The groundwater store is the only source of runoff in dry weather. That is, it provides the baseflow component of the streamflow. The quantity of groundwater runoff in a time interval is determined by the recession equation

 

where:

GR = groundwater runoff

KG = constant rate groundwater recession factor

GN = variable rate groundwater recession factor

TF = time interval (days)

GS = current value of groundwater storage (mm)

Equation (23) expresses the groundwater runoff as a depth over the pervious part of the catchment. It must be converted to a depth over the entire catchment. The conversion is made as follows:

 

where:

FPER = pervious fraction