The Lockyer Valley is an important agricultural area experiencing water insecurity, which causes a decrease in agricultural production. Regional authorities are initiating a wastewater reclamation project conveying treated municipal wastewater to water users, including potentially the Lockyer Valley. This additional (and essentially reliable) water source will change farming options and practices. A participatory modelling approach was used to analyse the consequences of changed water availability at the farm scale. This approach incorporates both farmer and scientist knowledge and gives due attention to non-technical issues like perception and acceptance. Two cropping patterns were worked out to forecast the effects on water deficit and crop yield in the projected situation.