Runoff occurs when there is more water than the land can absorb. The excess rainwater flows across the surface of the land and into nearby creeks, streams, or ponds.
It is pulled by gravity across the land’s surface. On percolating, it recharges groundwater, and on the surface, it feeds the water bodies like river, stream or watershed.  The source may be from rain, snowmelt, irrigation or other sources, comprising a significant element in the water cycle as well as the water supply when it drains into a watershed.
During runoff, it also erodes the surface of the land in its path carving out canyons, gorges and various other erosional landforms.
Depending on the amount of rainfall, the porosity of the soil, vegetation and slope, the amount of runoff may vary. Impervious surfaces or surfaces that cannot absorb water, increase runoff. Soil absorbs \(65\)% of the precipitation leaving only \(35\)% to reach the sea or ocean.
Types of Runoff
Runoff is classified into three types based on the time delay between the occurrence of rainfall and the generation of runoff.
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Surface Runoff
When the rainfall is longer, heavier and exceeds the rate of infiltration, surface runoff takes place. It is the portion of rainfall that enters the stream immediately after the rainfall. On having the rain continued even after all losses are satisfied, the excess water makes a head over the ground surface, which follows the land gradient moving from one place to another. It is also known as Surface flow.
Sub-Surface Runoff
Here, the rainfall enters into the soil and moves laterally without joining the water-table to the streams, rivers or oceans. It is also known as interflow.
Base Flow
It is delayed flow, in which water, after falling on the ground surface, infiltrates into the soil and meets to the water-table; and flow to the streams, oceans etc. In this type of runoff, the movement of water is very slow; that is why it is also referred to as delayed runoff. It takes a long time to join the rivers or oceans (says for like years). Sometimes, base flow is also known as groundwater flow.

It usually appears at a downstream location where the channel elevation is lower than the groundwater table. Even during dry seasons, groundwater provides streamflow.