movement of water across the earth’s surface owing to the force of gravity. A distinction is made between sheet flow and channel flow. Sheet flow is made up of rain and meltwater and moves down slopes, without fixed channels. Channel flow occurs in definite linear directions, in the channels of rivers and bottoms of ravines and gulleys. Subterranean water and groundwater sometimes also form part of channel flow.

Surface runoff is described by the volume of water flowing across a surface (the modulus of runoff) and is expressed either as l/sec/km² or as a depth in mm per year or some other period. In the USSR, the lowest modulus of runoff— in the arid regions of the Middle Asian plain— is 0–1 l/sec/km², while the highest goes up to 125 l/sec/km², in the mountains of the Western Caucasus. Surface runoff varies over time. The average annual modulus of runoff in the Vorskla River basin is 2.1 l/sec/km², but the maximum figure, during spring high water, is 220 l/sec/km². In the Primor’e, where the modulus of average runoff is 8-15 l/sec/km², the maximum modulus of direct runoff reaches 600–700 l/sec/km², and sometimes even more than 1,000 l/sec/km².