Deep drainage(DD) - water that passes beyond the - is an important process in irrigated cropping soils to ensure leaching of salts through the soil profile to deeper soil layers, the vadose zone ( the zone between the rootzone and the watertable) or to groundwater. Salt can either be naturally present within some soils or be added through low quality irrigation water. Furthermore, excessive DD may cause water table rise to the rootzone with associated salts, so precludig the growth of salt sensitive species.

DD is also an economic negative, as costs of pumping and storage are nto realised in increased yields or possible increased reas under production. The loss of irrigation waters to DD is particularily important in drought years where the rare water resource must be carefully utilised to ensure crops attain maximum yield per unit volume of applied water.

The study reported here, focused on DD water losses and the quality of those lost waters (in terms of salinity) on 7 irrigated cotton farms (all but one under traditional furrow irrigation management) in the Upper Murray Darling Basin (UMDB) near the towns of Boggabilla (2 sites) , Dalby, Goondiwindi, Macalister, Pampas and St George.

many regarded the advent of low volume irrigation devices (eg. lateral moves) with their known capacity to increase water use efficiency (WUE), (bales of cotton/unit water applied) as a 'win-win' situation, making minimum water go further, particularily as DD is almost zero. However, minimal or no DD equates to a reduced leaching fraction. This in turn can lead to a potential for increased rootzone salinity.

Several hydrological models were tested, to investigate their capacity to predict DD.