DECISION SUPPORT SYSTEM (DSS) MODELLING PROJECT

(Much of the text below is taken from: Zammit and Summers 2005, Modelling Catchment Processes in the Peel-Harvey Basin. The full report can be downloaded here in PDF format .)

The Peel-Harvey estuarine system has suffered from large-scale eutrophication problems since the 1960s, caused by the application of phosphatic fertilisers in rural and in urban landscapes. The poor water quality is a consequence of nutrient rich catchment water combined with tidal flushing in the lower rivers. Under the scope of the Australian Government's Coastal Catchments Initiative, a Decision Support System is being established for the Peel-Harvey basin. It is based on the Large Scale Catchment Model (LASCAM), which has been developed with the aim of predicting the impacts of land use and climatic change on the daily trends of stream flow and water quality (salinity, sediments, nutrients, etc.) in large catchments over long time periods. The model is used to test and predict the possible effects of urbanisation and the diverse agricultural pressures on the Peel Harvey basin over the next 100 years.

Application of LASCAM to the Peel Harvey Catchment

The Large Scale Catchment Model (LASCAM) is a daily time step complex conceptual model, which has been developed to predict the impacts of land use and climate changes on the daily trends of streamflow, salinity, sediment and nutrient yields in large catchments, over long time periods (Sivapalan et al., 1996 a,b,c; Viney and Sivapalan, 1999; Viney et al., 2000; Viney and Sivapalan, 2001, Sivapalan et al., 2002, Zammit et al., 2005).

For modelling purposes, the Peel Harvey catchment has been disaggregated into a network of 328 sub-catchments and 17 reporting catchments (Figure 2), which are the base of a Water Quality Improvement Plan (WQIP) (EPA 2005). The hydrological network used for calibration includes the presence of eight dams and a wetland. The model was applied and calibrated to simulate the water, sediment and phosphorus balances over a 35 years period between 1970 and 2004. The period 1970-1974 was used as a spin-up period for the model. Due to time constraints and the large size of the catchment, the model was calibrated on 10 flow and water quality gauge sites.

Phosphorus inputs on the catchment are from atmospheric deposition, septic tank release and fertilisation. The timing of the application of fertiliser is based on the result of the survey done by the Department of Agriculture, Western Australia (AgWA) and by the work of Viney and Sivapalan (2001) on two catchments north of the Peel Harvey catchment. The annual fertilisation rates are from three different sources:

Results from the survey (Lavell, et al., 2004) conducted by AgWA, which is one of the CCI projects. This survey contains for each of the main land uses, a P loss export, which has been converted into an application rate for each land use (Neville et al.., 2004)
Kelsey (2001), who published a literature review of fertilisation rates for land use on the Swan Coastal Plain.
JDA (2001), who derived fertilisation application rates for urban areas by aerial photography analyses.

Water Quality Scenarios

Considering the importance of the eutrophication problem within the Peel Harvey Estuary due to the large contributions of P from the three major catchments, two possible management scenarios are explored. The first scenario examines the consequences of urbanisation of the catchment. The second scenario examines different scenarios to achieve P load reduction to meet WQIP quality target.

As urbanisation involves adding greater nutrient inputs than pasture, the first scenario examines the consequences of the urbanisation outlined in the Metropolitan (MRS) and Peel (PRS) Regional Schemes (Ministry for Planning, 2000a, b) over the period 2005-2094. The proposed urbanisation seeks to create seven new developments located on the Coastal area of the Peel Harvey Inlet catchment (Figure 1). The settlements will be established in areas that are currently under rural zoning and are assumed to be established during the first ten years of the simulation. Land use and vegetation cover are modified on a yearly basis within the sub-catchments containing the development to reflect the establishment of the towns.

The second set of scenarios focuses on P load reduction scenarios to meet the WQIP water quality target at the outlet of the 17 reporting WQIP catchments. Different scenarios involving point source management, and/or septic tank management, and/or soil remediation, and/or fertiliser management were tested.

The DSS Modelling project, along with the water quality monitoring program form the basis on much many of the other P-H CCI projects rely in regards to developing and assessing the effectiveness of the proposed Best Management Practices and ultimately the recommendations that are included in the Peel-Harvey Water Quality Improvement Plan.

For further information contact Aquatic Science Branch, Department of Water (08) 6364 6500