This chapter focuses on the pathways taken by water in moving to streams and the mechanisms for runoff generation.  Water moves either over the surface as overland flow or infiltrates to flow through the subsurface within the soil matrix as well as through preferential pathways known as macropores.  Macropores include pipes that are open passageways in the soil caused by decaying roots and burrowing animals.  Macropores also include larger structural voids within the soil matrix that serve as preferential pathways for subsurface flow.  The permeability of the soil matrix may differ between soil horizons and this may lead to the build up of a saturated wedge above a soil horizon interface.  Water in these saturated wedges may flow laterally through the soil matrix, or enter macropores and be carried rapidly to the stream as subsurface stormflow in the form of interflow.  Research in hillslope hydrology has shown that rapid lateral flow through a network of macropores is responsible for significant quick flow runoff in humid areas.  This quick flow often has similar chemical and isotopic composition to old water, held in the soil matrix prior to rainfall.  The effusion of old water from the soil matrix into the lateral flow macropore network is triggered by soil moisture content exceeding a threshold. 

Pathways followed by subsurface runoff on hillslopes.  (From Kirkby, 1978)