Baseflow is the portion of streamflow

Streamflow

Streamflow, or channel runoff, is the flow of water in streams, rivers, and other channels, and is a major element of the water cycle. It is one component of the runoff of water from the land to waterbodies, the other component being surface runoff...

 that comes from "the sum of deep subsurface flow and delayed shallow subsurface flow". It should not be confused with groundwater flow

Groundwater flow

Groundwater flow is defined as the "...part of streamflow that has infiltrated the ground, has entered the phreatic zone, and has been discharged into a stream channel, via springs or seepage water". In hydrogeology it is measured by the Groundwater flow equation.- See also :*Subsurface...

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Certain parameters of baseflow, such as the mean residence time and the baseflow recession curve, can be useful in describing the mixing of waters (such as from precipitation and groundwater) and the level of groundwater contribution to streamflow in catchments.

Baseflow separation is often used to determine what portion of a streamflow hydrograph

Hydrograph

A hydrograph is a graph showing the rate of flow versus time past a specific point in a river, or other channel or conduit carrying flow...

 occurs from baseflow, and what portion occurs from overland flow. Common methods include using isotope tracing

Isotope hydrology

Isotope hydrology is a field of hydrology that uses isotopic dating to estimate the age and origins of water and of movement within the hydrologic cycle. The techniques are used for water-use policy, mapping aquifers, conserving water supplies, and controlling pollution...

and the software program HYSEP, among others.