The influence of tagasaste (Chamaecytisus proliferus Link.) trees on the water balance of an alley cropping system on deep sand in south-western Australia

Components of the water balance of an alley cropping system were measured t o assess the extent to which tree rows 30 m apart with access to a fresh, p erched watertable at 5 m depth were able to capture deep drainage from an i nter-cropped cereal-legume rotation. Neutron probe data showed that the 4-y ear-old trees, cut back to 0.6-m high at the beginning of the experiment, d epleted soil water to 2, 4, and 8 m laterally from the tree rows in their f irst, second, and third years of coppice regrowth, respectively. Combining data from soil water depletion in summer and comparisons of deuterium: hydr ogen ratios of groundwater, xylem sap of trees, and herbaceous plants, it w as shown that tagasaste trees drew on soil water for 80% of their transpira tion in the first winter and 40% in the second, while switching to near tot al dependence on groundwater each summer and early autumn. Tree water use o n a whole plot basis was 170 mm in 1997 (68% from groundwater) v. 167 mm in 1998 (73% from groundwater). Recharge to the perched watertable was estima ted to be 193 mm under sole crop in 1998 (52% of rainfall), reducing to 32 mm when uptake of groundwater by trees was included. The degree of compleme ntarity between tagasaste trees and crops in alley cropping used for water management is quantified for 1998 by calculating the ratio of the distance over which trees reduced drainage to zero to the distance over which they r educed crop yield to zero. It is concluded that segregated monocultures of trees and crops would be a more appropriate strategy than a closely integra ted system such as alley cropping in this case.