Drainage is the natural or artificial removal of a surface's water and sub-surface water from an area.
Surface drainage is the removal of excess water from the surface of the land. This is normally accomplished by shallow ditches, also called open drains. The shallow ditches discharge into larger and deeper collector drains.
Subsurface drainage is the removal of water from the rootzone. It is accomplished by deep open drains or buried pipe drains.
The function of the field drainage system is to control the water table, whereas the function of the main drainage system is to collect, transport, and dispose of the water through an outfall or outlet.