## 1 Introduction  
This study describes a new approach to assess the fitness for use (FFU) of irrigation water. An assessment of the FFU of available water sources forms an integral part of the successful planning and implementation of any irrigation project. Fortunately, the guidelines for assessing the FFU of traditional sources of water for irrigation are well established. The first widely accepted irrigation water classification system was developed by the United States Salinity Laboratory Staff [1]. They correctly identified that the FFU of irrigation water is largely determined by a combination of itsodicity and salinity. Their classification system has subsequently been refined and supplemented with guidance provided by the FAO [2] and learned societies [3]. A recent guide for assessing irrigation water quality [4] continued to make extensive use of the United States Salinity Laboratory Staff classification but found it necessary to increase the concentration of the highest salinity hazard class (C4), from 500 to 3000 mS/m, in order to provide for waters with extremely high salinity. The Queensland (Australia) Department of Environment and Resource Management [5] recently developed a steady-state Excel-based calculator,called SALF2, as a decision support tool to assist in evaluating the impacts of irrigation water quality on plants and soil, with an emphasis on its effect on soil permeability and salinity-affected crop yield. This represents a major improvement over earlier calculating procedures. A further application of FFU guidelines is to satisfy the need identified by managers of water resource quality, who wish to establish water quality levels that would not jeopardise irrigation and other users of a particular water source. By using these levels as targets for managing the quality of water resources, water quality managers try to ensure that water resource quality remains fit for its intended use. Several countries have developed similar water quality guidelines catering for a range of water uses, including irrigation [6; 7; 8]. These guidelines were primarily developed to establish water quality requirements (WQR) specifying concentrations of water constituents that would not be detrimental to a specific user. In addition to their primary purpose of providing guidance to water resource managers with establishing water quality levels that would be acceptable for irrigation purposes, these guidelines have also found application to assess the FFU of irrigation water. A common feature of the guidelines that were developed is that they are generic and conservative in nature and largely ignore site-specific considerations. An unintended consequence of using such guidelines to assess FFU is that the problems that may arise from irrigation with particular water are emphasised, while the identification of irrigation options that would be successful under appropriate site-specific conditions at an acceptable level of risk of negative effects is largely ignored. This brings to the fore the need for a system to assess the FFU of water for irrigation that is based on realistic assumptions of crop and soil responses to different site-specific conditions. This need is also not addressed by traditional water quality guidelines. Since irrigation has historically been practiced mainly in arid environments, guidelines to assess the fitness of water for irrigation were almost, without exception, based on the assumption that irrigation was the main, if not the only, source of water for plant use. Water quality guidelines for irrigation, therefore, assume minimal or no dilution of irrigation water constituents by rain and, furthermore, primarily consider the effects of constituents that occur naturally in surface and ground waters. Guidelines, therefore, have largely ignored the variability associated with site-specific conditions that may influence the fitness of water for irrigation. However, the situation facing irrigation has changed.