Groundwater flow modelling to assist dryland salinity management of a coastal plain of southern Australia

Abstract

Groundwater flow modelling has been undertaken for an area of 10 500 ha within the regional unconfined aquifer system of a coastal plain of southern Australia, in the vicinity of the town of Cooke Plains, to predict the impact of various land management options (including recharge reduction and discharge enhancement) on the extent of land salinisation caused by shallow saline watertables. The model was calibrated against field data collected over 6 years. Sensitivity analysis was performed to assess the influence of mesh size, boundary conditions, and aquifer parameters, and particularly rates of recharge and evaporative discharge, on groundwater levels. These were varied until the model was shown to be capable of simulating seasonal trends and regional and local flow patterns. The model was then used to predict the impact of the management options on groundwater levels. The results showed that continuing current annual crop–pasture rotations will result in watertable rises of approximately 0·2 m in 20 years (significant in this setting), with a further 50 ha of land salinised. A reduction in the rates of groundwater recharge through the establishment of high water-use perennial pastures (e.g. lucerne) showed the most promise for controlling groundwater levels. For example, a reduction in recharge by 90% would result in watertable declines of 0·6–1·0 m within 5–10 years, with the return to productivity of 180 ha of saline land. Small-scale (say <100 ha) efforts to reduce recharge were found to have no significant impact on groundwater levels. Enhanced groundwater discharge such as pumping from a windmill was found to be non-viable due to the relatively high aquifer transmissivity and specific yield. The modelling approach has enabled a relatively small area within a regional aquifer system to be modelled for a finite time (20 years) and has shown that extension of the boundaries of the model would not have altered the predicted outcomes. Furthermore, the analysis of sensitivity to cell size in an undulating landscape where net recharge areas can become net discharge areas with only small increases in groundwater level is novel, and has helped to build confidence in the model. Modelling has demonstrated that dryland salinisation can be controlled by reducing groundwater recharge over substantial tracts of land, and is not dependent on recharge reduction over an extensive area upgradient, at least over the next 20 years.