Modelling subsurface flow conditions in a salinized catchment in south-western Australia, with a view to improving management practices
✍ Scribed by W. J. Stolte; R. J. George; D. J. McFarlane
- Publisher
- John Wiley and Sons
- Year
- 1999
- Tongue
- English
- Weight
- 229 KB
- Volume
- 13
- Category
- Article
- ISSN
- 0885-6087
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✦ Synopsis
Finite element modelling of the saturated±unsaturated surface±subsurface ¯ow mechanisms operative in a small salinized catchment in south-western Australia was used to help de®ne the ¯ow system and explain the causes of waterlogging and salinization there. Data available at the site from a previous study were used to obtain a ®rst approximation to the ¯ow system. Altering the properties of some of the strata gave a closer calibration. It was found that the modelled saturated hydraulic conductivity of the B horizon in the duplex soil zone needed to be at least an order of magnitude lower than that measured in order to reproduce the perching conditions observed in the ®eld. Also, the model indicated the in¯uence of a doleritic dyke, whose presence was con®rmed by ®eld measurement.
Our analysis showed that there were two main ¯ow systems operating in the hillslope. The ®rst, and most dominant, was the recharge occurring through the upslope gradational soil zone and percolating down to both the deeply weathered regolith and the basal aquifer. The second ¯ow system is an unsaturated ¯ow system operating in the high permeability A horizon in the downslope duplex soil zone. The ®rst system is primarily responsible for the saline seepage zone in the valley bottom. The second contributes to the waterlogging and perching occurring upslope of the seepage zone.
Vertical ¯ow through the higher permeability B horizon in the gradational soil zone in the upper slopes is a major contributor of recharge. Recharge by ¯ow through macropores occurs where, but only where, perched aquifers develop and allow the macropores to be activated. Areas with perched aquifers occurred in downslope locations and near a doleritic dyke located upslope. Thus, the area where macropore recharge occurred was not large.
The recharge rate required to maintain the piezometric levels at present values is only about 30 mm/yr (about 5% of the annual rainfall). The piezometric levels under the upper part of the catchment varied greatly with only small changes in recharge rate. A 50% reduction in recharge rate had the eect of reducing the length of the seepage zone at the end of winter by 40%. Changes in recharge rate had little eect on the extent of the perched aquifer at the end of winter. Deep-rooted perennial forages, shrubs or trees on the gradational soil zone in the upper part of the catchment and on the zones upslope of geological barriers to ¯ow would be required to reduce the recharge and to allow for rehabilitation of the saline valley ¯oor. Waterlogging associated with the perched water table in the bottom part of the catchment would be best addressed by tree plantations located just upslope of the salinized zone in the valley ¯oor.