Hydrograph Separation

“Division of a hydrograph into direct and groundwater runoff as a basis for subsequent analysis is known as hydrograph separation or hydrograph analysis. Since there is no real basis for distinguishing between direct and groundwater flow in a stream at any instant, and since definitions of these two components are relatively arbitrary, the method of separation is usually equally arbitrary.” - Linsley et.al., 1975

The Idea

For hydrograph separation, it is generally assumed that total flow (\(q\)) at any particular time (\(t\)) of a streamflow hydrograph can be partitioned into two primary components:

  1. The slow flow component \((b)\), which is itself composed of the gradual release of water from watershed stores in addition to groundwater discharging into streams, the “groundwater runoff” in Linsley etal.). The slow flow component has been commonly referred as “baseflow.” and,
  2. The quick flow component (\(f\)), which originates from rainfall and/or snow melt events (i.e., “direct runoff” in Linsley et.al., 1975).

Together, the slow and quick flow components sum to total flow: \(q=b+f\). Conceptually, after a period of time following a precipitation event, streamflow continues to decrease at a predictable rate as it is composed entirely of slowflow (\(f=0\)). Upon the onset of a heavy rain event, the hydrograph quickly rises, as quick flow is added to the slowflow signature. One could imagine that should this rain event never occur, the underlying slowflow would have continued uninterrupted (such as in Reed et.al., 1975). The difference between total flow and this “underlying” slowflow is perceived as quickflow.