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The leaching of chloride from a de-watered, non-irrigated, saline soil profile

Bourgault du Coudray, Philip L. (1996) The leaching of chloride from a de-watered, non-irrigated, saline soil profile. PhD thesis, Murdoch University.

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Abstract

Extensive areas of non-irrigated agricultural land in Western Australia have become saline. Rehabilitation requires the immobilisation of salt at the catchment scale as well as the removal of excess salt from within the rooting zone. The latter issue requires the determination of the physical and temporal factors affecting the leaching of excess salt by rainfall.

The leaching of chloride from a saline de-watered soil profile near Kellerberrin, in the West Australian Wheatbelt, was investigated. The project involved three distinct phases that allowed a full analysis of the actual and potential leaching following the lowering of groundwater levels. Phase one involved a pedon-scale field experiment in which soil water tension and solute concentration were monitored in three stages:

A. With the dispersed saline soil surface untouched.
B. With deep cultivation of the soil surface to a depth of 250mm.
C. With deep cultivation of the soil surface and flooding with 70mm of water.

The pedon study revealed that surface crusting resulted in high amounts of runoff which was enhanced by a compacted layer at 100-250mm depth. Downward flow of water and solutes was prevented. The soil water tension only changed near the surface and no chloride displacement was observed. Following cultivation to 250mm, this compacted layer was broken. This resulted in more rainfall infiltration and water percolating through the profile was intercepted at a depth of 1.5m. Soil water tension remained the same throughout the profile, however, chloride displacement was observed. Flooding saturated the profile and resulted in further displacement of chloride.

Phase two involved a series of small laboratory scale exercises to explain field observations. These exercises concentrated on the role of preferred pathways on the leaching process. The preferred pathways conducted most of the infiltrating water and only marginally contributed to the leaching of chloride when compared with micropore (matrix) flow.

Phase three involved the use of the one-dimensional solute transport model, MACRO. Model parameters were derived from field and laboratory measurements and literature sources. i Most flow was identified as occurring within the micropore domain, with non-unform solute flow explained as conveyance through preferred pathways. The modelling allowed the predictive simulation of the time required to leach the chloride from the top 1.5m of the soil profile given three scenarios:

A. A bare uncultivated soil surface was maintained throughout the simulation.
B. Deep cultivation without surface sealing was maintained throughout the simulation.
C. Periodic deep cultivation followed by surface sealing.

Leaching would take at least 400 years if the soil remained uncultivated. For the second scenario, leaching could be achieved in as little as four years. The third scenario is a realistic option that enables farmers to achieve leaching within a generation.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Biological and Environmental Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: repository@murdoch.edu.au. Thank you.
Supervisor(s): Williamson, D. and Scott, William
URI: http://researchrepository.murdoch.edu.au/id/eprint/52471
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