Irrigation increases groundwater recharge in the Macquarie Valley

Abstract

Rising groundwater, and the potential for development of shallow watertables, were recognised in the Lower Macquarie Valley of New South Wales in the late 1980s. Irrigated agriculture was proposed as a possible source of the recharge causing the problem. This paper reports the increase 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, using temporally separated chloride profiles and mass balance modelling. These changes in long-term deep percolation rates were integrated over all years since irrigation commenced. Irrigation affected deep percolation on all soils, with an increase in long-term mean rates ranging from 17 to 202 mm/year. This equated to increased leaching rates ranging from 3 to 25%. Deep percolation appeared to be related to the clay content of the B horizon. The potential groundwater rise varied from 37 to 524 mm/year. The largest increases in deep percolation rates corresponded 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 Macquarie Valley.