Surface water

(hydrology) All bodies of water on the surface of the earth.
(oceanography) mixed layer

A term commonly used to designate the water flowing in stream channels. The term is sometimes used in a broader sense as opposed to “subsurface water.” In this sense, surface water includes water in lakes, marshes, glaciers, and reservoirs as well as that flowing in streams. In the broadest sense, surface water is all the water on the surface of the Earth and thus includes the water of the oceans. Subsurface water includes water in the root zone of the soil and ground water flowing or stored in the rock mantle of the Earth. Subsurface water differs from surface water in the mechanics of its movement as well as in its location. Surface and subsurface water are two stages of the movement of the Earth's water through the hydrologic cycle. The world's ocean and atmospheric moisture are two other main stages of the grand water cycle of the Earth. See also Hydrology.

The table gives estimates of the amounts of water in various parts of the hydrologic cycle and their detention periods. It may be noted that surface water on the continents is but a small part of the world's water and that the bulk of that is in fresh-water lakes. However, the detention period is also short. This means that the surface-water part, and especially the water in the streams, is rapidly discharged and replenished. That is why surface water, as well as the shallower ground water, is called a renewable resource. Water that has a detention period of more than a generation is not renewed within sufficient time to be so considered. See also Ground-water hydrology; River.

*109 acre-ft = 1.233 × 10⁸ ha · m = 1.233 × 10¹² m³.

†2500 ft = 750 m.

Precipitation that reaches the Earth is subdivided by processes of evaporation and infiltration into various routes of subsequent travel. Evaporation from wet land surfaces and from vegetation returns some of the water to the atmosphere immediately. Precipitation that falls at rates less than the local rate of infiltration enters the soil. Some of the infiltrated water is retained in the soil, sustaining plant life, and some reaches the ground water. The precipitation that exceeds the capacity of the soil to absorb water flows overland in the direction of the steepest slope and concentrates in rills and minor channels. During storms most of the water in surface streams is derived from that portion of the precipitation which fails to infiltrate the soil. See also Precipitation (meteorology).

The distinction between surface and subsurface water, though useful, should not obscure the fact that water on the surface and water underground is physically connected through pores, cracks, and joints in rock and soil material. In many areas, particularly in humid regions, surface water in stream channels is the visible part of a reservoir, which is partly underground; the water surface of a river is the visible extension of the surface of the ground water.

1. See surface moisture.
2. Rainfall which runs over the surface of the ground.
3. Water carried by an aggregate except that held by absorption within the aggregate particles themselves.