[ D:\PURSL Littleboy et al .doc ] Page 1 AN OVERVIEW OF MODELLING TECHNIQUES AND DECISION SUPPORT SYSTEMS AND THEIR APPLICATION FOR MANAGING SALINITY IN AUSTRALIA Dr Mark Littleboy 1,2 , Professor Rob Vertessy 1,3 and Dr Paul Lawrence 4 1 CRC for Catchment Hydrology, Canberra, ACT 2 Department of Infrastructure, Planning and Natural Resources, Queanbeyan, NSW 3 CSIRO Land and Water, Canberra, ACT 4 Department of Natural Resources and Mines, Indooroopilly, Qld INTRODUCTION In Australia, computer models are being used to support the development and implementation of salinity management strategies. These models facilitate the assessment of the impacts of salinity management options, and enable the outcomes of implementation to be quantified. Models can be used to estimate both potential benefits and any unwanted impacts of management actions. There is a myriad of salinity models developed or under development across Australia. A recent stock take of models conducted by URS for the National Action Plan for Salinity and Water Quality (URS Australia 2002) contained details of over 100 models that can be used to assess salinity management options. While it could be argued that such a vast number of models is overkill, many of these models focus on different processes and aspects of salinity. This paper reviews the range of modelling techniques and approaches used in Australia to improve the management and understanding of dryland salinity. The numerous modelling approaches being used in Australia have evolved to answer a variety of questions across different scales. While the main focus of this paper is on biophysical models, some case studies highlighting social and economic models and decision support tools currently being used in Australia are also be presented. GENERAL MODELLING CONCEPTS Computer models are mathematical representations of natural or economic systems. These can be very complex and highly variable in time and space. Often it is not possible or practical to represent these systems in great detail, and simplifying assumptions are commonly made. This is done for a number of reasons, including lack of basic data and that all the factors and processes affecting system behaviour are either not fully understood or are considered of less importance. Although usually applied to spatial data, scale can also be relevant in a temporal sense. The spatial scales we are most interested in from a salinity management perspective are: • property scale; • subcatchment or catchment scale; • river basin scale; • regional scale; • state scale; and • national scale.