Runoff is a broad term incorporating different types of flows on or below the Earth's surface.
Perhaps the common type of runoff is Overlandflow.Robert Horton implied that overlandflow occurred when precipitation was higher than the ability of water to infiltrate.As such water flows along the slope in a thin sheet of water as unconcetrated flow(sheetwash) or as concetrated flow into rills(tiny surface channels).Overlandflow mostly occur on slopes because on undulating terrains water is given time to infiltrate slowly.The type of surface dertemines Overlandflow,rocky surfaces as in arid regions disallow infiltration.Compacted soil such as in farming fields caused by tractors or hydrophobic soils that swell when in contact with water creating an impermable layer favour overlandflow.Short,sudden, bursts of rainfall have less time to infiltrate thoroughly causing overlandflow.This is common in deserts which can lead to flash floods.
Other experiments done by Hibbert and Hewlett shows that overlandflow occurs when the ground is already filled with water.They implied that water slowly infiltrates and percolate such that the water table rises to the surface(return flow).Overlandflow will occur as rainwater fall and flow with the already saturated ground.
Human Factors infuencing OverlandflowAgricultural Activities
Tractors,Cattles compress the soil creating an impermable layer which restrict infiltration and encourage overlandflow.Deforestation
Clearing trees means water readily reaches the surface without being intercepted.In addition, trees help by absorbing water and lessen runoff,in this case overlandflow.
Mining and quarying requires the removal of vegetaion which in turn results in overlandflow.
The increasing number of people in urban areas implies that more and more roads and pavements will be constructed which hinders infiltration.
Other activities such as leveling slopes by dredging or terracing impede overlandflow from occur.
Irrigation can cause water to accumulate on the surface and the water can flow with rainwater as saturation overlandflow.
Throughflow is the subsurface movement of water in the unsaturated zone.The water can travel downwards through natural pipes, percolines or along bedding planes in Throughflow mostly occurs when water is restricted to percolate downwards thus the water moves laterally downslope.
Water that flows in the saturated zone.
Water that slowly seeps to the bed or banks of streams from groundwater sources.It is thought to cause river flow during dry periods.