Water Quality and Sediment Behaviour of the Future: Predictions for the
21st Century (Proceedings of Symposium HS2005 at IUGG2007, Perugia, July
2007). IAHS Publ. 314, 2007, 239-250
Appropriate
use of catchment models for water-quality
target setting and land-use management
Bradford S. Sherman1, Jon Brodie2, Lex Cogle3 & Chris Carroll4
1 CSIRO Land and Water, GPO Box 1666,
Canberra, ACT 2601, Australia
brad.sherman@csiro.au
2 Australian Centre for Tropical Freshwater
Research, James Cook University, Douglas, Queensland 4811, Australia
3 Queensland Department of Natural Resources,
Mines and Water, Mareaba, Queensland 4880, Australia
4 Queensland Department of Natural Resources,
Mines and Water, Rockhamption, Queensland 4700, Australia
Abstract Management of land use and the setting of water-quality targets on the whole-catchment scale are typically constrained by a lack of observational data on which to base decisions. Typically, GIS-based models are used to interpolate and extrapolate a limited observational data set to cover an entire catchment and these derived values (pseudo-data) are used as input to catchment contaminant generation and transport models. We examine the impact of the intrinsic uncertainty in pseudo-data on predictions made using the catchment sediment and nutrient model, SedNet/ANNEX, and the consequences of this uncertainty for land-use management planning. The use of pseudo-data to estimate bulk soil properties (% clay, % phosphorus, % nitrogen) exhibited both a high bias and high standard deviation when compared with direct measurements. The high bias limits the usefulness of model predictions for setting quantitative targets. The high standard deviation requires model results to be aggregated over scales considerably larger than the intrinsic model grid to facilitate comparison of contaminant export rates between spatial units. Assessment of competing land management scenarios is not severely impacted by model uncertainty provided the interpretation of results is limited to relative changes in contaminant export.
Key
words catchment modelling; decision making;
uncertainty; Great Barrier Reef; water quality; sediment; nitrogen; phosphorus