Storm hydrograph, DRH, baseflow separation

Definitions

• Storm hydrograph: the graph of total discharge against time at a river section during and after a storm. It consists of the rising limb, crest and recession limb, and it includes base flow.
• Direct runoff hydrograph (DRH): the hydrograph of surface runoff (plus interflow), obtained from the storm hydrograph by removing the base flow.
• Base flow: the part of the stream flow that comes from groundwater and delayed sub-surface flow. It keeps the stream flowing between storms.

Methods of base flow separation

 Q |        /\ peak
   |       /  \
   |    A /    \       B = end of direct runoff
   |  ---/------\------B----
   |   (1) straight line AB
   +--------------------------- t

1. Straight-line method: join the point A, where the rising limb starts, to the point B on the recession limb where direct runoff ends, by a straight line (horizontal when base flow is level). It is simple and suits perennial streams.

 Q |        /\  peak
   |       /  \
   |     A/    \
   |  ---/__    \__ B      (2) A'-B with
   |   A' \ \__________ N days after peak
   +-------------------------- t

2. Fixed-base length method: the end of direct runoff B is taken $N$ days after the peak, $N = 0.83A^{0.2}$ ($A$ in km$^2$). Extend the pre-storm base flow recession forward from A to a point A' below the peak, and join A' to B by a straight line.

 Q |         /\ peak
   |        /  \
   |       /    \
   |   .--D      C   <- inflection
   |  /  extended recession
   |_/____________________ t

3. Variable-slope method: extend the pre-storm recession forward to a point D under the peak, extend the post-storm base flow recession backward to the inflection point C on the recession limb, and join D and C. It is considered the most realistic.

4. Master recession (semi-log) method: on a semi-log graph the base flow recession is a straight line. The point where the plot of the falling limb merges into this line is the end of direct runoff.