Brigalow Catchment Study

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Journal Papers

Arnold, S., Thornton, C., Baumgartl, T. (2012). Ecohydrological feedback as a land restoration tool in the semi-arid Brigalow Belt, QLD, Australia. Agriculture, Ecosystems and Environment. 163: 61-71.

In this study of native plant communities in the Brigalow Belt - a semi-arid bioregion of Queensland and New South Wales, Australia - an ecohydrological model was designed to investigate the complex feedback relationships existing between plant community traits and soil water dynamics among post-disturbance (i.e. mining and agricultural) landscapes. Two distinct locations (having similar composition and climatic environment, yet different soil water dynamics) were selected to compare the interaction and sensitivity of these traits towards water evaporation from soil or from plant transpiration. The model is constrained by soil physical attributes and climate data monitored at the Brigalow Catchment Study, and plant community parameters were derived using Latin hypercube sampling and pattern oriented modelling. Our findings indicate that, under the given soil-climate constraint, plant communities could only thrive if they were able to avoid prolonged periods of water stress (e.g., by minimising their physiological wilting point). Further, the influence of vegetation dynamics on evaporation from soil was deemed to be critical for the simulated soil water dynamics, whereas plant transpiration affected soil moisture only marginally. Moreover, both monitoring sites were dominated by the same species but co-dominated by different tree species, suggesting that evaporation from soil was probably influenced by the co-dominant species, whereas transpiration was probably controlled by the dominant species. For the re-establishment of native plant communities on post-mined landscapes and for agro-forestry and resource management in the Brigalow Belt, this implies that inherent ecosystem processes exist, which control plant community development and, hence, ecohydrological functions such as regulation of evapotranspiration. Therefore, restoration strategies should carefully reflect on species composition and their ecohydrological functions rather than attempting to re-establish the pre-disturbance ecosystem form and function - which could be less robust and even unsuccessful given post-disturbance conditions and under altered soil conditions of post-mined landscapes or the uncertainty of future climatic environments. © 2012 Elsevier B.V.

Bradshaw, C.J.A., Bowman, D.M.J.S., Bond, N.R., Murphy, B.P., Moore, A.D., Fordham, D.A., Thackway, R., Lawes, M.J., McCallum, H., Gregory, S.D., Dalal, R.C., Boer, M.M., Lynch, A.J.J., Bradstock, R.A., Brook, B.W., Henry, B.K., Hunt, L.P., Fisher, D.O., Hunter, D., Johnson, C.N., Keith, D.A., Lefroy, E.C., Penman, T.D., Meyer, W.S., Thomson, J.R., Thornton, C.M., VanDerWal, J., Williams, R.J., Keniger, L., Specht, A. (2013). Brave new green world - Consequences of a carbon economy for the conservation of Australian biodiversity. Biological Conservation. 161: 71-90.

Pricing greenhouse gas emissions is a burgeoning and possibly lucrative financial means for climate change mitigation. Emissions pricing is being used to fund emissions-abatement technologies and to modify land management to improve carbon sequestration and retention. Here we discuss the principal land-management options under existing and realistic future emissions-price legislation in Australia, and examine them with respect to their anticipated direct and indirect effects on biodiversity. The main ways in which emissions price-driven changes to land management can affect biodiversity are through policies and practices for (1) environmental plantings for carbon sequestration, (2) native regrowth, (3) fire management, (4) forestry, (5) agricultural practices (including cropping and grazing), and (6) feral animal control. While most land-management options available to reduce net greenhouse gas emissions offer clear advantages to increase the viability of native biodiversity, we describe several caveats regarding potentially negative outcomes, and outline components that need to be considered if biodiversity is also to benefit from the new carbon economy. Carbon plantings will only have real biodiversity value if they comprise appropriate native tree species and provide suitable habitats and resources for valued fauna. Such plantings also risk severely altering local hydrology and reducing water availability. Management of regrowth post-agricultural abandonment requires setting appropriate baselines and allowing for thinning in certain circumstances, and improvements to forestry rotation lengths would likely increase carbon-retention capacity and biodiversity value. Prescribed burning to reduce the frequency of high-intensity wildfires in northern Australia is being used as a tool to increase carbon retention. Fire management in southern Australia is not readily amenable for maximising carbon storage potential, but will become increasingly important for biodiversity conservation as the climate warms. Carbon price-based modifications to agriculture that would benefit biodiversity include reductions in tillage frequency and livestock densities, reductions in fertiliser use, and retention and regeneration of native shrubs; however, anticipated shifts to exotic perennial grass species such as buffel grass and kikuyu could have net negative implications for native biodiversity. Finally, it is unlikely that major reductions in greenhouse gas emissions arising from feral animal control are possible, even though reduced densities of feral herbivores will benefit Australian biodiversity greatly. © 2013 Elsevier Ltd.

Cowie, B.A., Thornton, C.M., Radford, B.J. (2007). The Brigalow Catchment Study: I. Overview of a 40-year study of the effects of land clearing in the brigalow bioregion of Australia. Australian Journal of Soil Research. 45 (7): 479-495.

This paper describes a long-term, paired-catchment study, its broad findings, and considerations for future resource management of brigalow lands in north-eastern Australia. The Brigalow Catchment Study (BCS) commenced in 1965 with a pre-clearing calibration phase of 17 years to define the hydrology of 3 adjoining catchments (12-17 ha). After 2 catchments were cleared in 1982, 3 land uses (brigalow forest Acacia harpophylla, cropping, and grazed pasture) were monitored for water balance, resource condition and productivity, providing information for scientific understanding and resource management of the major land uses of the brigalow bioregion. In addition, this paper draws upon several project reviews to highlight the value of the BCS as an 'outdoor laboratory', its data resource, and to reflect on the study's scientific rigor to support present and future value. An assessment of the BCS against national and international attributes of best practice for long-term studies showed the study to rate highly in aspects of design, implementation, monitoring, and data management, and moderately in formal publication, strategic management, and networking. The literature shows that Brigalow Catchment Study is the longest paired-catchment study in Australia, and continues to sample the interactions between climate, soils, water, land use, and management. Finally, this paper provides the context for component-specific papers on changes in hydrology, productivity, and salt balance. Results from the study to date include: a doubling of runoff after clearing, a reduction in wheat yield by more than 60% over 20 years, a halving of pasture availability 3 years after clearing, a decline in cattle liveweight gain of 4 kg/ha.year over an 8-year period with a constant stocking rate, and the leaching of 60% of the root-zone (0-1.5 m) chloride after clearing for cropping. Unanticipated applications of the data from the study include: (i) a crucial set of soil samples for calibration of the RothC soil carbon model used to estimate Australia's soil carbon emissions; and (ii) estimates of deep drainage as a basis for salinity risk assessment in the region. © CSIRO 2007

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Dalal, R.C., Thornton, C.M., Cowie, B.A. (2013). Turnover of organic carbon and nitrogen in soil assessed from δ13C and δ15N changes under pasture and cropping practices and estimates of greenhouse gas emissions. Science of the Total Environment. 465: 26-35.

The continuing clearance of native vegetation for pasture, and especially cropping, is a concern due to declines in soil organic C (SOC) and N, deteriorating soil health, and adverse environment impact such as increased emissions of major greenhouse gases (CO2, N2O and CH4). There is a need to quantify the rates of SOC and N budget changes, and the impact on greenhouse gas emissions from land use change in semi-arid subtropical regions where such data are scarce, so as to assist in developing appropriate management practices. We quantified the turnover rate of SOC from changes in δ13C following the conversion of C3 native vegetation to C4 perennial pasture and mixed C3/C4 cereal cropping (wheat/sorghum), as well as δ15N changes following the conversion of legume native vegetation to non-legume systems over 23years. Perennial pasture (Cenchrus ciliaris cv. Biloela) maintained SOC but lost total N by more than 20% in the top 0-0.3m depth of soil, resulting in reduced animal productivity from the grazed pasture. Annual cropping depleted both SOC and total soil N by 34% and 38%, respectively, and resulted in decreasing cereal crop yields. Most of these losses of SOC and total N occurred from the >250μm fraction of soil. Moreover, this fraction had almost a magnitude higher turnover rates than the 250-53μm and <53μm fractions. Loss of SOC during the cropping period contributed two-orders of magnitude more CO2-e to the atmosphere than the pasture system. Even then, the pasture system is not considered as a benchmark of agricultural sustainability because of its decreasing productivity in this semi-arid subtropical environment. Introduction of legumes (for N2 fixation) into perennial pastures may arrest the productivity decline of this system. Restoration of SOC in the cropped system will require land use change to perennial ecosystems such as legume-grass pastures or native vegetation. © 2013 Elsevier B.V.

Dalal RC, Thornton CM, Allen DE, Kopittke PM (2021). A study over 33 years shows that carbon and nitrogen stocks in a subtropical soil are increasing under native vegetation in a changing climate. Science of the Total Environment 722: 145019.

Soil plays a critical role in the global carbon (C) cycle. However, climate change and associated factors, such as warming, precipitation change, elevated carbon dioxide (CO2), and atmospheric nitrogen (N) deposition, will affect soil organic carbon (SOC) stocks markedly – a decrease in SOC stocks is predicted to drive further planetary warming, although whether changes in climate and associated factors (including atmospheric N deposition) will cause a net increase in SOC or a net decrease is less certain. Using a subtropical soil, we have directly examined how changes over the last three decades are already impacting upon SOC stocks and soil total nitrogen (STN) in a Vertisol supporting native brigalow (Acacia harpophylla L.) vegetation. It was observed that SOC stocks increased under native vegetation by 5.85 Mg C ha−1 (0.177 ± 0.059 Mg C ha−1 y−1) at a depth of 0–0.3 m over 33 years. This net increase in SOC stocks was not correlated with change in precipitation, which did not change during the study period. Net SOC stocks, however, were correlated with an increasing trend in mean annual temperatures, with an average increase of 0.89 °C. This occurred despite a likely co-occurrence of increased decomposition due to higher temperatures, presumably because the increase in the SOC was largely in the stable, mineral-associated fraction. The increases in CO2 from 338 ppmv to 395 ppmv likely contributed to an increase in biomass, especially root biomass, resulting in the net increase in SOC stocks. Furthermore, STN stocks increased by 0.57 Mg N ha−1 (0.0174 ± 0.0041 Mg N ha−1 y−1) at 0–0.3 m depth, due to increased atmospheric N deposition and potential N2 fixation. Since SOC losses are often predicted in many regions due to global warming, these observations are relevant for sustainability of SOC stocks for productivity and climate models in semi-arid subtropical regions.

https://doi.org/10.1016/j.scitotenv.2021.145019

Dalal RC, Thornton CM, Allen DE, Owens JS, Kopittke PM (2021). Long-term land use change in Australia from native forest decreases all fractions of soil organic carbon, including resistant organic carbon, for cropping but not sown pasture. Agriculture, Ecosystems and Environment 311: 107326.

Soil organic matter (SOM) performs an essential function in soil fertility, biomass and crop productivity, environmental sustainability, and climate change mitigation. We examined how land use change from native forest to either pasture [sown buffel (Cenchrus ciliaris cv. Biloela)] or cropping [primarily wheat (Triticum aestivum L.) and sorghum (Sorghum bicolor L.)] affected total soil organic C (SOC) stocks as well as stocks of three SOC fractions, particulate organic C, humus organic C and resistant organic C. Furthermore, for the cropping system, we also examined whether the use of a ley pasture phase could reverse the loss of SOC. It was found that land use change from native forest to pasture decreased SOC stocks by 12.2 % and soil total N (STN) stocks by 24.6 % during the land development to pasture establishment (≤ 1.75 y), although there were no significant (P > 0.05) changes thereafter up to 33 y and final values were generally similar to initial values. Furthermore, stocks of the three SOC fractions did not change with time in this pasture system. In contrast to these modest changes following conversion to pasture, for land use change to cropping, SOC decreased by 48 % at 0􀀀 0.1 m and 38 % (from 54 to 33 Mg ha􀀀 1) at 0􀀀 0.3 m, due mainly to insufficient C inputs to maintain SOM at steady state. Moreover, stocks of all three SOC fractions decreased with time, including the resistant organic C fraction, indicating that this fraction was not recalcitrant under cropping. The biomass C inputs by crops, mainly as root biomass, were not sufficient to reverse or slow down the rate of decrease of SOC in this soil. However, the introduction of pasture during the last 4 y indicated that the decreases in the stocks of SOC could be arrested by a ley pasture phase.

https://doi.org/10.1016/j.agee.2021.107326

Elledge, A., Thornton, C. (2017). Effect of changing land use from virgin brigalow (Acacia harpophylla) woodland to a crop or pasture system on sediment, nitrogen and phosphorus in runoff over 25 years in subtropical Australia. Agriculture, Ecosystems and Environment. 239: 119-131.

Native vegetation has been extensively cleared for agricultural systems worldwide, resulting in increased pollutant loads that often have adverse impacts downstream. This study uses 25 years of flow data and 10 years of sediment, nitrogen and phosphorus (total and dissolved) event mean concentrations from paired catchments to quantify the effect of changing land use from virgin brigalow (Acacia harpophylla) woodland in a semi-arid subtropical region of Australia into an unfertilised crop or conservatively grazed pasture system. Both the cropped and grazed catchments exported higher loads of sediment and phosphorus than the virgin brigalow catchment; however, the grazed catchment exported less total, oxidised and dissolved nitrogen than the virgin brigalow catchment. The cropped catchment exported higher loads of all water quality parameters compared to the grazed catchment. The simple hydrology and water quality model presented was effective for measuring the effect of land use change on runoff water quality. Variations in water quality between the three catchments are likely due to the presence of native legumes, ground cover, tillage practices and pasture rundown.

Huth, N.I., Thorburn, P.J., Radford, B.J., Thornton, C.M. (2010). Impacts of fertilisers and legumes on N₂O and CO₂ emissions from soils in subtropical agricultural systems: A simulation study. Agriculture, Ecosystems and Environment. 136 (3-4): 351-357.

There is increasing focus on greenhouse gas emissions from agricultural systems. One suggested method for increasing the sequestration of carbon (C) within agricultural soils is to increase crop productivity and therefore C input into the soil. However, if enhanced production is achieved via nitrogenous fertilisers, there is a potential tradeoff between decreased C emissions and increased nitrous oxide (N2O) emissions due to the increased soil C and nitrogen (N). An alternative is to incorporate leguminous crops into cereal cropping rotations to provide a biological source of N. However, the likely production of N2O from N released during the decomposition of leguminous residues is unknown as is the impact on C input into the soil when some cereal crops are replaced with grain legumes. Consequently, an analysis of the likely impacts has been undertaken for a subtropical dryland cropping system in Queensland, Australia where soil, climate and management are conducive to denitrification losses. A series of scenarios embracing a range of cropping rotations, N fertilisers and leguminous crops was tested using the Agricultural Production Systems Simulator (APSIM). The model configuration was tested using long term data from the Brigalow Catchment Study site near Theodore, Queensland, Australia (24.81°S, 149.80°E). A wide range of data was used in testing the model for the major terms in the C, N and water balances. Scenario analyses of alternative management systems including the use of fertiliser or legume grain or forage crops within cereal rotations demonstrated that soil C can be managed to some degree via simple changes in agronomic practice. The use of legumes within cereal rotations was not always as effective in reducing N2O emissions as improved fertiliser practice. For example, replacing wheat with chickpea did not reduce N2O emission relative to fertilised systems and did not assist in increasing soil C due to impacts on stubble cover over the important summer months. The fact that some interventions proved counterproductive due to complex feedback mechanisms highlights the need for detailed models which capture the links between water, C, N and management. © 2010 Elsevier B.V.

Radford, B.J., Thornton, C.M., Cowie, B.A., Stephens, M.L. (2007). The Brigalow Catchment Study: III. Productivity changes on brigalow land cleared for long-term cropping and for grazing. Australian Journal of Soil Research. 45 (7): 512-523.

Productivity of grain crops and grazed pastures inevitably declines without soil nutrient replacement and may eventually make these enterprises unprofitable. We monitored these declines in north-eastern Australia during 23 years after clearing 2 of 3 adjacent brigalow catchments, in order to define the productivity levels of developed brigalow land over time. One catchment (11.7 ha) was used for grain production and another (12.7 ha) for beef production from a sown buffel grass pasture. There was no upward or downward trend in annual rainfall amounts throughout the study period. In the cropped catchment, grain yield from 14 winter crops without added nutrients declined significantly in 20 years from 2.9 to 1.1 t/ha.year on the upper-slope clay soil (92 kg/ha.year) and from 2.4 to 0.6 t/ha.year on the Sodosol (88 kg/ha.year). Crop production per year declined by 20% between 2 successive 10-year periods. Wheat grain protein content also declined with time, falling below the critical value for adequate soil N supply (11.5%) 12 years after clearing on the Sodosol and 16 years after clearing on the clay soil. Such declines in grain quantity and quality without applied fertiliser reduce profitability. The initial pasture dry matter on offer of 8 t/ha had halved 3 years after clearing, and a decline in cattle liveweight gain of 4 kg/ha.year was observed over an 8-year period with constant stocking of 0.59 head/ha. Due to fluctuating stocking rate levels of 0.3-0.7 head/ha over the trial period, liveweight productivity trends are attributed to the multiple effects of stocking rate changes and fertility decline. The amount of nitrogen exported from the cleared catchments was 36.1 kg/ha.year in grain but only 1.6 kg/ha.year in cattle (as liveweight gain). Total soil N at 0-0.3 m declined by 84 kg/ha.year under cropping but there was no significant decline under grazing. The soil nutrients removed during grain and beef production need to be replaced in order to avert productivity decline post-clearing. © 2007 CSIRO

Silburn, D.M., Cowie, B.A., Thornton, C.M. (2009). The Brigalow Catchment Study revisited: Effects of land development on deep drainage determined from non-steady chloride profiles. Journal of Hydrology. 373 (3-4): 487-498.

A large area of woodland in the brigalow bioregion in semi-arid central Queensland was cleared for agriculture from the 1960s to the 1980s. To assess the risk of salinity associated with land clearing, soil chloride (Cl) was monitored at the Brigalow Catchment Study (BCS), in brigalow (Acacia harpophylla) scrub, a cropped catchment and a pasture catchment before and after clearing in 1982. The monitoring sites include three landscape positions, two on clay soils and one on a sodic duplex (Sodosol), within each catchment. An earlier report of deep drainage, using the early soil Cl profiles and steady-state and transient chloride (SODICS) mass balance, was revisited after a further 13 yr and four more sampling times. Profile Cl mass changed little in 18.4 yr at scrub sites, justifying the use of native vegetation sites to represent pre-clearing Cl for paired cleared sites. Steady-state Cl mass balance (CMB) gave deep drainage of 0.13-0.34 mm/yr for nine pre-clearing scrub sites. Large losses of soil Cl occurred under cropping and smaller losses occurred under pasture. Transient CMB gave average deep drainage of 59 and 32 mm/yr for crop and pasture catchments, respectively, during the development phase (1981-1983) when the land was bare following clearing of native vegetation and prior to establishment of crops or pastures. In the 16.7 yr following establishment of agricultural land uses (1983-2000), transient CMB gave average deep drainage of 19.8 (range 3.3-50) and 0.16 (-2.2 to 1.4) mm/yr, respectively, in crop and pasture catchments. The drainage rate under pasture was similar to that under brigalow scrub. In the cropped catchment, drainage for modern farming systems (less tillage, more summer/opportunity crops) was about half that of older farming systems (wheat-summer fallow, more tillage, less stubble retention). Drainage was greater for the Sodosol than for the clay soils under cropping. Deep drainage occurred under cropping even though the soils are considered to have low permeability and the climate is semi-arid, with potential evaporation exceeding rainfall, on average, in all months. Increased drainage at cropped sites has driven a clear exponential loss of soil Cl, as predicted by the transient CMB theory. One cropped site is at or near a new steady-state and the others will reach a new steady-state 50-200 yr after clearing. The leachate is saline with an average Cl concentration of 7000 mg/L and would salinised any groundwater it entered. The salinity risk associated with the drainage is not well understood as yet and will depend on local hydrogeological conditions, which are poorly mapped in the Fitzroy. Effects of these losses of salts on sodicity, soil structure and permeability should also be investigated. © 2009 Elsevier B.V.

Thornton, C.M., Cowie, B.A., Freebairn, D.M., Playford, C.L. (2007). The Brigalow Catchment Study: II. Clearing brigalow (Acacia harpophylla) for cropping or pasture increases runoff. Australian Journal of Soil Research. 45 (7): 496-511.

The Brigalow Catchment Study (BCS) was established to determine the impact on hydrology when brigalow land is cleared for cropping and grazing. The paired catchment study was commenced in 1965 using catchments of approximately 15 ha, with natural vegetation dominated by brigalow scrub (Acacia harpophylla). Three contiguous catchments were selected near Theodore in central Queensland to represent the extensive brigalow bioregion of central and southern Queensland and northern New South Wales (∼40 Mha). The hydrology of the 3 catchments was characterised during a 17-year calibration period (1965-81). The catchments were considered hydrologically similar, with sufficient data available for an empirical comparison between catchments. In 1982, two of the catchments were cleared, with one developed for cropping and the other sown to improved pasture. The third catchment was used as an uncleared control. Hydrologic characteristics were then compared for the following 21 years. In their virgin state, the catchments behaved similarly, with average annual runoff being 5% of annual rainfall. Once cleared, total runoff from the cropping catchment increased to 11% of annual rainfall and total runoff from the pasture catchment increased to 9% of annual rainfall; however, timing of the individual runoff events varied between land uses. In order to confirm that changes in hydrology were a function of land use and not just seasonal variability or sampling error, several analytic techniques were used: a simple comparison of runoff totals, comparison of events, comparison of probability of exceedance for daily runoff, and comparison of predicted and observed runoff using a water balance modelling approach. © CSIRO 2007

Thornton, C.M., Elledge, A.E. (2016). Tebuthiuron movement via leaching and runoff from grazed Vertisol and Alfisol Soils in the Brigalow Belt bioregion of Central Queensland, Australia. Journal of Agricultural and Food Chemistry. 65 (20): 3949-3959.

Tebuthiuron is one of five priority herbicides identified as a water pollutant entering the Great Barrier Reef. A review of tebuthiuron research in Australia found 13 papers, 6 of which focused on water quality at the basin scale (>10,000 km2) with little focus on process understanding. This study examined the movement of tebuthiuron in soil and runoff at the plot (1.7 m2) and small catchment (12.7 ha) scales. The greatest concentration and mass in soil occurred from 0 to 0.05 m depth 30-57 days after application. Concentrations at all depths tended to decrease after 55-104 days. Runoff at the small catchment scale contained high concentrations of tebuthiuron (average = 103 g/L) 100 days after application, being 0.05% of the amount applied. Tebuthiuron concentrations in runoff declined over time with the majority of the chemical in the dissolved phase. © 2016 American Chemical Society.

Thornton, C.M., Shrestha, K. (2020). The Brigalow Catchment Study: V*. Clearing and burning brigalow (Acacia harpophylla) in Queensland, Australia, temporarily increases surface soil fertility prior to nutrient decline under cropping or grazing. Soil Research.

In the Brigalow Belt bioregion of Australia, clearing of brigalow (Acacia harpophylla) scrub vegetation for agriculture has altered nutrient cycling over millions of hectares. In order to quantify the effect of this vegetation clearing and land use change on soil fertility, the Brigalow Catchment Study commenced in 1965. Initial clearing and burning of brigalow scrub resulted in a temporary increase of mineral nitrogen, total and available phosphorus, total and exchangeable potassium and total sulfur in the surface soil (0–0.1 m) as a result of soil heating and the ash bed effect. Soil pH also increased, but did not peak immediately after burning. Soil fertility declined significantly over the subsequent 32 years. Under cropping, organic carbon declined by 46%, total nitrogen by 55%, total phosphorus by 29%, bicarbonate-extractable phosphorus by 54%, acid-extractable phosphorus by 59%, total sulfur by 49%, total potassium by 9% and exchangeable potassium by 63% from post-burn, pre-cropping concentrations. Fertility also declined under grazing but in a different pattern to that observed under cropping. Organic carbon showed clear fluctuation but it was not until the natural variation in soil fertility over time was separated from the anthropogenic effects of land use change that a significant decline was observed. Total nitrogen declined by 22%. Total phosphorus declined by 14%, equating to only half of the decline under cropping. Bicarbonate-extractable phosphorus declined by 64% and acid-extractable phosphorus by 66%; both greater than the decline observed under cropping. Total sulfur declined by 23%; less than half of the decline under cropping. A similar decline in total potassium was observed under both land uses, with a 10% decline under grazing. Exchangeable potassium declined by 59%. The primary mechanism of nutrient loss depended on the specific land use and nutrient in question.

https://doi.org/10.1071/SR20088

Thornton, C.M., Yu, B. (2016). The Brigalow Catchment Study: IV. Clearing brigalow (Acacia harpophylla) for cropping or grazing increases peak runoff rate. Soil Research. 54 (6): 749-759.

In Queensland, Australia, large tracts of native vegetation have been cleared for agriculture, resulting in substantial hydrological changes in the landscape. Australia's longest-running paired catchment study, the Brigalow Catchment Study (BCS), was established in 1965 to monitor hydrological changes associated with land development, particularly that of the 1960s Land Development Fitzroy Basin Scheme. The BCS has unequivocally shown that developing brigalow (Acacia harpophylla) for cropping or for grazing doubles runoff volume. However, to date little research had been undertaken to quantify the changes in peak runoff rate when brigalow is cleared for cropping or grazing. The present study compared peak runoff rates from three brigalow catchments, two of which were subsequently cleared for cropping and pasture. Prior to land development, average peak runoff rates from the three brigalow scrub catchments were 3.2, 5 and 2mmh-1 for catchments 1 to 3 respectively. After development, these rates increased to 6.6mmh-1 from the brigalow scrub control catchment (catchment 1), 8.3mmh-1 from the cropping catchment (catchment 2) and 5.6mmh-1 from the pasture catchment (catchment 3). Peak runoff rate increased significantly from both the cropping and pasture catchments after adjusting for the underlying variation in peak runoff rate due to climatic variation between the pre- and post-development periods. The average peak runoff rate increased by 5.4mmh-1 (96%) for the cropping catchment and by 2.6mmh-1 (47%) for the pasture catchment. Increases in peak runoff rate were most prevalent in smaller events with an average recurrence interval of less than 2 years under cropping and 4 years under pasture. © CSIRO 2016

Technical Reports

Landsberg, L., Cox, H., Nothard, B., Thornton, C., Moravek, T. (2020). Gross margin analysis of grain cropping at the Brigalow Catchment Study with APSIM simulations to evaluate the effect of nitrogen fertiliser application. The State of Queensland, Queensland.

This report presents the methodology and findings from an economic investigation into the Brigalow Catchment Study (BCS) (The Study) grain cropping program. The project had three key outcomes which were:

A historical gross margin analysis of grain cropping at the BCS.

A calibrated Agricultural Production Systems sIMulator (APSIM) model of the BCS.

A gross margin analysis on simulated nitrogen (N) fertiliser rate applications.

The BCS is a paired, calibrated catchment study conducted by the Department of Resources and is located in the Fitzroy Basin, Central Queensland. A cropping site was cleared, developed and cropped for 26 years (1984 to 2010). Crops grown included wheat and sorghum, as well as a barley and a chickpea crop. Crop selection was reflective of the commercially grown crops in the region at the time of planting. There was no fertiliser added to the site during this time. As such, all results should be interpreted only within the context of the crop sequences that were analysed.

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Thornton, C.M., Elledge, A.E. (2013). Runoff nitrogen, phosphorus and sediment generation rates from pasture legumes: An enhancement to reef catchment modelling (Project RRRD009). Report to the Reef Rescue Water Quality Research and Development Program. Reef and Rainforst Research Centre Limited, Cairns.

The Fitzroy and Burdekin basins are Queensland’s largest coastal catchments, and both drain directly to the Great Barrier Reef. Greater than 80% of the catchment area in each basin is impacted by grazing, on both native and improved pastures. Despite substantial historical plantings of legume pasture species and ongoing programs of pasture improvement incorporating legumes, no information exists on the potential for increased exports of nitrogen in runoff from these systems to the Great Barrier Reef. This study sought to determine if broad-scale plantings of pasture legumes, particularly leucaena and butterfly pea, pose a risk to Great Barrier Reef water quality by increasing loads of nitrogen in runoff waters compared to grass only pastures or the virgin brigalow scrub landscape.
Comparison of pasture type effects on water quality at the paddock scale was undertaken in the Fitzroy Basin using a paired, calibrated catchment study approach combined with simple regression based modelling developed from the long-term Brigalow Catchment Study. This work was complimented by plot scale rainfall simulation experiments in the Burdekin and Burnett-Mary Basins.
At the paddock scale (12 to 24 ha) in the hydrological years 2010 and 2011, loads of total suspended solids and nitrogen in runoff from grass only and leucaena pastures were typically lower than or equal to loads from virgin brigalow scrub. Phosphorus loads from the pastures showed the opposite trend, being typically equal to or higher than loads from virgin brigalow scrub. In the same period, all parameters from butterfly pea ley pasture were equal to or higher than loads from virgin brigalow scrub. High event mean concentrations did not necessarily equate to high loads. During 2012, no runoff occurred from the virgin brigalow scrub, so loads from all catchments were an absolute increase compared with their pre-European condition.
Plot scale rainfall simulation conducted in the Burdekin and Burnett-Mary Basins showed that runoff in the late dry season typically had higher loads of total nutrients than runoff in the late-wet season. No significant interaction between pasture type and season for total suspended solids was observed. Rainfall simulations at the plot scale in the Burdekin and Burnett-Mary Basins also indicated that results from paddock scale catchment studies at the long-term Brigalow Catchment Study in the Fitzroy Basin are applicable to other grazing areas in the Brigalow Belt Bioregion.
Modelling flow and water quality from cropping and grazed buffel grass pasture between 1984 and 2012 showed similar trends to the rainfall simulation studies. Using virgin brigalow scrub as a reference, cropping exported more dissolved inorganic nitrogen and phosphorus, total phosphorus and total suspended solids; whilst grazing exported less total nitrogen and dissolved inorganic nitrogen, but more total and dissolved inorganic phosphorus and more total suspended solids. Cropping exports were always greater than grazing exports.
Within the grazing landscape, soil and pasture nutrient concentrations exhibited high variability and limited temporal response within the study. With no discernible period of potential high nutrient availability during the year, soil and pasture management should focus on minimising runoff, rather than manipulation of the natural nutrient cycle to reduce risks to water quality.
Newly planted legume based ley pastures pose a risk to water quality as they contribute higher nutrient loads than grass only pasture systems, established leucaena pastures, and the virgin brigalow scrub landscape representative of the environment in its pre-European condition. However, they do reduce total suspended sediment loads compared with the cropping system that they replaced. Dissolved inorganic nitrogen loads from well-established leucaena exceeded those from grass, indicating a potential risk to water quality from the legume component of permanent pasture. This may have implications for parts of northern Australia, such as the Burdekin Basin, with large areas of naturalised Stylosanthes spp. pasture. These findings have been synthesised into a series of values and trends suitable for use in model development and validation to further refine estimations of the impact of changed land use, management and the adoption of leguminous pastures on water quality.

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Thornton, C.M., Elledge, A.E. (2014). Runoff nitrogen, phosphorus and sediment generation rates from pasture legumes: Addendum to paddock scale water quality monitoring for 2013 and 2014 (Project RRRD009). Report to the Reef Rescue Water Quality Research and Development Program. Reef and Rainforest Research Centre Limited, Cairns.

Nitrogen, phosphorus and sediment were monitored in runoff from virgin brigalow scrub, grass pasture and leguminous pastures from 2010 to 2012 at the Brigalow Catchment Study, located in the Fitzroy Basin. Brigalow scrub is representative of the landscape in its pre-European condition. It was hypothesised that nutrient and sediment loads from a newly established ley pasture (previously cropping) would decline over time as plant cover and biomass increased. The data did not clearly demonstrate this, with trends confounded due to record breaking rainfall and runoff. Consequently the applicability of this data to reflect catchment responses in more typical seasons was unknown. Thus, an additional two years of monitoring was undertaken to capture water quality responses to less extreme climatic sequences.
Rainfall in the period 2013 to 2014 was much closer to the long-term annual average (660 mm) and did not exceed the 60th percentile in either year. Runoff during 2013 supported the 2010 to 2012 result that clearing brigalow scrub for either cropping or grazing increased runoff. Grass pasture continued to display this trend in 2014; however, butterfly pea ley pasture had similar runoff to brigalow scrub. Loads of total, oxidised and dissolved inorganic nitrogen from butterfly pea, grass and leucaena pastures were all lower than virgin brigalow scrub. However, the greatest load of dissolved inorganic phosphorus came from butterfly pea in both years.
There was little change in the relativity of loads between brigalow scrub, grass pasture and leucaena pasture between 2010 and 2014; however, loads from butterfly pea ley pasture showed quiet different dynamics. This catchment typically had the highest nutrient and sediment loads during 2010 to 2012. Conversely, loads from butterfly pea during 2013 and 2014 were similar to the other pasture land uses with loads of total, oxidised and dissolved inorganic nitrogen, total phosphorus and total suspended sediment all less than brigalow scrub.
No temporal trends were detected in the event mean concentrations of nutrients or sediment during 2010 to 2014 from brigalow scrub, grass pasture or leucaena pasture. However, a declining trend was observed for total, oxidised and dissolved inorganic nitrogen and total suspended sediment from butterfly pea ley pasture.
These findings support the hypothesis that higher nutrient and sediment loads are exported from ley pasture during the development phase and then decline over time towards that of long-term grazed landscapes. Loads of nutrients and sediment from long-term grazed landscapes were lower than that of virgin brigalow scrub. No temporal trends were detected in the event mean concentration of nutrients and sediment from brigalow scrub or the established grass and leucaena pastures from wet to dry years. This indicates that not only do these land uses maintain their specific flow signatures in extreme wet seasons, but they also maintain their specific water quality signatures.
The dynamics of dissolved inorganic nitrogen in runoff from established legume and non-legume pastures is still not clear. The risk posed to water quality is likely to be of concern given the concentration of dissolved inorganic nitrogen in runoff from butterfly pea is equal to that reported for some sugar cane systems. However, these concentrations are typically an order of magnitude less than those from brigalow scrub.
When considering on-ground management action, this study indicates that the establishment stage of a ley pasture is, not unexpectedly, the period of greatest risk to water quality. Conservative grazing management combined with spelling should be promoted in the first year to coincide with the highest risk of total, oxidised and dissolved inorganic nitrogen loss in runoff. Continued management for high cover and biomass will deliver reductions in nutrients and sediment loads past the first year.
The incorporation of a legume ley pasture into a farming system compared to a more permanent legume pasture, such as leucaena, needs to be carefully considered from both an economic and environmental perspective. Switching in and out of legume pastures, particularly ley pastures in cropping enterprises, is a substantial financial investment with the establishment phase proving the greatest risk to water quality.

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Thornton, C.M., Elledge, A.E. (2014). Pesticide dynamics in the Great Barrier Reef catchment and lagoon: Management practices (grazing, bananas and grain crops) and risk assessments (Project RRRD038). Tebuthiuron management in grazing lands (Subproject 2). Report to the Reef Rescue Water Quality Research and Development Program. Reef and Rainforest Research Centre Limited, Cairns.

The Brigalow Belt Bioregion of Queensland has been extensively cleared for agriculture, with the majority of cleared areas being utilised for grazing (Thornton and Elledge 2013). If left uncontrolled, the proportion of woody species in grazed pastures increases over time, leading to a decrease in pasture productivity (Burrows 2002, Scanlan 2002). This productivity decline has negative implications for both the environment and the viability of grazing enterprises. In order to avert productivity decline, the regrowth of woody species in grazed pastures is typically controlled by either mechanical intervention, such as blade ploughing or stick raking, or by the use of herbicides, such as tebuthiuron (Partridge et al. 1994).
Under Objective 1 of Reef Plan 2009 (Department of the Premier and Cabinet 2009), tebuthiuron is one of the five priority herbicides identified as main pollutants in water entering the Great Barrier Reef lagoon. A literature review of tebuthiuron research in Australia found 12 published papers; six of which focused on end of system water quality loads and concentrations at the basin scale (>10,000 km2) with little focus on process understanding. Thus, this study sought to examine the movement of tebuthiuron in runoff at both the plot (1.7 m2) and paddock (12.7 ha) scales, in addition to movement in soil.
At the paddock scale, runoff 100 days after application showed high concentrations of tebuthiuron (mean 103 µg/L); however, the total lost in runoff was only 0.05% of the amount applied. Runoff was monitored up to 472 days after application with losses of applied tebuthiuron ranging between 0.05 and 0.45%. The highest loss was associated with the highest discharge runoff event. Comparison of individual event hydrographs showed that tebuthiuron concentrations declined exponentially with time. Furthermore, results indicated that tebuthiuron movement in runoff was in a dissolved form rather than being transported adsorbed to suspended sediments.
At the plot scale, maximum load of tebuthiuron in the soil from 0 to 5 cm deep occurred 28 days after application. This is likely due to the movement of tebuthiuron from trash into the soil following rainfall. Maximum loads in the soil from 5 to 20 cm deep occurred between 56 and 98 days after application. After 98 days tebuthiuron concentrations at all depths tended to decrease over time. Losses of tebuthiuron in runoff were greater from Vertosols than Sodosols.
This study has filled a significant knowledge gap about the behaviour of tebuthiuron at the plot and paddock scale, and provides data where none previously existed. The data also improves understanding of tebuthiuron behaviour from predominantly grazed catchments that discharge into the Great Barrier Reef lagoon. Limitations of this study are: (1) data were sourced solely from the Fitzroy Basin; (2) only Vertosol and Sodosol soil types were investigated; (3) investigation of persistence in soil was limited to particular profile depths and time intervals; and (4) runoff at the paddock scale was limited to 17 months data with only small runoff events immediately after application. These limitations could be easily addressed by replicating the plot components of this study on additional soil types in other Great Barrier Reef catchments. Opportunities also exist for replicating the paddock scale work in additional catchments of the Brigalow Catchment Study.

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Thornton, C.M., Elledge, A.E. (2018). Paddock scale water quality monitoring of grazing management practices in the Fitzroy Basin: Technical report on the effect of grazing pressure on water quality for the 2015 to 2018 hydrological years. Report to the Paddock to Reef Program. Department of Natural Resources, Mines and Energy, Rockhampton.

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Thornton, C.M., Elledge, A.E. (2019). Agricultural land management practices and water quality in the Fitzroy Basin: Technical report for the 2015 to 2019 hydrological years. Addendum to "Paddock scale water quality monitoring of grazing management practices in the Fitzroy Basin: Technical report on the effect of grazing pressure on water quality for the 2015 to 2018 hydrological years". Report to the Paddock to Reef Program. Department of Natural Resources, Mines and Energy, Rockhampton.

Loss of sediment, particulate nitrogen and particulate phosphorus in runoff from the extensive grazing lands of the Fitzroy Basin, central Queensland, continue to contribute to the declining health of the Great Barrier Reef. Substantial investment has been made by the Australian and Queensland Governments to improve runoff water quality from grazing land; however, there is little data directly comparing the effect of grazing pressure on hydrology and water quality. This is further confounded by the difficulty of separating the impacts of climate variability from the anthropogenic impacts of changing land use from native vegetation to grazing. This study measured changes in hydrology and water quality from conservative and heavy cattle grazing pressures on rundown improved grass pastures. Conservative grazing pressure reflected the safe long-term carrying capacity for rundown buffel grass pastures, whereas heavy grazing pressure reflected stocking rates recommended for newly established pastures. This study also considered the anthropogenic effect of changing land use from brigalow scrub to an improved grass pasture with a conservative grazing pressure.
After four below-average rainfall years from 2015 to 2018 (Appendix 1.1), heavy grazing resulted in 3.6 times more total runoff and 3.3 times greater average peak runoff rate compared to conservative grazing. No runoff occurred from brigalow scrub in two of the four years, which means that no runoff would have occurred from the conservatively grazed pasture had it remained uncleared. Mean annual loads of total suspended solids, nitrogen and phosphorus (total and dissolved) in runoff were greater from the two grass pastures than from brigalow scrub, while loads from heavy grazing were greater than from conservative grazing. In contrast, event mean concentrations were lower from heavy than conservative grazing due to the dilution effect of increased runoff. In the two years with no runoff from brigalow scrub, total runoff and pollutant loads from conservatively grazed pasture were an absolute anthropogenic increase attributable to land use change.
Hydrology and water quality monitoring continued for the first six months of the 2019 hydrological year. Mean annual rainfall for this period was also below the long-term average; however, rainfall in the month of October, when runoff occurred, was the second-highest October total on record. This resulted in both the highest mean annual and event based runoff from all three catchments compared to 2015 to 2018. During 2019, loads of total suspended solids, particulate nitrogen and all phosphorus parameters remained higher from heavily than conservatively grazed pasture. However, loads of total and dissolved nitrogen were lower from heavily than conservatively grazed pasture, which is in contrast to the 2015 to 2018 period where loads were greater from heavily grazed pasture. Event mean concentrations were consistently lower from heavily grazed pasture compared to conservatively grazed pasture for both reporting periods.
Modelling of the long-term hydrology and water quality data from the Brigalow Catchment Study has shown that an unfertilised cropping system exports higher loads of total suspended solids, nitrogen and phosphorus (total and dissolved) compared to a conservatively grazed pasture (Appendix 1.2). Furthermore, grazed pasture exports higher loads of total suspended solids and phosphorus compared to brigalow scrub, but less total and dissolved inorganic nitrogen. One explanation for the variation in the magnitude and direction of pollutant differences between treatments is dilution. That is, increased runoff from either above average rainfall or a treatment effect, such as grazing pressure or a bare fallow, results in the dilution of pollutants in runoff which leads to lower event mean concentrations. This highlights the importance of reporting runoff data, as high loads are not necessarily related to high event mean concentrations.
Other research at the Brigalow Catchment Study (Appendix 1.3) investigated changes in soil fertility when changing land use from brigalow scrub to either an unfertilised cropping system or a conservatively grazed pasture. Increases in mineral nitrogen and both total and available phosphorus were found in surface soil due to ash deposition from clearing and burning native vegetation. However, total and available nitrogen and phosphorus under both agricultural systems declined over the subsequent 32 years since land use change. The effective depth of interaction for rainfall, runoff and soil is 0.1 to 4.0 cm (Sharpley 1985), so the cumulative loss of sediment and nutrients in runoff and the subsequent decline in surface soil fertility over time are interrelated. This highlights the importance of not just monitoring runoff pollutants, but also the fertility of the soil surface to improve understanding of agricultural land management impacts.
Determination of particle size distribution in both runoff and deposited material was undertaken at the Brigalow Catchment Study for the first time during the 2019 hydrological year. Land uses with high cover and high biomass had the lowest proportion of fine particles less than 16 μm in runoff. No correlation was found between loads of total suspended solids and fine particles. The proportion of ultrasonically dispersed fine particles from land uses with low cover and low biomass was 94%, which is the same as that reported at the end of catchment scale for the Fitzroy Basin. A fine particle sediment enrichment ratio of 1.6 was observed from deposited material to runoff. Despite clear trends, this data only represents a single point in time and ongoing monitoring will be essential to improve confidence in these findings.
Long-term data from the Brigalow Catchment Study has also been used to develop methods for estimation of peak runoff rate to improve erosion modelling activities in Great Barrier Reef catchments (Appendix 1.4). Four methods of estimating peak runoff rate were compared using data from three catchments, both prior to clearing brigalow scrub (1965 to 1982) and after conversion of two catchments to either cropping or grazing, while the third catchment was retained as brigalow scrub (1985 to 2004). Despite different data requirements and complexity, all four methods were easily applied with parameter values derived from widely available rainfall data, easily measured or estimated runoff volume data, and basic physical descriptors of the catchment.
In summary, the long-term Brigalow Catchment Study dataset has been fundamental for addressing numerous knowledge gaps through: (1) the provision of empirical data to support the adoption of improved agricultural land management practices; and (2) collaboration with modellers funded by the Paddock to Reef Integrated Monitoring, Modelling and Reporting Program to further refine parameters used to report progress towards achieving the Reef 2050 Water Quality Improvement Plan 2017 to 2022 water quality targets. A conceptual model of the outputs from the Brigalow Catchment Study and how they have delivered on the objectives of the Paddock to Reef Integrated Monitoring, Modelling and Reporting Program highlights these achievements.

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Conference Papers

Buck, S., Thornton, C., Dixon, R. (2011). Diet quality of cattle grazing grass or Leucaena-grass pastures in Central Qld. Proceedings of the Northern Beef Research Update Conference, Darwin, Australia.

Beef production from Leucaena-grass pasture can be double that from grass-only pasture due to both increased amount and nutritive value of the forage. However limited information is available on diet selection and nutrient intake of cattle grazing Leucaena-grass or grass-only pastures.

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Elledge, A., Thornton, C. (2012). The Brigalow Catchment Study: Comparison of soil fertility, forage quality and beef production from buffel grass vs. leucaena-buffel grass pastures. Proceedings of the 5th Joint Australian and New Zealand Soil Science Conference: Soil solutions for diverse landscapes. Tasmania, Australia.

This paper assesses the fertility status of soils and forages in two grazed pasture systems in central Queensland: 1) buffel grass only, and 2) leucaena-buffel grass. Trends observed in our preliminary results indicate that the paddock with leucaena had similar or lower soil fertility (0 to 10 cm) than the grass only paddock based on concentrations of available phosphorus, organic carbon, nitrate and ammonia; however, the leucaena paddock had similar or better pasture quality based on crude protein and total phosphorus. This can be partly explained by the deeper root system of leucaena which can utilize nutrients further down the soil profile than grass pastures. Furthermore, greater live weight gains of cattle in the leucaena-grass paddock can be attributed to higher crude protein concentrations found in leucaena vs. grass leaves.

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Elledge, A., Thornton, C. (2012). The Brigalow Catchment Study: Nitrogen runoff generation rates from pasture legumes and changes since land development. Proceedings of the 34th Hydrology and Water Resources Symposium, Sydney, Australia.

Grazing is the dominant agricultural land use in reef catchments of central Queensland, and despite the rapid and broad-scale inclusion of pasture legumes into grazing systems, there is currently no information available on the loads and impacts of nitrogen on the Great Barrier Reef (GBR) lagoon from legume based pastures. This paper assesses the risk of nitrogen runoff from legume pastures to water quality in the GBR by comparing runoff water quality from grass pastures with and without nitrogen fixing legumes, namely leucaena and butterfly pea. Furthermore, historical data from the long-term internationally recognised Brigalow Catchment Study (BCS), near Theodore in central Queensland, is used to provide estimations of pre-European nitrogen loads in runoff waters to determine how nitrogen dynamics have changed with land development. Trends observed in these results indicate that pastures with leucaena and buffel grass pose a smaller threat to water quality in the GBR than both buffel grass only pastures and ley pastures of butterfly pea, as less nitrogen and sediments are exported in runoff waters. Furthermore, based on 20 years of historical data from the BCS, the cumulative effect of grazing on total and oxidised nitrogen was smaller than the effect from cropping and the predicted effect had the native brigalow scrub not been cleared.

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Robinson, J.B., Shaw, M., Silburn, D.M., Roberts, A., Viagak, O., Thornton, C.M., McClymont, D. (2011). An improved model for linking phosphorus loads in runoff to climate, soil and agricultural management. 19th International Congress on Modelling and Simulation, Perth, Australia.

Soil phosphorus (P) is one of the nutrients that contribute to eutrophication of waterways, algal blooms and damage to coral reefs. Reducing P movement from farm paddock to the broader environment often requires comprehensive, catchment-wide approaches to management. The substantial complexity of the physical and managerial causes of P pollution has led to a role for simulation models in evaluating the efficacy of regulations and management changes. In Queensland, the Reef Plan program (http://www.reefplan.qld.gov.au) addresses issues of sediment, nutrient and pesticide impact on the health and future of the Great Barrier Reef. Some of the associated regulations are concerned with the use of P fertilizer. The HowLeaky? model is one of few biophysical models that represent agricultural management, biophysical conditions and P exports. As reported at MODSIM07, information about P export was added to an existing model of water balance and sediment export (HowLeaky?) to create a useful model of P export from agriculture. However, the predictive power of the model was modest, especially over short periods (e.g. individual days). To improve the predictive power of the model, several changes have been made, including (i) additional empirical functions for estimating the enrichment of total P in sediment, and (ii) additional functions for estimating concentration of soluble P in runoff (mg P/L). Soil adsorption of P (P buffering), which affects the soluble P concentration in runoff, is now estimated from the widely available phosphorus buffering index test (PBI) rather than phosphorus buffering capacity (PBC). Large, high-quality datasets are used to evaluate these relationships. A case study is presented where HowLeaky? has been used to estimate P loads generated at a site in the GBR catchment. The Brigalow Catchment Study is a long-term study of the effects of land clearing and agriculture on the water balance and water quality. It is located in central Queensland, has a subcatchment that is cropped, has representative soils, and is part of the Fitzroy Catchment. HowLeaky? reproduced daily runoff amounts very well, and the revised P model in HowLeaky? was acceptably accurate at predicting daily P loads. Modelling predicted the long-term totals (6 years) of soluble and particulate P export from the field site with less than 50% error. This is a better-than-expected result with an un-calibrated model, as P exports are notoriously difficult to predict.

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Shaw, M., Silburn, D.M., Thornton, C., Robinson, B., McClymont, D. (2011). Modelling pesticide runoff from paddocks in the Great Barrier Reef using HowLeaky. 19th International Congress on Modelling and Simulation, Perth, Australia.

Herbicides applied in cropping in the Great Barrier Reef catchment in Queensland, pose a threat to water quality in riverine environments and the reef lagoon. With the introduction of Reef Plan in 2009, which sets a target of a 50% reduction in herbicides entering the Great Barrier Reef lagoon by 2013, there is a need to investigate water quality implications of weed management practices. Models are the most logical approach to extrapolate from the relatively small experimental database to a wider range of conditions and also investigate the impact of management options. Towards this end, a pesticide module has been added to the HowLeaky crop and soil water balance model. This pesticide module is based on algorithms from GLEAMS. An extraction coefficient to predict concentrations of chemicals in runoff as a function of soil concentrations is a central concept. Good agreement has been found between model predictions and measured data for pesticide runoff from experimental field sites. Sensitivity analysis shows major factors affecting transport potential are the application rate and degradation rate of the pesticide. Using modelled simulations, the water quality impacts of weed management practices incorporating a heavy reliance on residual herbicides (atrazine, diuron) in grains farming have been explored and compared to the use of knockdowns herbicides (e.g. 2,4-D, glyphosate). Several pesticides that are being increasingly used in the grains and cane Queensland are also assessed industry in (e.g. isoxaflutole, s-metolachlor, pendimethalin). Simulation results demonstrate that reducing the use of residuals in favour of knockdown herbicides would result in an improvement in runoff water quality. However, caution is needed when selecting ‘emerging’ traditionally applied pesticides to replace the residuals, as use of these products may not lead to improved water quality. The examples presented demonstrate the utility of the HowLeaky pesticides module water quality implications from weed management practice options in cropping lands.

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Thornton, C.M., Radford, B., Silburn, D.M., Cowie, B.A. (2010). The Brigalow Catchment Study: More than 20 years of monitoring water balance and soil fertility of brigalow lands after clearing for cropping or pasture. 19th World Congress of Soil Science; Soil Solutions for a Changing World, Brisbane, Australia.

The Brigalow Catchment Study was established to determine the impact on hydrology and soil fertility when brigalow land is cleared for cropping or pasture. This paired catchment study commenced in 1965, when three catchments were selected in central Queensland, Australia, to represent the extensive brigalow bioregion of approximately 40 million hectares. Catchment hydrology was characterised during a 17-year calibration period (1965–81). In 1982, two of the three catchments were cleared, with one developed for cropping and the other sown to improved pasture. The third catchment was retained as an uncleared control. Soil sampling on 13 occasions from 1981-2007 allowed changes in soil fertility to be characterised. Land development for either cropping or grazing has doubled runoff and increased peak runoff rates. Deep drainage increased dramatically during land development and significant amounts of soil chloride were leached. This continues to occur under cropping. Cropping has resulted in a decline in soil organic carbon, total nitrogen and bicarbonate-extractable phosphorus. Grazing beef cattle on improved pasture however has maintained soil organic carbon and total nitrogen levels, but has shown a greater decline in bicarbonate-extractable phosphorus than cropping.

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Thornton, C., Buck, S. (2011). Beef production from buffel grass pasture compared to leucaena-buffel grass pasture in the brigalow belt of Central Queensland. Proceedings of the Northern Beef Research Update Conference, Darwin, Australia.

Establishing leucaena pastures requires significant financial investment, which is expected to be recouped via increased beef production. This study compares beef productivity from grass-only pasture and grass-leucaena pasture at two set stocking rates.

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Thornton, C., Cowie, B., Silburn, M. (2012). The Brigalow Catchment Study: forty-five years of paired catchment monitoring in the Brigalow Belt of Australia. Proceedings of a Workshop held during the XXV IUGG General Assembly, Melbourne, Australia.

The Brigalow Catchment Study was established primarily to determine the impact on hydrology when brigalow land is cleared for cropping or pasture. This paired catchment study commenced in 1965, when three catchments were selected in central Queensland, Australia, to represent the extensive brigalow bioregion of approximately 37 million hectares. After a 17-year calibration period (1965–1981) two of the three catchments were cleared, with one developed for cropping, another sown to improved pasture, and the third retained as an uncleared control. Monitoring of salinity, water quality, soil fertility and productivity also commenced at this time and analysis of these long-term data sets clearly indicates that paired catchment studies are capable of answering questions beyond their initial scope of hydrological change.

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Thornton, C., Elledge, A. (2012).The Brigalow Catchment Study: Increases in runoff associated with land development can still be detected in flood events at a small catchment scale. Proceedings of the 34th Hydrology and Water Reources Symposium, Sydney, Australia.

The Brigalow Catchment Study has unequivocally shown that developing brigalow lands for cropping or for pasture doubles runoff volume and more than doubles peak runoff rate at the small catchment scale (<20 ha). However, the persistence of these land use effects on catchment response during high flow conditions are hotly debated within the international literature. Typical hypotheses are that these changes are reflected in smaller events and that the effects of forests, land cover and land use tend to converge during high flows, which have been defined as events with a return period of as short as two and ten years. During 2010 and 2011, much of Queensland, Australia was subjected to consecutive record wet seasons. Data from the Brigalow Catchment Study, near Theodore in central Queensland, showed 2010 to be the third wettest year since records commenced in 1965; this was eclipsed by 2011 rainfall totals. These extreme seasons provided a unique opportunity to investigate the impacts of land use change on runoff and peak runoff rate in high flow conditions using a paired, calibrated catchment approach. During the height of flooding in 2011, the brigalow scrub yielded 183 mm of runoff, cropping yielded 224 mm and pasture yielded 197 mm. Gross increases in runoff were 28% from the cropping and 50% from the pasture compared to that expected had they remained brigalow scrub. Increases in runoff as a result of land development were found in the three wettest years on record using a number of analytical approaches. These findings lend strong support to the hypothesis that increases in runoff associated with land development can still be detected in flood events at a small catchment scale.

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Thornton, C., Yu, B. (2012).The Brigalow Catchment Study: Effects of land development on peak runoff rate and its prediction in central Queensland, Australia. Proceedings of the 34th Hydrology and Water Resources Symposium, Sydney, Australia.

Commencing in 1965 and continuing today, the Brigalow Catchment Study in central Queensland has measured both runoff volume and peak runoff rate from three catchments (11.7 to 16.8 ha) which were initially covered with native brigalow scrub. Thirty-eight years of data were used to assess the accuracy of three different methods for estimating peak runoff rate, and then, to quantify the changes in peak runoff rate as a result of clearing two of the three catchments for either cropping or pasture. Three methods were used to estimate the peak runoff rate for the 3 catchments: 1) multi-variable regression; 2) Soil Conservation Service Curve Number (SCS-CN) method; and 3) Spatially Variable Infiltration model (SVIM). Regression analysis showed that peak runoff rate was strongly correlated with total runoff volume, in addition to 3 rainfall related variables. Regression method yielded the best results of the three methods tested. Estimates using the SCS-CN method were strongly correlated with observed peak runoff rate for all catchments. However, the method typically underestimated for small events and overestimated for large events. Estimates using SVIM with linear kinematic wave routing were strongly correlated with observed peak runoff rate for all catchments. Prior to land use change, peak runoff rate from the three brigalow scrub catchments averaged 3.4 mm/hr. A significant increase in peak runoff rate from both the cropping and pasture catchments was clearly evident and attributed to land use change, with the average peak runoff rate increased by 9.1 mm/hr for the cropping catchment and 3.4 mm/hr for the pasture catchment. This equates to an increase of 263% and 164%, respectively, to estimates of their peak runoff rate had they not been cleared. Increases in the peak runoff rate were largest for smaller storm events with an average recurrence interval of less than two years under cropping and less than four years under pasture.

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