Integrating agroforestry and perennial pastures to mitigate water logging and secondary salinity

The studies at the ‘Ucarro’ site have identified rapid transmission of vertical drainage from the root zone to re-charge a transmissive aquifer that conveys virtually all the hill-slope outflow to the riparian zone of the local stream. This feature of a predominance of deep drainage in total outflow contrasts with other regional and national studies on duplex soils where lateral flow within the root zone constitutes a significant component of the total outflow. Hydraulic properties of the ‘Ucarro’ subsoil matrix are consistent with those of the other studies, suggesting that bypass flow, operative during phases of soil saturation, is the critical mechanism in determining this distinctive hydrologic response. These observations at ‘Ucarro’ serve to indicate that partitioning of rainfall excess between lateral and vertical flow is fraught with difficulty. Simulation of hydrologic response, reliant on hydraulic characterization of duplex soils for drainage prediction, needs to be qualified with large bounds of uncertainty. Differences in annual evapotranspiration were evaluated for three categories of plant community to reveal a ranking according to depth of root uptake of soil water. The woody community used water in excess of annual rainfall by 100 mm due to uptake of groundwater. The herbaceous perennial, lucerne, used marginally less than annual rainfall whilst the annual pasture exploited less of the annual rainfall in most years. Additional evapotranspiration was achieved mainly during the summer/autumn season when differences in soil water uptake between communities were apparent. These seasonal differences in soil water uptake also explained the paradox that annual evapotranspiration was inversely related to above-ground growth and also to the maximum leaf area attained by the community during the year. Persistence of plant activity over the year was the major determinant of annual evapotranspiration and depth of rooting was a significant factor to extend the period of plant activity in supplying additional evapotranspiration. The amount of re-charge to groundwater for each community was approximately commensurate with the difference between annual values of rainfall and evapotranspiration with average annual values estimated to range from −100 to 35 mm. Calculations based on observed evapotranspiration by each category of community provided a range of hill-slope outflow for various combinations of tree cover, lucerne and annual agricultural communities as crop and pasture. An optimal mix comprising 12% tree cover, 30% lucerne and 58% annual crop and pasture offered a viable agricultural enterprise with outflow control at 5 mm per year, approaching that of the natural water balance. Integration of persistent perennial species, woody and herbaceous, with traditional agriculture provides satisfactory drainage control in this environment to ameliorate existing outbreaks of salinity.