ABSTRACT
Excessive soil water is a major concern on soils with seasonally shallow water tables. Drainage is the practice of removing excess water from land in order to facilitate seedbed preparation and planting and to provide adequate aeration following excessive rainfall. Several techniques are available to improve drainage and reduce excess water-related crop stress. These include both surface practices[1] and subsurface practices.[2-4] While wetness is the major concern, soil moisture under rainfed conditions varies such that crops periodically suffer from drought stresses even on traditionally shallow water table soils. Intensive drainage systems that are often necessary to remove excess water during extreme wet periods, tend to remove more water than necessary during drier periods, a condition referred to as temporary overdrainage.[5] To reduce the occurrence of overdrainage and improve crop utilization of rainfall, a water control structure may be installed in the drainage outlet to regulate or ‘‘control’’ the rate and amount of drainage, Fig. 1. The decline in the drainage volume often results in a reduction in the nutrient load being discharged with the drainage water.[7,8] While recent growth in the use of controlled drainage has been to conserve water and enhance drainage water quality, controlled drainage has been used historically to reduce subsidence in drained organic soil.[9] This application continues in places such as the Everglades agricultural area in Florida, the Wester Johor area in Malaysia, and several other locations around the world.[10]
HOW CONTROLLED DRAINAGE WORKS
Controlled drainage involves the use of some type of adjustable, flow-retarding structure placed in the drainage outlet that allows the water level in the outlet to be artificially set. Many types of structures can be used depending on the layout of the drainage system. Controlled drainage may be practiced with either surface or subsurface drainage systems, although the benefits of drainage control are closely correlated to subsurface drainage intensity. In other words,
controlled drainage effectiveness increases as the subsurface drainage intensity increases. Where drain tubing or field ditches outlet directly to an open channel such as a canal or stream, the system is referred to as an open system. Water control structures for open systems may range from simple, stop-log, weir type structures often referred to as flashboard risers,[11] Fig. 2, to automated inflatable dam type structures.[5] Where drain tubes outlet to main drains rather than open channels, the system is referred to as a closed system.[12] Several tubing manufacturers have designed and marketed barrel type structures for use in closed systems that function as a weir in the main drain line and allow the water level to be controlled.
