Sugarcane, banana and grain (cropping) industries
The sugarcane, banana and grain industries are modelled using paddock soil water balance models. These are designed specifically to simulate the effects of management practices such as crop rotations, intensity of tillage or traffic, and the application method, rate and timing of herbicides and fertilisers.
The Agricultural Production Systems sIMulator (APSIM) model is used for the sugarcane industry and the HowLeaky model is used for the banana and grain industry simulations.
The paddock soil water balance model simulations used in P2R Projector are summaries of those generated by the Paddock to Reef – Paddock Modelling Team to support the Reef Water Quality Report Card reporting process. Read more about the Paddock to Reef Integrated Monitoring, Modelling and Reporting Program.
A 48-year, daily timestep modelling simulation is conducted for each soil type in each climate zone, for each scenario of management practice. For the sugarcane industry this equates to around 6 million daily modelling simulations.
To prepare the data for use in the P2R Projector, the modelling simulation results are averaged to provide a long term annual average pollutant loss (kg/ha/yr) summarised by soil type, prevailing climate (wet, medium, dry) and management practice at the sub-catchment scale.
Only fine sediment loads are estimated for the grazing industry through two separate modelling exercises (streambank and pasture management). Gully management results are calculated through the Gully Toolbox.
Estimates of reductions in fine sediment from streambank erosion are based on the results of SOURCE catchment models. The modelled anthropogenic streambank-derived fine sediment load and stream length is used to calculate river reach scale average fine sediment loads per km. The fine sediment yield is scaled based on the user defined stream size (small streams deliver less fine sediment than large streams). The reduction in fine sediment yield resulting from management practice change is based on a proportional reduction identified in the water quality risk framework.
Estimates of reductions in fine sediment resulting from improved pasture management are based on a revised universal soil loss equation (RUSLE) approach. This uses the same functionality (PEPER) as used to determine erosion rates in the VegMachine® application. End-of-dry season cover is derived from satellite imagery. The PEPER erosion rate is determined each year for 10 years at the end of the dry season (2005-2014). These rates are averaged to estimate the long-term erosion rate under current practice. Pasture management practice change survey results are then used to create a modified cover factor. The 10 end-of-dry season erosion rate calculations are rerun using the modified cover factor. The average of these results represents the ‘after’ erosion rate. The erosion rates are scaled to represent the percentage of fine sediment delivered to the stream.
Gully saving estimates for P2R projector are derived from the Gully Toolbox.
Known limitations for paddock water balance models
- P2R Projector does not directly model management practice on an individual farm.
- The catchment level averaging of soil type/climate/management practice combination means that local subtleties of the position of a farm within the landscape and the effect of slope are not represented in the predictions for each project.
- P2R Projector results represent the ‘average’ water quality improvement for a management practice change and soil type combination within the project catchment. Some projects will be better and some will be worse than the P2R Predictions.
The modelling simulations that are used in P2R Projector generate sediment and nitrogen yield at the paddock edge. Before these modelling simulations are added to the P2R simulation lookup tables, a scaling factor (delivery ratio) is applied to account for losses and transformation between the paddock and stream network.
P2R Projector reports load reduction at the end of the catchment. To achieve the end of the catchment estimate, load delivered to the stream is scaled by a delivery ratio to account for losses and transformation in the river network. This regional delivery ratio is reported in the detailed results file.
Learn more about delivery ratios