Modelling unseen flow pathways of water and contaminants in the Wet Tropics: the role of alluvial palaeochannels
Other Publication ResearchOnline@JCUAbstract
Nutrients from agriculture in catchments draining to the Great Barrier Reef (GBR) are a stressor of this important ecosystem. Current GBR catchment models do not mechanistically link movement of nutrients from paddocks to rivers. An understanding of these water and nutrient flow pathways is crucial in any attempt to model and manage the GBR catchments. Conduits of water transport include surface drains and subsurface features such as palaeochannels. The presence of palaeochannels results in heterogeneity of soil and aquifer properties, which need to be accounted for in any attempt to assess water and nutrient transport in these flat agricultural landscapes. This project addressed this gap by looking at preferential flow pathways, via palaeochannels, of water and nutrients at the paddock to catchment scale using the Babinda Swamp Drainage Area (BSDA) in the Wet Tropics as a study site. This site has high modelled export of dissolved inorganic nitrogen (DIN) as well as many palaeochannels and drains, both of which have recently been mapped. A better understanding of the palaeochannel system in relation to the individual paddocks and surface drains was achieved by spatial analysis. Approximately half of the mapped paddocks had palaeochannels covering at least 14.5% of their area, with a small number of paddocks along the Russell River having over 50% of the paddock area covered by palaeochannels. The length of surface drains that intersected palaeochannels per paddock is, on average, 887.3 m which corresponds to approximately 74% of the total paddock boundary. For paddocks with palaeochannels, almost half (44.3%) of the paddock area was closer to the palaeochannel than to a drain. The shallow nature of the mapped palaeochannels, which were within 1.5 m of the surface, meant that they likely intersected with surface drains, especially the deeper trunk drains in the BSDA. A literature review of existing models highlighted the need for coupled models to capture the surface-subsurface water interactions in an environment with preferential flow paths such as palaeochannels. To enable an assessment of the likely influence of paleochannels on water and nutrient flows to surface drains, a framework for virtual experiments was set up by coupling the crop model APSIM with the groundwater model MODFLOW. In a preliminary virtual experiment, three scenarios with different palaeochannel configurations were modelled to assess the impact of these preferential flow pathways on water flux from a paddock. The MODFLOW simulations were run with steady state conditions, palaeochannels (when present), running parallel to a hydraulic gradient, and lateral Ks of the palaeochannel being 10 times higher than non-palaeochannel soils. The experiments showed that palaeochannels altered subsurface flow patterns and water flux. A subsurface palaeochannel increased water flux by 4.4% and a surface palaeochannel increased it by 3.5%. Palaeochannels caused a slight divergence of flow towards them. With some finessing, the coupled model set-up will allow more virtual experiments to simulate the effects of different palaeochannel scenarios that are likely encountered in the BSDA landscape. Such experiments could be used to determine sensitivity of flows to palaeochannel density and characteristics and thereby enable design of efficient models to account for their effects.
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93
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Queensland Department of Environment and Science
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Brisbane, QLD, Australia
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