A catchment sediment and nutrient budget for the Mitchell River Queensland. A report to the Tropical River and Coastal Knowledge (TRaCK) reesearch program

Abstract

This report presents a sediment and nutrient budget for the Mitchell River catchment, located on the western flanks of Cape York and draining to the Gulf of Carpentaria. A catchment sediment (or nutrient) budget accounts for the major sources, transport pathways and sinks of sediments and nutrients within a catchment. It is difficult to measure all of the relevant components of a catchment sediment budget across space and time. Consequently a modelling framework is useful to bring together the individual components of the budget in a coherent manner, whether derived from local empirical data or theoretical models. The SedNet and ANNEX models have been used in this study; 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 (terrain, hillslope erosion, colluvial gully density and surface soil nutrient concentrations), state-wide data sets (foliage projected cover) and locally-derived models (alluvial gully erosion, floodplain extent) have been used for input data. For some aspects of the model, such as the dissolved nutrient concentrations in overland flow, no local data were available and values have been derived from reference to the literature. Some budget components (particularly floodplain deposition, and also partially hillslope erosion) have been derived as “residual value” terms; that is model parameters and/or terms have been adjusted to match other data with limited independent constraint on the exact magnitude of that budget term. These residual terms contain not only the data attributed to them, but also unmeasured components of the sediment budget and the associated error from the known or directly measured components, such as total load at gauges and alluvial gully erosion.