In the general progression of the hydrological cycle beginning with atmospheric water vapour and ensuing precipitation (Chapters 1 and 3), it is then subsurface flow that transports most of the residual rainfall (rainfall less evapotranspiration) to rivers. Overland flow on hillslopes (Section 6.5) transports a much smaller proportion of the flow to rivers. The study of subsurface flow within soil, regolith and rock strata is of vital importance to the assessment of available groundwater resources (Chapter 17), themigration of nutrients and contaminants (Section 8.1), and in the regulation of both evapotranspiration (Chapter 10) and runoff generation, as simulated by catchment models (Chapter 12). While the proportion of precipitation travelling to channels by overland flow is now considered to be much less than that travelling to channels by subsurface pathways (Section 1.2), overland flow observations are of fundamental importance to the quantification of floodplain inundation (Bates et al., 2006), hillslope erosion and sediment delivery (Owens and Collins, 2006), and the migration of phosphorus across agricultural landscapes (Withers and Bailey, 2003).