E2 is a software product for whole-of catchment modelling. It is designed to allow modellers and researchers to construct models by selecting and linking component models from a range of available choices. E2 enables a flexible modelling approach, allowing the attributes and detail of the model to vary in accordance with modelling objectives

In E2, the model structure and algorithms are not fixed. They are defined by the user, who can choose from a suite of available options. Model selection requires the user to be familiar with the detail, applicability and data requirements of component models, and the implications of joining component models. E2 is therefore intended to be a tool for experienced catchment modellers.

E2 is designed for researchers, modellers and consultants who are constructing tailored models for particular catchment management problems.
Users of E2 should have considerable knowledge and experience with integrated catchment modelling.

E2 is complex, requiring users to be familiar with modelling issues such as:

• detail/resolution requirements and capabilities
• choice and application of suitable rainfall-runoff models
• choice and application of suitable models for constituent generation and filtering
• choice and application of suitable models for runoff routing and in-channel processing
• implications of joining component models
• uncertainty in data, and the propogation of uncertainty due to application of an integrated model

It is highly recommended that potential users attend an E2 training course, so as to become familiar with both the concepts and operation of E2.

E2 is designed for application in a range of catchment sizes, from backyards to many 100,000 km². It provides output at various temporal and spatial scales.

Examples of application of E2 include:

• Bushfire damage assessment in alpine areas, Victoria
• Pumicestone Passage catchments modelling to predict nutrient fate and tranpsort contributing to Lyngbya majuscula blooms in Deception Bay, South East Queensland
• Urban development and sewage treatment plant impacts, Hornsby Shire
• Prediction of Nutrient Loads from Dairying Catchments in North East Tasmania
• Decision support for receiving water quality improvement in Port Phillip and Western Port Bays, Victoria
• North Pine Dam Sustainable Loads Assessment, linking catchment and receving water quality models
• Water quality improvement plans for regional NRM groups, Queensland
• Water quality management activities, Burnett River, Qld
• Development of local water quality guidelines, Condamine region
• Policy analysis and sediment and nutrient load assessment

E2 can simulate the effects of the above scenarios on outputs such as fluxes and yields of both water and constituents such as Total Nitrogen.

E2 provides a flexible approach to whole-of-catchment modelling, allowing selection and linking of models of a complexity appropriate to the management questions being addressed, and the available data and knowledge.

Models created using E2 will be able to predict the flow and load of constituents, such as sediment and nutrients, at any point in a river network over time, operating at daily (or sub-daily) time steps and reporting on a variety of time scales.

The building blocks of an E2 model are sub-catchments, nodes and links:

• sub-catchments: The sub-catchment is the basic spatial unit in E2, although it can be divided into "functional units" based on a common response or behaviour (eg. based on landuse). Within each functional unit, three models may be assigned - a rainfall-runoff model, a constituent generation model and a filter model.
• nodes: Nodes represent subcatchment outlets, stream confluences, or other places of interest (eg. stream guages, dam walls). Nodes are connected by links, forming a representation of the stream network.
• links: Links represent river reaches, dams, or floodplains. Within each link, three models may be assigned - a routing model, a source/sink model and a decay/enrichment model.

When applying each of the models mentioned above, the user is given a choice between multiple modelling options. Therefore, E2 provides a unique opportunity to create an overall integrated model that is highly tailored to the problem at hand.