Estimates of deep percolation beneath cotton in the Macquarie Valley

Expansion of flood irrigation in the Lower Macquarie Valley of New South Wales, Australia, has been suggested as a major cause of increased groundwater recharge. The aim of this study was to estimate deep percolation under irrigation on two soils in the valley, in order to infer groundwater recharge. Three methods were used; water balance, Darcian flux calculations and chloride mass balance modelling. Chloride mass balance modelling and the water balance method gave comparable estimates of deep percolation for each soil. Chloride mass balance modelling was identified as the most reliable method for estimating deep percolation, but only gave an estimate for the entire growing season. These estimates were 214 and 104 mm for a cracking clay and red brown earth, respectively. While there is potentially greater error associated with estimates obtained using the water balance, this technique provided estimates of deep percolation for each individual irrigation. Results of the water balance indicated that deep percolation was greatest early in the growing season, following initial wetting of the soil, when the crop had a low leaf area index. Results calculated using Darcian flux equations were highly variable, and were therefore unreliable estimates of deep percolation. Groundwater recharge, inferred from estimates of deep percolation determined with the chloride mass balance model, was used to estimate the magnitude of potential annual groundwater rise. The potential groundwater rise during the 1992/1993 cotton growing season ranged from 465 mm beneath the cracking clay to 267 mm under the red brown earth. It is suggested that groundwater recharge and rise were highly dependent on the weather conditions prevailing during this period. Received: 24 January 1997