## 4 Discussion  
IrrigWQ described herein is considered to be a first to integrate the various components in the soil-plant-atmosphere continuum into an electronic tool that can predict the effect of irrigation water constituents on a range of indicators that define their effect on soil quality, crop yield and quality, and irrigation infrastructure. While the Queensland SALF2 decision support tool [5] already presents a major improvement over more traditional approaches, it still relies on steady-state calculations and is currently limited in the range of suitability indicators that are considered. Without access to electronic computational tools, traditional water quality guidelines had to use simpler computational approaches to predict these effects and could not consider the site-specific factors incorporated in IrrigWQ. However, IrrigWQ uses established criteria similar to those used by traditional water quality guidelines to quantify the effect of irrigation water constituents on its FFU. Differences in FFU evaluations between traditional guidelines and IrrigWQ, thus, derive from the more sophisticated, dynamic, and site-specific approach of IrrigWQ. Each effect on FFU is categorised as being either ideal, acceptable, tolerable, or unacceptable and reported on in a user-friendly, colour-coded format. IrrigWQ thereby provides the user with an improved assessment of how a given water will affect a range of suitability indicators, each describing different components of soil quality, crop yield and quality, or irrigation infrastructure. Because of this functionality, IrrigWQ allows us to fundamentally change the question normally asked, \"is this water fit for irrigation?\" to \"are there conditions under which this water is fit for irrigation?\". IrrigWQ further provides the user with the ability to conduct either a rapid conservative (Tier 1) or more rigorous site-specific (Tier 2) evaluation of the suitability of specific water for irrigation. IrrigWQ also provides for the determination of WQRs, which are required for the management of water resources. Should the Tier 1 FFU assessment (that makes use of several conservative assumptions) not indicate potential problems with any of the suitability indicators, the water is deemed fit for use on all crops under all but the most exceptional circumstancesHowever, should a Tier 1 assessment identify potential problems with one or more of the suitability indicators, a more detailed, site-specific Tier 2 assessment is indicated. Tier 2 assessments allow users to select more appropriate site-specific variables (such as crop species, irrigation system and management, soil texture and weather data) to produce a much more rigorous assessment of soil-crop-water interactions. Running  
Figure 5: IrrigWQ simulated drainage during the maize cropping (red bars) and fallow periods (white bars) over a 40-year period in Riebeek Wes. the SWB model over several years, enables some expression of the level of risk of negative effects, by calculating the likelihood of yield and other parameters falling into different suitability categories. This paper provides some validation of Tier 2 IrrigWQ results. Crop yields that are statistically similar were obtained when the crop yields reported by the FAO guidelines [2] using a widely accepted traditional approach were compared with those calculated for Tier 2 in a virtual experiment using assumptions that approximated those of the FAO guidelines [2]. It is believed that the possibility to assess the suitability of irrigation water at different levels or Tiers is unique to IrrigWQ described herein. The ability to determine WQR is also considered novel in the field of water quality assessment. Soil profile salinity, one of the suitability indicators of soil quality, was used to illustrate the differences between the output of Tiers 1 and 2 for FFU assessments and to illustrate how FFU output differs from that of WQR determinations.