eWater CRC is working on local water management issues in partnership projects with organisations in NSW, at their request. Focusing on the Nattai and Cattai river catchments in the Hawkesbury-Nepean basin, the project teams are applying eWater software to simulate in detail the movement of water and contaminants from areas of mixed land-use in a range of climatic conditions.

This ‘road-test’ challenges the software to see if it can model runoff patterns at small scale. The aim is to provide detail on the variable source areas during very wet or moderately wet weather, and the infl uences of landscape, land-use and land management factors on runoff and contaminant loads.

About the Focus Catchment

The Hawkesbury-Nepean catchment, which covers an area of some 22,000 km2, supplies drinking water for Sydney’s 4 million people, providing over 95% of Sydney’s water supply. More than half of the catchment area is bushland.

The Nattai sub-catchment, south-west of Sydney near Mittagong and Bowral in the Hawkesbury-Nepean region, is the site for one detailed trial of the software. Annual rainfall is 1200-800 mm, and over two-thirds of the catchment is covered by natural forest or woodland. The urban development, part of the ‘southern highlands’, is mostly in the upper catchment.

Contamination sources include sewage (from one treatment plant, sewer overfl ows and septic tanks in the catchment), as well as moderately heavy grazing pressure from mixed livestock (cattle, sheep, horses and alpacas), though there is generally good groundcover in the pastured areas.

Work is also commencing at a further trial site, the Cattai sub-catchment, north-north-west of Sydney, near Blacktown, Windsor and Penrith.

This eWater application is seeking to demonstrate the functionality and capability of the CRC’s software, so that partner organisations can compare it to other commercial software that is currently in use.


Only one of eWater CRC’s next-generation software tools is being trialed here: the catchment modelling framework Water Cast, which models movement of runoff and contaminants across and out of a catchment.

Sources of contaminants and runoff

Several of the characteristics of the Nattai catchment can influence the risks to a drinking-water catchment, and are of interest for this trial. For example:

  • Sewage contamination;
  • Septic tanks;
  • Urban development, particularly with different degrees of impervious area;
  • Livestock;
  • Fertiliser;
  • Sediment transport in runoff;
  • Farm dams which can act as pollution detention basins;
  • Steep forested areas which can be signifi cant sources of sediment and nutrients in large events.

How eWater is helping

Using the considerable quantity of data available for Nattai catchment, the eWater team has begun work to meet these challenges to the catchment modelling software.

The model works with a range of climate and spatial data relevant to subdivisions of a catchment. Rainfall data have been collected from records held by Sydney Catchment Authority and the Bureau of Meteorology.

Subcatchments are further divided into functional units that enable this software tool to work with variable yields of contaminants, and an initial set of functional units has been selected on the basis of major land-use categories. It is intended to test whether further refi nements of these units based on soil, land-capability or differences in management practices will improve the model sensitivity.

The software will be working with detailed data in the Nattai catchment, including: digital elevation data at 25 m spacing; mapping of land-use and land-capability, soil types, impervious paved areas, distribution of farm dams and unsewered residences, and rainfall variability; and a pollutant hillslope hazard index for total phosphorus, total nitrogen, pathogens and sediment.


This application will give partner organisations in NSW, especially Sydney Catchment Authority, a sound basis for comparing eWater’s ‘WaterCAST’ catchment modelling framework against other software currently in use.

The application will also further develop the tool’s functionality and precision simulation, which will be of value in other situations elsewhere.

All eWater tools demonstrated in focus catchments are part
of our integrated modelling suite.

Organisations involved

These eWater partner organisations are involved in this project:

See Related Technical Report:

Nattai River Catchment Source Catchments Application Project.