Deep drainage (DD), defined as water that passes beyond the root zone, can be an important contributor in terms of recharging ground water and also in transporting certain amounts of soluble salts away from the principal root zone. As such, DD is not necessarily an adverse component in the soil or landscape water balance. However, excessive DD is not only economically poor practice but may also lead to rising ground water tables and increased solute concentrations elsewhere in the landscape. The Australian cotton industry has become increasingly aware of the large amount of deep drainage (DD) in many of the heavy textured soils (Vertosols) that are flood irrigated (Silburn et al., 2004). This information contradicted earlier beliefs in the cotton industry that 'clay soils don't drain' (Hearn, 1998). A review by Silburn and Montgomery (2003) was instrumental in helping the industry change their view of "leak proof" Vertosols, though work by Dalton (2003) stated that 'deep drainage in surface irrigated cotton systems has remained a contentious issue and one that has not been well understood or grasped by the Industry'. Seasonal deep drainage values under irrigated cotton of 100 mm and 200 mm have been calculated using volume balance measurements (Dalton, 2003), and Gordon (2000) measured seasonal deep drainage (using a large lysimeter) of 95 to 305 mm/year under drip irrigation and 165 to 180 mm/year under furrow irrigation on the Darling Downs, Queensland