IRRIGATION INCREASES GROUNDWATER RECHARGE IN THE MACQUARIE VALLEY

Rising groundwater, and the potential for development of shallow water tables, were recognised in the Lower Macquarie Valley of New South Wal es in the late 1980s. Irrigated agriculture was proposed as a possible source of the recharge causing the problem. This paper reports the in crease in deep percolation rates resulting from cotton irrigation on 4 soils in the Lower Macquarie Valley, New South Wales. Changes in deep percolation rates were measured on these soils over the long-term, us ing temporally separated chloride profiles and mass balance modelling. These changes in long-term deep percolation rates were integrated ove r all years since irrigation commenced. Irrigation affected deep perco lation on all soils, with an increase in long-term mean rates ranging from 17 to 202 mm/year. This equated to increased leaching rates rangi ng from 3 to 25%. Deep percolation appeared to be related to the clay content of the B horizon. The potential groundwater rise varied from 3 7 to 524 mm/year. The largest increases in deep percolation rates corr esponded to sites where the watertable was closest to the soil surface . This suggests that the development of shallow watertables is related to recharge resulting from irrigated agriculture. Detailed studies of deep percolation under irrigated agriculture are required in the Macq uarie Valley.