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Paddock scale modelling to assess effectiveness of agricultural management practice in improving water quality in the Great Barrier Reef Catchments

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Shaw, M., Silburn, D., Ellis, R., Searle, R., Biggs, J., Thorburn, P. and Whish, G. (2013) Paddock scale modelling to assess effectiveness of agricultural management practice in improving water quality in the Great Barrier Reef Catchments. In: MODSIM 2013, 20th International Congress on Modelling and Simulation, 1–6 December 2013, Adelaide, Australia.



Agriculture in the catchment areas adjacent to the World Heritage listed Great Barrier Reef (GBR) Marine Park generates pollutants that are a concern for the health of the Reef. Under the Paddock to Reef Integrated Monitoring, Modelling and Reporting program (P2R) of the Reef Plan, the impacts of improved agricultural management practices on water quality entering the GBR are modelled to evaluate the effectiveness of Government water quality improvement policies. The Source Catchments modelling framework estimates loads of pollutants entering the GBR lagoon from rivers. However, Source Catchments does not have the capacity to represent the collection of management practices available to farmers that affect water quality in runoff and drainage at a paddock scale. Therefore, paddock scale agricultural systems models were used to demonstrate the effects of management practice adoption and to provide input to the catchment scale models. Paddock scale models were used because they represent a level of process detail compatible with the management practice investments and implementation on-ground. A fit-for-purpose modelling approach was used, where the paddock model most suited to a given land use and/or water quality pollutant was applied. Three one-dimensional agricultural systems models were employed; HowLeaky in grains, APSIM in sugarcane with HowLeaky post-processing for herbicides and phosphorous and GRASP in grazing lands. These models share similar soil water balance, ground cover and runoff sub-models. However, they vary in the level of detail, particularly in terms of representing crop growth and in the processes considered, such as pesticide degradation and export. In grains and sugarcane cropping, the pollutant time-series (e.g. load per day per unit area) in the Source Catchments models was replaced with an output time-series from HowLeaky or APSIM for each soil-climate spatial combination. Management practices were grouped into systems classed as A, B, C or D. The proportion of each of these management systems contributing to the modelled loads was adjusted to reflect data on the prevalence of adoption of improved management practices in the GBR catchment. In grazing lands, GRASP pasture utilisation and ground cover time-series outputs were interrogated to derive relationships between changes in grazing system management and changes in the USLE C-factors. The USLE is used to predict hillslope erosion in the Source Catchments model. Scaling indices derived from GRASP outputs were used to adjust the USLE C-factors applied in Source Catchments where management practices had changed. The P2R program has demonstrated the effectiveness of linking paddock scale models or emergent models derived from them with catchment scale models. This has enabled detailed management options to be simulated to investigate broad scale water quality impacts of the adoption of improved agricultural practices.

Item Type:Conference or Workshop Item (Paper)
Business groups:Animal Science
Keywords:Paddock to Reef, water balance modelling, HowLeaky, APSIM, GRASP
Subjects:Agriculture > Agriculture (General) > Agricultural economics
Science > Statistics > Simulation modelling
Agriculture > Agriculture (General) > Agriculture and the environment
Aquaculture and Fisheries > Fisheries > By region or country > Australia > Great Barrier Reef
Agriculture > By region or country > Australia > Queensland
Live Archive:12 Apr 2022 06:21
Last Modified:12 Apr 2022 06:21

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