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Shallow groundwater that returns quickly (relative to deep groundwater flow) to contribute to stream flow during a precipitation event is classified as base flow. Four different base flow methods are available for estimating this return of infiltrated precipitation to the channel.


The recession method is generally more applicable to shorter duration periods for watersheds where the volume and timing of the base flow is strongly influenced by the precipitation event itself. The input parameters for the recession base flow method are as follows:

  • Initial flow at the start of the storm in cfs or cfs/sq. mile (cms or cms/sq. km for metric units).
  • Recession constant describing the rate of base flow decay. The constant represents the ratio of base flow at the present time to the flow one day earlier and consequently ranges between 0 and 1.
  • Threshold flow in cfs (cms) below which base flow recession occurs in accordance with the recession constant and corresponds to the point on the hydrograph where base flow replaces overland or runoff flow as the source from the sub-basin. This can be specified as an absolute value or as a ratio of the peak flow.

Constant Monthly

The constant monthly method uses a constant base flow in cfs (cms) at all simulation time steps that fall within a given month. The method is intended for use in long term simulations and requires a separate value for each month that is part of the overall simulation time.

Linear Reservoir

The linear reservoir method computes base flow from groundwater storage and can only be used in conjunction with the soil moisture accounting loss method. Available water from each groundwater layer (there are two layers defined) is converted to base flow through a specified number of linear reservoirs. The storage coefficient and number of linear reservoirs is required for each of the two layers.

Bounded Recession

The bounded recession method is similar to the recession, but includes a bounding recession value for each month so that it can handle individual events within a long term simulation better.

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