Runoff or runoff is called the current of water that is discharged when it exceeds its natural or artificial reservoir or channel. In hydrology, runoff refers to the sheet of water that circulates on the surface in a drainage basin, that is, the height in millimeters of the drained and extended rainwater. It is normally considered as precipitation minus actual evapotranspiration and infiltration of the soil system.

According to Horton's theory, it is formed when precipitation exceeds the infiltration capacity of the soil. This is only applicable in soils in arid areas and with torrential rainfall. This deficiency is corrected with the saturation theory, applicable to soils in areas of high and constant rainfall. According to this theory, runoff will form when soil compartments are saturated with water.

Runoff is one of the basic processes of the water cycle. It refers to the flow of water from rain or snow melt that circulates on the surface of the soil once it exceeds its evaporation and infiltration capacity. "Infiltration (hydrology)"), as explained by the United Nations Development Program (UNDP). There are different types ranging from superficial, hypodermic and underground.

Surface runoff is one of the main causes of erosion worldwide. It tends to be particularly harmful in poorly permeable soils, such as clay, and in areas with scarce plant cover.

The proportion of water that follows each of these paths depends on factors such as climate, the type of rock or the slope of the terrain. Similarly, in places where there is an abundance of loose or very porous materials, the percentage of water that stagnates is very high.

The main parameters that affect runoff are:[1]​.

The comparison between these variables allows obtaining information about the processes that can occur under different situations. The conditions in which the soil is found at the time precipitation occurs will substantially affect runoff. The following cases can be distinguished: