An ecological optimality approach for predicting deep drainage from tree belts of alley farms in water-limited environments

The clearing of natural vegetation for agriculture in southern Australia has increased deep drainage, led to increased groundwater recharge and, hence, the salinisation of land and streams. Alley farming systems, comprising alternate belts of trees and crops, have been proposed for reducing deep drainage but their effectiveness is unknown. This paper describes an application of ecological optimality theory to estimate the equivalent no drainage (ENOD) width B (m) for a tree belt. The relative drainage RD from an alley farm, compared to conventional agriculture is, therefore, 1 − B/W, where W is the centre spacing of the belts. We present a method for estimating BLA from the leaf area per unit length of belt LLA (m2 m−1), divided by the leaf area index LAI (m2 m−2) of nearby natural vegetation. Preliminary evaluation of BLA showed good agreement with BWB measured from water balance and BDD measured from deep drainage. The estimation of BLA for calculation of RD allows rapid estimates of the relative drainage reduction expected from alley farms in water-limited environments.