- discharge not associated with the storm (i.e. from groundwater) is termed baseflow (bottom frame, Fig. 2).
- hydrograph or baseflow separation is performed to determine the portion of the hydrograph attributable to baseflow
- Methods (Fig. 3):
- Constant-discharge method:
- assume baseflow constant regardless of stream height (discharge)
- project from minimum value immediately prior to beginning of storm hydrograph.

- Constant-slope:
- connect inflection point on receeding limb of storm hydrograph to beginning of storm hydrograph
- Assumes flow from aquifers began prior to start of current storm, arbitrarily sets it to inflection point
- for large watersheds set inflection point at at , where N is number of days after hydrograph peak, (mi) is watershed area

- Concave (most realistic):
- assume baseflow decreases while streamflow increases (i.e. to peak of storm hydrograph)
- project hydrograph trend from minimum discharge value immediately prior to beginning of storm hydrograph to directly beneath hydrograph peak
- connect that point to inflection point on receeding limb of storm hydrograph

- Master depletion curve method
- use when the most accurate model of hydrograph recessions is needed
- combine data from several recessions to make general recession model (Fig. 4)
- from this an equation of the form can be derived, which gives discharge at any time after discharge is measured

- Constant-discharge method:

Dr. T. Brikowski, UTD. All rights reserved.