What is surface and subsurface drainage?

The first requisite in the prevention or elimination of waterlogging and salinity problems is an adequate drainage system. Very often the natural drainage in an area, along with good water management, is sufficient to eliminate excess water and preclude the need for expensive subsurface drainage systems. Almost every farmer who applies water by surface irrigation, however, or who deals with significant rainfall, should have adequate surface drainage facilities to remove excess water. This will allow the farmer to avoid waterlogging and possible salinity problems at the tail end of borders, furrows, or basins after irrigation or intense rainstorms. Drainage facilities will also allow the prevention of erosion associated with natural movement of excess water over the soil surface.
Surface drains are open channels that collect water as it runs off or into irrigated fields. These drains convey water to a stream or channel where it can be carried safely. The design procedures for these drains are the same as for any open channel (see Chapter 5). The main requirement is that they are able to convey the maximum expected flow rate without erosion. At the tail end of irrigated fields, these drains are often broad and shallow to allow farm machinery to operate efficiently.