The precipitation discharged into stream channels from an area. The water that flows off the surface of the land without sinking into the soil is called surface runoff. Water that enters the soil before reaching surface streams is called groundwater runoff or seepage flow from groundwater.

Overland flow

Excess water leaving a field horizontally across the soil surface because it cannot infiltrate into the soil, eventually ending up in a ditch or stream (= surface runoff).


Inundation of land beside a watercourse, as a result of an excessive water table. This may incur addition of sediment onto the land surface as well as into the water.


Process through which a waterbody, such as a lake or a soil solution, becomes enriched with dissolved nutrients. This can be natural, but is often due to pollution. Eutrophication may result in algal blooms which finally promote anaerobic conditions which may harm fish life.

Drainage (artificial)

Man-made adjustments to a field directed at the removal of excess water by ditches, subsoiling, pipes.

Buffering of fields

The presence of terraces, treelines, buffer zones, riparian zones, which all contribute to intercepting overland flow.


The process by which materials in rocks or other deposits are broken down into smaller parts and ultimately their constituents. An example is ‘freeze thaw’ expansion and cracking. There are physical, chemical and biological weathering processes.


The movement of water through the soil.


The movement of water through the soil.


The addition of hydrogen, removal of oxygen, or the addition of electrons to an element or compound. Under anaerobic conditions (where there is no dissolved oxygen present) such as in ‘gley’ soils, sulphur compounds are reduced to odour-producing hydrogen sulphide (H2S) and other compounds. Reduction is the opposite of oxidation.