Subsurface drainage design and management in irrigated areas of Australia

Subsurface drainage to protect irrigated cropping has been practised in some areas of Australia since the 1920s. and most irrigation districts have large land areas protected by some form of subsurface drainage. Across the irrigated areas, a broad spectrum of practices were developed that suited the conditions at the time of development. This paper assesses the performance of these subsurface drainage systems in terms of long-term sustainability of irrigated agriculture, based on the results of a detailed review of all the subsurface drainage systems in use in Australian irrigation areas. The long-term sustainability of irrigated agriculture depends upon controlling the salinity levels in the crop root zone and maintaining the ability to dispose of drainage water. This requires that subsurface drainage systems are efficient in terms of removing the minimum amount of water with the lowest salinity possible, given the existing conditions, while still maintaining crop productivity. Analysis of the current drainage system operation showed that many systems were draining greater volumes of water than designed for, leading to excessively high leaching fractions, and reduced irrigation water-use efficiency. The salt load removed by these systems was also often found to be far greater than the salt applied by irrigation, indicating a mining of stored salt. This is necessary from a salinised root zone but not if the salt is from below the root zone. The extra salt load above that required to maintain a salt balance in the root zone leads to increased difficulties in the disposal of the drainage water due to downstream impacts. Suggestions are discussed for adaptive management and new design considerations that may help make subsurface drainage more efficient, leading to reduced negative downstream effects and reduced costs of disposal.