Reducing sediment export from the Burdekin Catchment

Summary

Hotspots of hillslope erosion are located in the NE reaches of the Upper Burdekin and within the Bowen catchments. Hotspots of gully erosion occur around and to the N and SW of Charters Towers. High hillslope erosion hazard (erosion rates >10 t ha-1 yr-1) is found in 8% of the catchment, and less than 1% of the catchment has high gully erosion hazard (gully densities >2 km km-2). If future efforts at minimising soil loss are targeted towards these hotspots using refined grazing management guidelines, a comparatively large benefit in reducing sediment loads delivered downstream can be achieved with less effort. Impacts on rivers by in-stream sediment deposition are predicted to be comparatively small, given that most river reaches in the Burdekin catchment have a high sediment transport capacity.

The past wet season indicated that the newly developed gauging stations perform well even during major cyclone events. Typical values of suspended solids measured at both gauging sites ranged between 0.5 and 3.5 g/L, which are comparatively high. Gully head progression was only noted at sites where contributing area to gully notches was > 0.6 ha, indicating that in many cases gully head progression has probably reached an equilibrium. However, erosion from gully sidewalls is still occurring where these have remained without ground cover.

Significant progress has been made on the development of a spatial model for predicting the location and intensity of cattle grazing at the paddock scale, based on collation of existing data and data collected as part of this project. Analysis of the 99/00 wet season data shows that there are large differences between paddocks in terms of vegetation type and distribution, but there was no significant indication of any strong spatial signatures of grazing distribution. This is expected to change markedly during the dry season. Work has focussed on the modelling framework and obtaining the key model input parameters using rainfall simulation and data from the Cardigan runoff plots. A 2-dimensional hillslope runoff and sediment discharge model has been developed and successfully tested. Rainfall simulation experiments have yielded strong relationships between runoff and sediment generation and soil surface condition. Perennial basal area and soil bulk density and clay content were the main factors associated. Cover threshold levels were observed at 40 %, corroborating earlier work.