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The Environmental Management Support System (EMSS) uses the lumped conceptual catchment scale model to estimate daily runoff and pollutant load from 175 catchments in the south-east Queensland region. The model has two components – a hydrologic model and a pollutant export load model. The climate data input into the model are daily rainfall and mean monthly area potential evapotranspiration and the main model outputs are total daily runoff and total daily pollutant loads (total suspended solids, total phosphorus and total nitrogen). There are seven parameters in the hydrologic model and two parameters in the pollutant export load model. Although there are nine land use categories in the EMSS, the limited local data only allows for meaningful differentiation between some of the land use categories.
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The runoff and pollutant export model operates on each sub-catchment, providing daily estimates of runoff, total suspended solids (TSS), total phosphorus (TP) an total nitrogen (TN). The sub-catchments are linked to one another using a ‘node-link’ system to represent the river network. Flow and pollutant loads are conveyed down the river network using a routing model. There is also a storage model in the EMSS that regulates river flows, traps pollutants and accounts for evaporative losses from large storages in the region.
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The hydrologic model has three parts – an impervious area model, a “forest” pervious land use model and an “other pervious” land use model. The natural bush, national park, managed forest and plantation land use categories in the EMSS are considered as “forest” land use and modelled using a daily lumped conceptual hydrologic.The remaining land use categories in the EMSS (grazing, intensive agriculture, broadacre agriculture and the pervious fraction of the dense urban and suburban land uses) are considered as “other pervious” land use and modelled using the same daily conceptual hydrologic model but with different parameters values.
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The lumped catchment scale modelling is carried out on a daily time step. The model, as used in the south-east Queensland EMSS, is called Colobus. The model has two components – a hydrologic model and a pollutant export load model. The input data into the model are daily rainfall, mean monthly areal potential evapotranspiration, parameters for the hydrologic model and EMC and DWC values for TSS, TP and TN. The model outputs total daily runoff and the runoff components, and total TSS, TP and TN loads.
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A properly calibrated model can also be used for sensitivity and scenario studies of issues such as the impacts of changes in land use practices or impacts of climate change. However, local data must be available to support the modelling because the model simulations can only be as good as the data used to calibrate the model. Sensitivity runs using a model can also indicate where additional local data is required.
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There are numerous rainfall-runoff and water quality models, and the choice of the model should ultimately depend on the objectives of the study and the available resources and data.For more detailed discussion on model types and selection, refer to these papers (Grayson and Chiew, 1984; McMahon and Chiew, 1998; and Chiew and McMahon, 1999).
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| Ecosystem Health Monitoring Program |
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