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A catchment sediment and nutrient budget for the Daly River, Northern Territory

Rustomji, Paul and Caitcheon, Gary (2010). A catchment sediment and nutrient budget for the Daly River, Northern Territory<br />. Australia: CSIRO.

Document type: Research Report
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Author Rustomji, Paul
Caitcheon, Gary
Title of Report A catchment sediment and nutrient budget for the Daly River, Northern Territory
Publication Date 2010
ISBN ISSN: 1835-095X - Water for a Healthy Country Report Series   (check CDU catalogue  open catalogue search in new window)
Publisher CSIRO
Place of Publication Australia
Total Pages 64
Field of Research 300800 Environmental Sciences
Abstract Executive Summary

This report presents a catchment sediment and nutrient budget for the Northern Territory’s Daly River catchment. A catchment sediment or nutrient budget accounts for the major sources, transport pathways and sinks of sediments and nutrients within a catchment. A catchment sediment or nutrient budget is not something that can be directly measured, consequently a modelling framework is required to bring the individual components of the budget (which themselves are sub-models) together in a coherent manner. The SedNet and ANNEX models have been used in this case; these models have been widely applied in tropical Queensland settings and elsewhere in Australia. Each sediment budget term comprises its own sub-model and here, a combination of national-scale models (e.g. terrain hillslope erosion, surface soil nutrient concentrations), and locally-derived models (bank erosion, floodplain extent) have been used for input data. For some aspects of the model, such as sub-soil sediment generation from “gully erosion” processes, no catchment-wide local data was available and values have been inferred from comparisons between model predictions and observational data.

The model predicts budgets for fine sediment readily transported in suspension, and dissolved and particulate nutrients. The model has been calibrated to (a) station-based load estimates at three locations, (b) geochemical tracer data indicating relative tributary contributions at river confluences, and (c) fallout radionuclide tracer data indicating the proportion of surface soil in-transit at a variety of locations. Calibration to these observational data sets are critical to obtaining realistic model results. The calibration process involved capping predicted hillslope erosion rates, reducing the hillslope sediment delivery ratio, modifying the bank erosion and overbank sediment settling velocity terms, applying a spatially uniform rate of sub-soil sediment generation from gully erosion and modifying sub-soil nitrogen concentrations. The model was consequently able to predict erosion intensity moderately well for two of the three load estimate stations, was able to correctly predict the dominant tributary contributions, and predicted surface-soil to be a minor component of the transported load, consistent with the tracer data. Predicted floodplain deposition rates were consistent with eight floodplain aggradation rate measurements from the catchment.

The calibrated model predicts 503 kt per year of fine sediment export from the catchment. Bank erosion is predicted to be the main sediment input term, exceeding the combined estimates of hillslope and gully erosion. Bank erosion also contributes strongly to the sediment load exported from the catchment. Sediment deposition upon floodplains accounts for approximately 81% of the catchment sediment supply. Areas strongly contributing to sediment export from the catchment are predominantly located in the north-west of the catchment (including the Douglas River) and along the stems of the major channels. The upper Dry River and Katherine Rivers are predicted to contribute relatively modestly to the catchment export. Sensitivity modelling of load variations to spatially uniform variations in sediment supply indicated a percentage change in hillslope and gully erosion applied across the catchment yielded ∼0.16 percentage variation in sediment load at the catchment outlet, whereas for bank erosion, this coefficient was ∼0.65. This reflects the high rate of sediment input from bank erosion and the strong contribution of this input to the catchment export. The high rate of bank erosion is likely a product of a shift to wetter conditions since 1996, with runoff estimated to be 66% higher than the mean for previous decades, resulting in systematic channel widening along at least the main stem of the Daly River, however this remains to be confirmed by more detailed field investigations.

The modelled nutrient budgets indicated 855–1369 and 178 t/yr of nitrogen and phosphorus export respectively. In the case of phosphorus this is approximately evenly split between dissolved and particulate phosphorus, though nitrogen may comprise a particulate-dominated load, depending on model parameterisation. The spatial pattern of nutrient contribution to catchment export is again dominated by input from the north-west of the catchment.
Additional Notes Report to Tropical Rivers and Coastal Knowledge Research Program
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