Surface Runoff and Subsurface Drainage

Abstract Runoff is the movement of water across or through the land surface. Sometimes, the word runoff is used simply to refer to river discharge. However, there are many different types of runoff, including overland flow (surface runoff) or throughflow (subsurface runoff, sometimes also called interflow). Precipitation can either hit the surface of the hillslope directly or be intercepted by vegetation. This intercepted water can be stored on leaves and tree trunks which shelter the ground beneath, or it can trickle down to reach the surface via stem flow. There are then two possibilities for direct precipitation or stem flow, once it reaches the hillslope, either to infiltrate into the soil or to pond up and flow over the surface as overland flow. Water that enters the soil may percolate down or move laterally through the soil as throughflow, or it may return to the surface where the soil becomes saturated. The following sections provide details on each of the main interlinked overland flow and subsurface drainage mechanisms.