Drainage from below the plant root zone is not only a waste of water resources but a potential driver of rising water tables and salinity. Better estimates of the quantity and timing of drainage are required in the clay plains of the northern Murray Darling Basin to improve understanding of drainage processes, in particular under irrigation. This will allow better assessment both of how and where improvements can be made in irrigation management and of the risk of salinity. Current drainage estimates are based on indirect measurements - such as chloride mass balance - or on calculations using measurements or estimates of other components of the water balance. Both approaches lead to large uncertainties in drainage estimates. Direct measurements of drainage are difficult because most instruments disturb the hydraulic gradient in the soil, which is the main driver for drainage, and therefore affect the amount of drainage measured.A variable tension drainage lysimeter was built at the Australian Cotton Research Institute near Narrabri to provide accurate measurements of drainage under an irrigated cotton-wheat rotation on a Grey Vertosol typical of the region. Such lysimeters are designed not to disrupt the hydraulic gradient by being 'hydraulically invisible' so as not to interfere with the rate of drainage. The lysimeter consists of an array of six steel boxes whose upper surface is made of a porous, sintered steel sheet that, once saturated, allows water to flow but can hold a vacuum of up to 32 kPa. The boxes were installed at 2.1 m depth via tunnels excavated horizontally from a concrete access shaft. Thus the soil over the trays is undisturbed. The trays cover an area of 1.8 x 0.9 m. A method of preparing the soil on the ceiling of the tunnel was developed that uses a resin peel to remove any smearing. A contact material to connect the upper surface of the tray to the soil ceiling was designed and manufactured by grading silica flour.The system mimics the hydraulic gradient in the soil by regularly measuring the soil water potential at 2.1 m depth with tensiometers and then applying a vacuum equal to this potential to the inside of the trays. A data logger continuously adjusts the vacuum to the soil water potential. Water in the soil above the trays therefore experiences the same hydraulic gradient as if the trays were not there and flows at the same velocity into the trays. The trays have sloping floors which direct the water to a drain and thence to cylindrical collection vessels that are continuously weighed allowing the rate of drainage to continuously measured. The system is designed to be fully automated.Apart from providing accurate measurements of drainage over time, the lysimeter will be used as a benchmark against which to test other simpler - and less expensive methods - that can be deployed in a wider range of situations. Barrel lysimeters, wetting front detectors, and instruments to measure other water balance components have been installed near the lysimeter. Chloride mass balance measurements are also regularly made nearby.In addition, the data from the lysimeter will be used to improve water balance models that provide the only way of estimating the long-term drainage of current and alternative management systems for a variety of soil and climatic conditions.