storm recession is extended forward to the time of the peak, and the post-storm base flow recession is extended backward to the inflection point on the recession limb. The two points are joined by a straight line.
4. Master recession curve (semi-log) method: the point on the semi-log recession plot where the slope changes marks the end of direct runoff.

The area between the hydrograph and the base-flow line gives the direct runoff volume.