Effect of land development on groundwater recharge determined from non-steady chloride profiles

The effect of clearing and subsequent crop and pasture growth on recharge to ground waters was investigated in three experimental catchments in the brigalow ( Acacia harpophylla) lands of north-eastern Australia. Recharge was calculated from soil chloride data, using a simple transient solute mass balance model. Clearing had a substantial initial effect on groundwater recharge, with average recharge rates of 29 to 70 mm year −1 in two cleared catchments, compared with 7 mm year −1 in an uncleared catchment. These results were attributed to record high rains that fell while both cleared catchments were bare of vegetation, before crops or pastures were established. The effect was only short lived, however, with no significant recharge occurring in any of the three catchments during the period in which crops and pastures were fully established. This lack of recharge was contrary to the general belief that clearing and establishment of crops or pastures causes a sustained increase in groundwater recharge. The low recharge rates at this site were attributed to the slowly permeable soils and the climate of the study site, where potential evaporation exceeds average rainfall in all months, and to the water use characteristic of brigalow. Brigalow is shallow-rooted, and at this site generated lower soil water deficits than either crops or pastures. Clearing is unlikely to result in high water-tables in these soils under pastures or opportunity cropping systems under the average climatic conditions of the region. The simple transient solute mass balance model used to estimate recharge rates gave important and significant differences in recharge when compared with a more commonly used steady-state model. All recharge rates calculated with the steady-state model were ⩽ 1.8 mm year −1 (most < 0.3 mm year −1), and so this model could not identify the gross short-term recharge response to clearing at this site.