Soil-water content characterisation in a modified Jarvis-Stewart model: a case study of a conifer forest on a shallow unconfined aquifer

Groundwater-vegetation-atmosphere fluxes were monitored for a subtropical coastal conifer forest in South-East Queensland, Australia. Observations were used to quantify seasonal changes in transpiration rates with respect to temporal fluctuations of the local water table depth. The applicability of a Modified Jarvis-Stewart transpiration model (MJS), which requires soil-water content data, was assessed for this system. The influence of single depth values compared to use of vertically averaged soil-water content data on MJS-modelled transpiration was assessed over both a wet and a dry season, where the water table depth varied from the surface to a depth of 1.4 m below the surface

Pore Water Exchange Processes in Offshore Intertidal Sandbanks

In recent years blooms of the toxic marine cyanobacteria Lyngbya majuscula have been frequently observed in a system of offshore intertidal sandbanks in Moreton Bay, Australia. Past research suggests that these blooms are linked to the presence of bioavailable forms of iron. Submarine groundwater discharge (SGD) is thought to provide an important nutrient (particularly iron) transport mechanism at locations where L. majuscula blooms are observed. In this study the feasibility of the current conceptual model for SGD to offshore L. majuscula bloom sites was examined using a combination of field measurements and numerical models. Numerical simulations indicated that SGD could occur several kilometres offshore within the sandbank system if a continuous, low hydraulic conductivity aquitard extends from the onshore aquifer to the offshore sandbank. Models that incorporated significant discontinuities or gaps in the aquitard unit resulted in SGD at the location of these discontinuities before reaching the offshore sandbank system. Improved hydrogeological data, particularly in relation to the presence and continuity of any aquitard units, is required to enable a better assessment of SGD mechanisms in this area. Simulated maximum instantaneous vertical flow velocities in the near-surface zone of the sediments around the offshore exit point were an order of magnitude lower the velocities associated with local-scale pore water flow processes that are driven by tidal and subtidal pumping. As such these local-scale flow processes (instead of SGD) are likely to provide the dominant pore water exchange mechanisms in the offshore sandbank systems. Field data was collected from a L. majuscula bloom site at an offshore intertidal sandbank within the Eastern Banks of the Bay to examine these local-scale flow processes. The measurements identified a tidally-induced residual pore water flow system in the sandbank, with seawater entering the upper sandbank platform and discharging through the bank edge. Furthermore the field data revealed concentrations of bio-available iron in pore waters that were more than four times greater than concentrations previously shown to stimulate L. majuscula growth. Upward flow and elevated near-surface dissolved Fe(II) concentrations (> 20 µM Fe(II) at -0.05 m depth) were measured simultaneously in the discharge zones at the sandbank edge. Analysis of dissolved Fe(II) profiles within pore waters at the site showed that measurable concentrations of dissolved Fe(II) were absent at the base (-2 m depth) of all monitoring stations. This result seems to indicate that terrestrially derived SGD is not an iron source at the site. Numerical simulations of the tidally-induced pore water flow at the field site generally showed good agreement with the field data. Simulation results revealed a more complex residual pore water flow system in the sandbank than shown by the field data. Both numerical simulations and field measurements suggest that the residual flow might provide a mechanism for transport of bio-available iron across the sediment-water interface. During periods when these sandbanks are submerged a number of pore water exchange processes, collectively termed subtidal pumping, are likely to be operating at the site. Numerical models were developed to examine one particular form of subtidal pumping, i.e., current-bedform interactions. These models were based on a one-way sequentially coupled model of surface water and porous medium flow. Interactions between unidirectional and bi-directional currents with simple two-dimensional fixed bedforms were considered. Results suggested that while pore water exchange was likely in distinct zones around the sandbank edges, a whole-of-bank scale pore water circulation was unlikely to occur. Smaller scale (< 1.0 m) variations in the sandbank topography might exert a stronger influence on the pore water flow and associated pore water exchange. Simulations showed more complex pore water flow paths and longer pore water residence times in systems under bi-directional flow conditions compared to systems subject to unidirectional flows. The consideration of bi-directional flows is important in modelling current-bedform interactions in tidally influenced environments. The findings of this study have implications for the current conceptual model for L. majuscula blooms in offshore intertidal sandbanks within Moreton Bay. This conceptual model should be updated to include tidal and subtidal pumping as potential delivery mechanisms for bio-available iron. Further investigations are required to fully understand the role of tidal and subtidal pumping in controlling the export of bio-available iron to coastal waters at the field site. In particular there is a need to better assess the link between the pore water flows and the geochemical reactions that might occur along the flow path. Improved data on pore water chemistry as well as the solid phase mineralogy is required to develop reactive transport models for these flow systems. Such models would allow further investigation of the importance of tidal and subtidal pumping as a mechanism for exporting dissolved chemicals across the sediment-water interface

Oceanic pulse forcing of a beach groundwater system

This paper presents the findings of a pilot study into oceanic pulse forcing of a sandy coastal aquifer. Field observations of the response of the aquifer to a wave setup induced pulse in shoreline levels reveal that the pulse penetrates further into the aquifer than the semi-diurnal tidal signal. The data are then used to evaluate the abilities of an existing analytical solution and a numerical model to predict the propagation of the pulse in the interior of the aquifer. Both the analytical and numerical models predict the growth of the time lag with landward distance of the pulse adequately but fail to reproduce the decay of the pulse, significantly over predicting the decay rates. In this instance, the presence of tidal fluctuations and a sloping ocean boundary have been eliminated as potential causes but another possible candidate that warrants further investigation in the future is the infiltration from wave runup which would act to increase groundwater levels

Field-scale numerical modeling of a dense multiport diffuser outfall in crossflow

A numerical investigation of near-field brine discharge dynamics is reported for the Gold Coast Desalination Plant offshore inclined multiport brine diffuser. Quasi-steady computational fluid dynamics simulations were performed using the Reynolds Averaged Navier Stokes equations with a k-ω Shear Stress Transport turbulence closure scheme. Simulations used an iterative mesh domain with length of 400 m, width of 200 m, and average depth of 24.2 m. Longshore crossflow conditions were examined with a current velocities range of 0.03-0.26 m/s. The alternating port orientation of the diffuser resulted in simultaneous co- and counterflowing discharges. Impact distance, impact dilution, and terminal rise locations were compared against the existing literature, and dimensionless empirical equations were fitted as functions of the current speed. Transverse spread and resulting salinity increases were also assessed against field measurements. For the first time, the areal extent of seafloor salinity increase is examined, with the quasi-quiescent regime holistically presenting the worst-case conditions. Plume trajectory, dilution, areal salinity intensity, and plume dispersion after impact each reflect distinct variations between jet- and crossflow-dictated regimes at a threshold value of urF≈0.8 (ur = ambient to jet velocity ratio; F = jet densimetric Froude number). This behavior depends on the presence of the arrested upstream sublayer that, in turn, has consequences for the application of empirical models to multiport discharges under low-crossflow regimes. This study demonstrates significant advancements over existing empirical and integral modeling methods, with strong application potential for designers, plant operators, and regulators

Turbulent mixing and sediment processes in Peri-urban estuaries in south-east Queensland (Australia)

An estuary is formed at the mouth of a river where the tides meet a freshwater flow and it may be classified as a function of the salinity distribution and density stratification. An overview of the broad characteristics of the estuaries of South-East Queensland(Australia) is presented herein, where the small peri-urban estuaries may provide an useful indicator of potential changes which might occur in larger systems with growing urbanisation. Small peri-urban estuaries exhibits many key hydrological features and associated with ecosystem types of larger estuaries, albeit at smaller scales, often with a greater extent of urban development as a proportion of catchment area. We explore the potential for some smaller peri-urban estuaries to be used as natural laboratories to gain some much needed information on the estuarine processes, although any dynamics similarity is presently limited by critical absence of in-depth physical investigation in larger estuarine systems. The absence of the detailed turbulence and sedimentary data hampers the understanding and modelling of the estuarine zones. The interactions between the various stake holders are likely to define the vision for the future of South-East Queensland's peri-urban estuaries. This will require a solid understanding of the bio-physical function and capacity of the peri-urban estuaries. Based upon the knowledge gap, it is recommended that an adaptive trial and error approach be adopted for the future of investigation and management strategies

Does fire alter soil water repellency in subtropical coastal sandy environments?

Irregular wetting, water repellency, and preferential flow are well-documented properties of coastal sandy podzols, though little is known about the effect of fire on unsaturated zone processes in this environment. This study investigates water repellency at and below the soil surface in two coastal sandy podzols following bushfire. Water drop penetration time tests were applied to burned and unburned soils at a high dune field site in South East Queensland, Australia. It was found that the mean water drop penetration time of the burned soil was four times that of the unburned soil, but both soils were largely non-repellent. Post-fire repellency peaked below the surface in a patchy layer, in contrast to the laterally extensive layer reported in other studies, and high organic matter content in the soil did not appear to significantly influence repellency post-burn. Non-parametric statistics were used to quantify the high spatial variability in water repellency, which was ultimately insufficiently captured by atypically large (n = 1000 drop) datasets. This study confirms the presence of naturally occurring repellency and patchy infiltration in sandy soils while demonstrating that conclusively describing the influence of fire is challenging in a soil with heterogeneous infiltration characteristics. With respect to this uncertainty, it appears that fire does not increase soil water repellency such that infiltration and runoff processes due to fire-induced water repellency would differ post-burn