As part of the Burdekin WQIP target setting process, the Burdekin CCI commissioned CSIRO to undertake a study of the sources of sediments and nutrients from erosional processes within the grazed lands of the Burdekin catchment to better understand water quality issues in these areas.  This was undertaken using the SedNet –Annex model.

The SedNet model simulates river sediment loads for catchments by constructing material budgets that account for the main sources and stores of sediment. To do this the model makes estimates of erosion rates (gully, hillslope and bank) for available climate, soil, topography and landuse data. Data relating to the catchment’s hydrological processes are also employed.

Outputs from the model include maps and tables of sediment sources, stream loads and areas of deposition within the system. The contribution of sediment from each subcatchment to the end of the river can be traced back through the system.

The focus of the first part of the SedNet modelling study was to:

• Identify "hot spot" areas that supply sediments and nutrients to the Burdekin catchment waterways
• Estimate, on a subcatchment basis, the quantities of sediment (loads) currently being supplied through gully, hillslope and river/stream bank erosional processes
• Determine how the total maximum pollutant load differs from the current estimated pollutant loads (based on actual water quality monitoring field data).

The second part of this study involved estimating the impacts of selected land management strategies on sediment loads/water quality  in the catchment. Management practices were based on the draft Best Management Practice Guidelines for Grazing Lands produced for the Burdekin CCI.

The management strategies modelled for each erosion type include:

• • hillslope erosion: increasing cover levels in rangelands by adjusting the cover to three minimum cover levels – 50% cover, 60% cover and 70% cover

  • gully erosion : reducing gully erosion by 50%

  • stream bank erosion : improving riparian zone cover levels to their pre-European assumed levels of 95%.

First these strategies were simulated for selected priority sub-catchments. These priority sub-catchments were selected on the basis of their sediment generation potential and contribution to end-of-catchment loads for each erosion type (hillslope, gully and bank).

Secondly, a further modelling scenario was undertaken to determine aspirational water quality  targets at a “whole of catchment” level. For this scenario the Sednet model applied grazing management strategies  to the catchment as a whole to estimate the impacts on water quality on a catchment level.

Read the Report

Improved SedNet Modelling of Grazng Lands in the Burdekin Catchment. Kinsey-Henderson, A., Sherman, B. and Bartley, R. 2007.

Modelling of management scenarios for water quality outcomes for sugar has been undertaken by CSIRO using the APSIM model.  The scenarios used in the model focused on a range of  the five most current farm management practices employed by cane farmers in the Burdekin region.   These scenarios were developed through consultation with cane farmers, researchers and extension staff in workshops organised by the Burdekin CCI.  These management systems range from the worst to the best practice farming systems, each of which include combinations of different practices in tillage, plant crop fertiliser splitting and amount, legume fallow crops amongst others. 

The five management practice scenarios were simulated for:

Model simulation results will be presented as water and nitrogen balances. 

NB. Nitrogen (N)  will comprise the proportions of  N fresh weight, N applied, N lost in surface runoff, N lost to deep drainage and N lost to denitrification.

The results from the modelling will be used to identify the  best on-farm  management practices (BMPs) which will deliver the best water quality outcomes without diminishing productivity.

Report in preparation