|Ayars, James - Jim|
Submitted to: Irrigation Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/31/2001
Publication Date: N/A
Citation: N/A Interpretive Summary: Subsurface drainage to protect irrigated cropping has been practiced in some areas of Australia since the 1920's and most irrigation districts have large land areas protected by some form of subsurface drainage. Across the irrigation 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 irrigation areas in Australia. The long term sustainability of irrigated agriculture depends upon 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 of the lowest salinity possible whilst still maintaining crop productivity. Analysis of the current drainage system operation determined that many were draining volumes of water greater than the designed capacity, 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. If the salt is from below the root zone this may be unnecessary. The extra salt load above that required to maintain a salt balance in the root zone leads to increased difficulties in disposal of drainage water due to downstream impacts. Suggestions for adaptive management and new design considerations are given that may help make subsurface drainage more efficient, leading to reduced negative down stream effects and reduced costs
Technical Abstract: Australia is one of the driest continents on earth and water resource management is critical to sustain irrigated agriculture while minimizing the environmental impacts associated with irrigation, i.e. return flows both surface and subsurface. A workshop was held to determine the state of irrigation and drainage management in Australia and representatives from each of the major irrigation areas in Australia participated. The data gathered in this workshop were used to analyze the existing conditions. The results showed that many irrigation districts were being over drained, more water was being discharged than was needed to maintain production. This was a result of either establishing very conservative drainage coefficients or the type of drainage system selected, pumped not horizontal. The secondary impact was higher salt discharge than was needed to maintain salinity levels in the soil profile. Changes in drainage system operation and irrigation management were recommended that will result in reduced drainage and salt load and improved irrigation efficiency. These recommendations include; controlling the water table, relaxing the depth to water table requirement, and improved irrigation scheduling.