## 2 Materials and Methods  
### Overview  
The topic of this paper is of a nature that is different from most papers published in scientific journals. Although its layout tries to follow the traditional format, the presentation of material and methods as well as results may come across as somewhat unconventional. The project on which this paper is based was designed to achieve the general aim of developing a software-based DSS that is able to provide both generic and site-specific, risk-based irrigation water quality guidelines, which is, in essence, a new approach to assessing irrigation water quality. The two main components of the project were, thus, firstly, to identify and describe the technical considerations that determine the water quality that is required for irrigation water use and, secondly, to capture these technical considerations in a DSS. The DSS (hereafter called IrrigWQ) was designed to provide water resource managers and irrigation water users with guidance about the risk of negative effects associated with using water of a specific composition for irrigation under both site-specific and generic conditions. The DSS that was developed provides, as far as possible, for quantitative FFU assessments, as well as for determining WQR for irrigation water use [10]. A basic premise of IrrigWQ is that the FFU of water for irrigation is determined by the extent to which water constituents determine the success of an irrigation development. The three most important components of an irrigation development that can be affected by water composition, were identified as soil quality, crop yield and quality, and the irrigation infrastructure used to convey and distribute water. Problems with any of these three components can jeopardise the success of an irrigation development. The FFU of irrigation water is, thus, deemed to be determined by the impact it has on these three components. For ease of pinpointing the exact problem with a water's FFU, each of the three components was subdivided into a number of so-called \"suitability indicators\", each of which describes a different aspect of the three success-determining components. The more a suitability indicator is negatively affected by a single or combination of irrigation water constituents, the less fit the water is deemed to be for irrigation. As part of a literature review, the current state of knowledge on how water constituents affect each of the suitability indicators was assessed, and approaches were developed to evaluate them. Next, the results of the literature review were used to decide on the determination of the criteria that define the boundaries between FFU categories for each suitability indicator, and these have been made explicit in the IrrigWQ output. More detail is given elsewhere [10] on the selection of suitability indicators and the definitions of FFU categories, and in the interest of brevity, this is not covered in much detail in this paper. The fact that suitability indicators and the criteria used to define FFU categories are stated explicitly enables IrrigWQ users who may wish to use different indicators or criteria to attach a modified interpretation to its output. Should a user consider some suitability indicators as inappropriate or of less concern, the indicator may be ignoredor less importance attached to its output. A user may likewise modify the importance attached to the output of a particular suitability indicator when the criteria used in IrrigWQ are considered inappropriate. However, against the background of the current state of knowledge, the authors consider the criteria used in IrrigWQ to be appropriate. The primary tool for evaluating FFU or establishing WQRs is a software-based DSS, which operates at two different levels or tiers. These levels of assessment vary in their input requirements, calculating procedures and output. 1. Tier 1 resembles the current South African generic guidelines (Department of Water Affairs and Forestry, 1996), with some modifications, and is similar to guidelines developed for other countries.