Runoff is a term used to describe the movement of water over the land surface, which occurs when precipitation falls on the land and flows overland towards streams and rivers. Runoff is an important component of the water cycle, as it replenishes groundwater and surface water as it percolates into an aquifer or moves into a river, stream, or watershed.

Runoff can be caused by various factors, including the intensity and distribution of precipitation, the slope, vegetation, and topography of the land over which it travels, and the type of soil present. Climate change can also affect the patterns of runoff, as more precipitation falls as rain and less as snow, and warmer temperatures change runoff patterns.

Runoff can be divided into three types: surface runoff, interflow, and baseflow. Surface runoff comes from overland flow and saturation excess overland flow, while interflow usually comes about after a large amount of precipitation, and baseflow is the slow movement of water through soil and rock towards streams and rivers.

In summary, runoff is an important part of the water cycle, and it occurs when precipitation falls on the land and flows overland towards streams and rivers. It can be caused by various factors, and it can be divided into three types: surface runoff, interflow, and baseflow.