of water from below the root zone is the most elusive component of the water balance to measure. Attention on drainage has increased because of concerns both about the efficiency with which irrigation water is used and about environmental damage caused by excess drainage through waterlogging, salinity and the movement of agrochemicals into waterways. However, most work on drainage has either used indirect measurements based on calculation of fluxes from the soil water profile measurements, chloride mass balance, or modelling to estimate its magnitude. This project attempts to directly measure drainage under an irrigated cotton system at ACRI using an equilibrium tension drainage lysimeter modified from a design of Brye et al. (1999).

The lysimeter facility has three objectives. The first is to measure drainage and better understand when it occurs during the crop rotation. The second is to act as a benchmark against which to test other, less expensive methods of measuring or estimating drainage which can be used in many more locations. Finally, data from the facility will be used to improve water balance models that can be used in conjunction with farming systems models to estimate drainage at a range of locations over long time periods (decades) and under a range of management systems. Such models can then be used to design more efficient and environmentally benign irrigation systems.