ABSTRACT
Although, by its nature, irrigated land will generally feature in a more arid climatic/physiographic setting than land used for rainfed agriculture, the occurrence of excess rainfall is still quite common. Rainfall in arid climates falls typically as intense storms, creating surface drainage requirements, which are easily 5 to 10 times greater than those due to the irrigation losses. For this reason design discharges for surface drainage systems for irrigated land may, in most cases, be based solely on rainfall. This also applies to subsurface drainage when the drainage is exclusively for the control of excess rainfall (as may for example be the case with supplemental irrigation in semi-humid climates). Under the typical and to semi-arid setting of most irrigated land where subsurface drainage is mainly needed for salinity control, different design criteria apply.
