Jim Wallace

Calibration and correction procedures for cosmic-ray neutron soil moisture probes located across Australia

The cosmic-ray probe (CRP) provides continuous estimates of soil moisture over an area of ~30 ha by counting fast neutrons produced from cosmic rays which are predominantly moderated by water molecules in the soil. This paper describes the setup, measurement correction procedures, and field calibration of CRPs at nine locations across Australia with contrasting soil type, climate, and land cover. These probes form the inaugural Australian CRP network, which is known as CosmOz. CRP measurements require neutron count rates to be corrected for effects of atmospheric pressure, water vapor pressure changes, and variations in incoming neutron intensity. We assess the magnitude and importance of these corrections and present standardized approaches for network-wide analysis. In particular, we present a new approach to correct for incoming neutron intensity variations and test its performance against existing procedures used in other studies. Our field calibration results indicate that a generalized calibration function for relating neutron counts to soil moisture is suitable for all soil types, with the possible exception of very sandy soils with low water content. Using multiple calibration data sets, we demonstrate that the generalized calibration function only applies after accounting for persistent sources of hydrogen in the soil profile. Finally, we demonstrate that by following standardized correction procedures and scaling neutron counting rates of all CRPs to a single reference location, differences in calibrations between sites are related to site biomass. This observation provides a means for estimating biomass at a given location or for deriving coefficients for the calibration function in the absence of field calibration data.

Flood water quality and marine sediment and nutrient loads from the Tully and Murray catchments in north Queensland, Australia

Current estimates of sediment and nutrient loads from the Tully–Murray floodplain to the Great Barrier Reef lagoon are updated by taking explicit account of flood events. New estimates of flood discharge that include over-bank flows are combined with direct measurements of sediment and nutrient concentrations in flood waters to calculate the loads of sediment and nutrient delivered to the ocean during 13 floods that occurred between 2006 and 2008. Although absolute concentrations of sediment and nutrient were quite low, the large volume of water discharged during floods means that they make a large contribution (30–50%) to the marine load. By not accounting for flood flows correctly, previous estimates of the annual average discharge are 15% too low and annual loads of nitrogen and phosphorus are 47% and 32% too low respectively. However, because sediments may be source-limited, accounting for flood flows simply dilutes their concentration and the resulting annual average load is similar to that previously estimated. Flood waters also carry more dissolved organic nitrogen than dissolved inorganic nitrogen and this is the opposite of their concentrations in river water. Consequently, dissolved organic nitrogen loads to the ocean may be around twice those previously estimated from riverine data.

Biophysical status of remnant freshwater floodplain lagoons in the Great Barrier Reef catchment: a challenge for assessment and monitoring

We investigated the biophysical environment, invertebrate fauna and ecosystem health of lagoons on the Tully–Murray floodplain in the Queensland Wet Tropics bioregion. These wetlands are biologically rich but have declined in area and condition with agricultural development and are poorly protected, despite being located between two World Heritage areas. Lagoons varied in size, habitats and water quality, with increasing signatures of agriculture (e.g. elevated nutrient concentrations) from the upper to lower floodplain. Zooplankton were abundant, but not diverse, and correlated variously with environmental variables, so were not useful in assessing lagoon condition. Benthic macroinvertebrates were abundant and diverse and correlated strongly with riparian condition, habitats, water quality and degree of agriculture in the catchment, but gradients in assemblage structure were not strong because the flow regime, with multiple annual floods, maintains higher water quality than in some tropical systems. The absence of pristine reference lagoons and the limited availability of replicate sites hamper the development of monitoring systems. Nevertheless, we show that appropriate sampling, analysis and knowledge of comparable systems allow inferences to be drawn regarding ecological condition. This is important because environmental managers need best available and timely advice whatever the opportunities for rigorous study design.

The temperature regimes of dry-season waterholes in tropical northern Australia: potential effects on fish refugia

The ephemeral rivers in northern Australia break up into a series of waterholes during the long, dry summer season. These in-stream waterholes provide vital habitat for the survival of aquatic biota during this period. We describe how high-time-resolution (20 min) waterhole temperature measurements made in the Flinders and Gilbert Rivers in tropical northern Australia were used to derive thermal frequency curves that show how often waterhole temperature exceeded any given temperature threshold. During the summer period, temperatures near the surfaces of waterholes were often above that suitable for the optimum growth of some tropical fish (31°C). At the bottom of waterholes, this exceedance occurred less often, and in turbid waterholes that were stratified, temperatures rarely exceeded this threshold. Temperatures that could be lethal to some fish (34°C) also were exceeded at the surface of waterholes, but rarely, if ever, at the bottom of waterholes. An energy-balance model was used to estimate daily mean waterhole temperature with good accuracy (±1 K) at all but the sites where wind speed may have been >2 m/s (assumed in the model). The model also was used to predict the effects of climate change on waterhole temperature and the change in exceedance of thermal thresholds. A 2 K climate warming raised waterhole temperature by ∼1 K. However small this increase might seem, it led to a doubling of the length of time water temperatures were in excess of thresholds around 31°C.

Persistence of in-stream waterholes in ephemeral rivers of tropical northern Australia and potential impacts of climate change

Many northern Australian rivers have limited or non-existent dry season flow and rivers tend to dry to a series of pools, or waterholes, which become particularly important refugial habitat for aquatic biota during the periods between streamflow events. The present study developed techniques to identify in-stream waterholes across large and inaccessible areas of the Flinders and Gilbert catchments using Landsat imagery. Application of this technique to 400 scenes between 2003 and 2010 facilitated the identification of key waterhole refugia that are likely to persist during all years. Relationships for predicting total waterhole area from streamflow characteristics were produced for four river reaches. Using these relationships and streamflow predictions based upon climate data scaled using 15 global climate models, the potential impacts of future climate on waterhole persistence was assessed. Reductions in waterhole area of more than 60% were modelled in some years under drier scenarios and this represents a large reduction in available habitat for areas that already have limited in-stream refugia. Conversely, under wetter future climates the total area of waterholes increased. The approach developed here has applicability in other catchments, both in Australia and globally, and for assessing the impacts of changed flow resulting from water resource development.

Modelling and dynamic simulation of struvite precipitation from source-separated urine

A model of a mixed-mode nutrient recovery reactor is developed for a urine feed, incorporating complex solution thermodynamics, dynamic conservation relations and a power-law kinetic expression for crystal growth from seed crystals. Simulations at nominal operating conditions predict phosphorus recoveries greater than 99%, based on existing process kinetic parameters and operating conditions employed in previously published studies. The overall rate of nutrient recovery depends on the saturation index of the precipitating solid, the available surface area for mass transfer and the feed rate of the limiting constituent ion. Under the conditions considered, the nutrient feed rate appears to be the limiting factor for precipitation. Simulations demonstrate that diurnal feed flow variations of ±50% have a small effect on the rate of nutrient recovery. Overall, the study shows that valuable insights are gained in relation to process performance predictions, which should lead to more confident process design, operation and control.

A comparison of temperature regimes in dry-season waterholes in the Flinders and Gilbert catchments in northern Australia

Dry-season waterholes in ephemeral rivers provide vital habitat for aquatic biota, whose survival is dependent on the waterholes lasting throughout the dry season with temperatures that are not lethal. To examine this in the Flinders and Gilbert Rivers, 20-min temperature measurements were taken during the 2012–2013 dry season in 10 waterholes in each catchment. These data were used to derive thermal-frequency curves that quantify how often waterhole temperature exceeds thresholds for (1) the optimum growth of tropical fish and (2) their lethal temperature. Waterholes that remained deeper than ~0.5m throughout the dry season provided thermally suitable fish refugia, especially if they were turbid, because this decreased the risk of exposure to undesirable temperatures at the bottom of the waterhole. However, surface temperatures in these waterholes often exceeded optimal and even potentially lethal temperatures, so fish may have had to move to cooler water at the bottom of these waterholes. The risk to aquatic species in waterhole refugia is, therefore, primarily determined by depth, with shallow waterholes presenting the greatest risk because they become thermally unsuitable well before (1–2 months) they fully dry out.

Fish larvae and recruitment patterns in floodplain lagoons of the Australian Wet Tropics

Floodplain lagoons in the Queensland Wet Tropics bioregion, Australia, are important and threatened habitats for fish. As part of studies to assess their ecological condition and functions, we examined patterns of occurrence of fish larvae, juveniles and adults in 10 permanent lagoons on the Tully–Murray floodplain. Lagoons contained early life-history stages of 15 of the 21 native species present, including 11 species that complete their life cycle in fresh waters and 4 that require access to saline habitats for larval development. Lagoon connectivity to the rivers, distance from the coast and flood dynamics influenced temporal variation in fish abundance, population size structures and recruitment patterns. This study and the literature show that wet, post-wet and dry-season habitats are utilised by small opportunists (e.g. Melanotaenia splendida), an equilibrium species (Glossamia aprion) and larger periodic strategists (neosilurid catfishes). Maintenance of natural seasonal patterns of flow and connectivity, and active protection of permanent floodplain lagoons from riparian and land-use disturbance, will be essential if their roles in fish recruitment are to be sustained.