DAF Paddock modelling program

The Department of Agriculture and Fisheries (DAF) Paddock modelling program was part of a joint initiative of the Australian and Queensland governments to improve water quality in the Great Barrier Reef (the Reef) through improved land management in catchments adjacent to the Reef (Reef Plan 2009). Under the Reef Plan Paddock to Reef program the impacts of investment in improved agricultural management practices on water quality entering the Reef were modelled. Paddock scaled models were used to demonstrate the effects of management practice adoption and to inform the catchment scale models. The catchment scale models represent loads of pollutants entering the Great Barrier Reef lagoon from rivers.

The Department of Agriculture and Fisheries project involved modelling of the grazing systems across the Reef catchments using the GRASP pasture growth model to provide links between ABCD management practices (PDF 558 kB), water quality and reef health. A comprehensive modelling structure was developed to capture the entire spectrum of grazing and environmental variability within the Burnett-Mary, Fitzroy, Mackay Whitsunday and Burdekin regions. The GRASP paddock modelling approach simulated the impact of grazing on a range of land types from various locations within the Reef catchments over a 31-year simulation period (1980-2010). Improvements in water quality through the adoption of improved grazing management practices (as defined under the ABCD framework) was determined by linking GRASP paddock times series outputs to catchment models. Department of Environment and Resource Management modellers used GRASP pasture utilisation and ground cover time-series outputs to relate changes in grazing system management to changes in the Universal Soil Loss Equation (USLE) cover factors. The USLE was used to predict hillslope erosion in the Source Catchments model (PDF 605 kB). Scaling indices derived from GRASP outputs were used to adjust the USLE cover factors applied in Source Catchments where management practices have changed.

Generally, soil surface cover was higher on lightly grazed, more productive land types with less tree cover, at more favourable climate locations (higher annual rainfall, less seasonal variation). Overall, pasture production and, hence, soil surface cover was less under trees, with the production of pasture growth decreasing as water and nutrients become scarcer in the drier, more heavily grazed systems. Runoff was greater at the more northerly locations (Charters Towers, Mt Fox, Proserpine), where the amount and intensity of rain was higher, and from the low productivity and more heavily treed land types. Highest soil losses occurred with highest rainfall, heaviest stocking rate and with lowest productivity.

Achieving a minimum of 50% late dry season ground cover is an important Reef Plan and Reef Rescue target to improve water quality in the Reef. Modelling suggests that achieving a minimum of 50% late dry season ground cover every year is unlikely for conservatively grazed pastures in the variable climate of the Reef catchments, and that runoff and soil loss could be significant during the drier years when surface covers are below 30%.

Managing grazing pressure in the variable climate of the Reef remains an effective practice in controlling water quality in the Reef catchments, and is particularly important in the higher rainfall areas where sediment losses can be high if ground cover is low.

Identification of grazing lands that are at risk of high runoff and sediment loads could highlight where changes to management practices could be most effective in achieving desired water quality outcomes.

Adoption of management practices that improve or maintain land condition (e.g. varying stocking rates in response variability in rainfall), and reduce pasture utilisation whilst maintaining animal productivity (strategic burning to reduce patch grazing, inclusion of water points, spelling to improve pasture productivity, control regrowth) should reduce the risk of pasture degradation during drier periods, maintain surface cover at sufficient levels to minimise runoff and soil loss for most years, assist in achieving the Reef Plan and Reef Rescue minimum of 50% late dry season ground cover target and enhance enterprise profitability.

To learn more, please read Paddock to Reef Program – Integrated monitoring, modelling and reporting (PDF 2 MB).

When: 1 October 2009 to 30 June 2013

Contact: Giselle Whish

Collaborators: Department of Agriculture and Fisheries and Department of Environment and Resource Management