Submitted to: Encyclopedia of Water Science
Publication Type: Book / Chapter
Publication Acceptance Date: May 14, 2001
Publication Date: N/A
Interpretive Summary: Irrigated agriculture is going to play an ever increasing role in meeting the world food supply and productivity will have to be maintained if it is to meet the challenge. Waterlogging and root zone salinity are the significant threats to production in many irrigated areas due to poor existing irrigation water management and artificial subsurface drainage is required to control these threats. Under these conditions it is critica that the population at large and not just drainage and irrigation professionals understand the design principles, management methods, and impacts of subsurface drainage. Transient design of drainage systems is recommended in irrigated areas since this replicates the intermittent application of water expected in irrigation. A steady-state analysis can be used but assumes a more uniform monthly application that is not typical of most irrigated regions. In the past drainage systems were operated as free flowing systems and the need to control these systems was highlighted. The use of controlled drainage in arid areas is relatively new concept and is recommended in response to the need to minimize the negative effects of drainage water disposal to a water course. Subsurface drainage water may contain salt, fertilizer, pesticides, herbicides, and trace elements and careful management of the drainage water disposal is required. This article provides the basis for understanding the design, management, and impacts of artificial subsurface drainage in irrigated agriculture.
Technical Abstract: Food production statistics indicate that approximately 30% of the world's food supply is produced by irrigated agriculture on 20% of the world's arable land. As such, irrigated agriculture has an important role to play in meeting the world's future food demand. Soil drainage is needed to provide adequate aeration and salinity control for the production of agricultural crop either in areas where crops are grown under conditions of natural precipitation or artificial irrigation. Drainage systems can be characterized as either horizontal or vertical. The horizontal systems are made up of clay or concrete tile or plastic pipes that are installed parallel to the soil surface to collect water and let it flow by gravity to an outlet. A vertical system is a pumped well that is used for drainage. The design objective for a good drainage system is to remove water from the soil; that is to either lower the water table to specific depth in a given period of time or to prevent the water from rising in the soil above a specified depth. Drainage system design is either by a transient or a steady state method. The transient method accounts for the soil type, crop, and uses intermittent application of irrigation water and rainfall. The steady state method has been adapted from procedures used in humid areas. Active management of subsurface drainage systems is a relatively new concept for drains installed in irrigated agricultural areas. The diversion of irrigation water from a stream or river reduces the total flow of that watercourse thus decreasing the dilution potential of the stream. Drainage water returned to the stream may have been degraded by salt, fertilizers, pesticides, herbicides, and other elements that are in solution.