SIMRAT is an ARCINFO GIS tool used to assess the changes in salt load discharged from an aquifer at some distance from the area of changed water application (increase or decrease). Developed as a tool for assessing the salinity impacts of interstate water trade along the River Murray from Nyah to Lake Alexandrina, SIMRAT incorporates methods to account for floodplain salt storage and incomplete aquifer connectivity with the River, as well as the use of the BIGMOD Ready Reckoner to assess the salinity impacts of water trade in common units (EC and $) at Morgan. As no detailed modelling or calibration is required, SIMRAT can be used to quickly assess the relative merits of applying or removing water on different parcels of land. Conceptually, SIMRAT moves rainfall and applied water excess vertically through the soil zone to the aquifer using wetting or drying functions. In the aquifer, the Unit Response Equation is used to esimate the aquifer discharge at some point remote from the area of application. Allowance can be made for the loss of salt to floodplain processes, or partial connectivity of the aquifer to a river. SIMRAT can be applied to a wide range of physical situations provided some basic assumptions are met.

The key target group is State Agencies and MDBC for use in salinity accounts. Other target groups include: - land and water managers along the River Murray from Nyah to Lake Alexandrina, and in the Lower Darling River downstream of Weir 32. - researchers who may wish to utilise the core algorithms for catchment scale applications, and - developers who may wish to undertake a preliminary assessment of the impacts of irrigation or other developments on salinity. - people interested in aquifer discharge at locations outside the lower River Murray provided the basic model assumptions can be verified.

As a Rapid Assessment Tool, SIMRAT has been developed to be as conceptually simple as possible while retaining the ability to predict salinity impacts to a reasonable level of precision and conservatism. Basic hydrogeological knowledge is all that is required to run SIMRAT. Before running SIMRAT it is recommended that users become familiar with the basic model assumptions and limitations as provided in the comprehensive companion documents. It is recommended that expert hydrogeological knowledge is sought to review data inputs and interpret model outputs appropriately.

Impacts of water trade. Impacts of changes in water use efficiency. Impacts of changes in land use on infiltration rates (eg revegetation) Impacts from environmental watering.

Advantages: - Rapid assessment of potential salinity impacts. - No modelling required. - Consistent approach across State boundaries. - Rapid comparison of the salinity effects of applying water to different areas. - Robust simplifying assumptions. Disadvantages: - Detailed modelling of individual sites is not undertaken. For final design of new irrigation schemes local models are recommended. - Simplifying assumptions may be inappropriate in some areas.