USE OF A MODEL TO ANALYSE THE ROLE OF TREES IN SOIL WATER DISTRIBUTION AND UTILISATION

A physically based model was developed to simulate changes in plant water use and soil water distribution over time. It was built from two main submodels which dealt with soil water flow and plant water uptake. The soil water flow submodel employed the Richards equation combined with vapour flow procedures to predict soil water distribution, deep drainage, and soil evaporation. The plant water uptake submodel used a potential driven approach in that it depended on the water potential gradient between the roots and soil matrix. The model was used to analyse the effects of rooting patterns on deep drainage, which is one of the main differences in plant properties between trees and annual crops or grasses. Root distribution affects plant water uptake, soil water distribution, and deep drainage. The deep drainage rate under Armidale (NSW, Australia) weather conditions is episodic in character and root distribution influences deep drainage during wet periods. From discussion of the optimal root distribution for an agroforestry system it was concluded that to prevent too much deep drainage, it may not be necessary to have only deep-rooted species present. Simulation can indicate the appropriate ratio of deep-rooted trees to shallow-rooted crops or grasses in an agroforestry system where the aim is to minimise deep drainage while maximising the percentage of productive shallow-rooted crops or grasses