Also known as subsurface flow, this is the movement diagonally downslope of water through the soil, as opposed to the vertical movement known as percolation. It may follow natural percolines in the soil. Throughflow involves micro-pores (less than 0.1 mm in diameter) and macro-pores, natural soil pipes, displacement flow, and groundwater discharge. Throughflow is a major factor in the hydrology of a drainage basin where the rocks underlying the soil are impermeable. See Kirkby and Chorley (1967) Bull. Int. Assoc. Sci. Hydrol. 12. See also Borga et al. (2002) Hydrol. Procs 16.