Computer-based techniques for assessment of irrigation water supply and demand are illustrated using a 140 x 75 km discrete element representation of the Canterbury Plain, New Zealand. A versatile soil moisture accounting model is developed and used to simulate soil moisture status throughout the Plain, and various irrigation management policies are modelled. Detailed statistics are compiled on soil moisture levels and net irrigation water requirements at Winchmore in central Canterbury. In a specific study of the Rakaia River, a major water resource of Canterbury, the stochastic component of stream flow under different climatic regimes are investigated. Water demand is then compared with available water supply on a daily basis to establish the potential of irrigation by direct diversion of stream flow.