## 4 Discussion
For Tier 2 output to be meaningfully compared with that of Tier 1, the site-specific conditions used for Tier 2 simulations were adapted to emulate those of Tier 1. This mainly required the removal of the rainfall component from the weather data and, since Tier 1 operates using a 10% leaching fraction, employing an irrigation management regime that produced a mean leaching fraction of about 10% over a 40-year simulation period. The same irrigation water composition, having an electrical conductivity of 100 mS/m, was used for both Tiers 1 and 2 assessments of FFU. Estimated crop yield is one of the suitability indicators used in IrrigWQ to assess the effect water constituents have on crop yield and quality. IrrigWQ estimates how crop yield is affected by irrigation water salinity, chloride, sodium, and boron contents. In line with the approach to provide a conservative FFU assessment for Tier 1, the yield response of a generic crop sensitive to water constituents is derived from mean concentrations in the root zone calculated using the Tier 1 steady-state procedure assuming a 10% leaching fraction and no rain. Tier 2 estimates of crop yield are calculated with the SWB model, which has been customized to make use of user-selected default site-specific variables. In line with the approach to establishing irrigation quality guidelines [2], crop yield is estimated from mean root zone salinity, boron, chloride, and sodium contents during the crop growing season. Catering primarily for irrigation in arid regions, traditional water quality guidelines disregard the effect of climate when establishing criteria for the effect of irrigation water salinity on crop yield [1; 2; 3]. However, a qualitative discussion of the role climate plays in modifying the effect of salinity may often form part of an explanatory narrative. Nevertheless, the desirable leaching fraction calculated with the FAO procedure for a target crop yield/irrigation water EC combination would be the same, irrespective of climate. Unless somehow modified by the irrigation practitioner, this would, thus, lead to applying too much water, resulting in more deep drainage below the root zone than is actually required in higher rainfall areas. The Tier 2 assessments of IrrigWQ, however, incorporate climate as an integral factor that modifies the effect irrigation water constituents have on crop yield and other suitability indicators. The amount of water applied would be determined by site-specific irrigation management rules, which may, e.g., be aimed at minimising leaching. The contribution of climate in modifying the role that irrigation water salinity plays in determining crop yield was selected to illustrate the usefulness of IrrigWQ to consider and combine contributions of the various components of the soil-plant-atmosphere continuum, when assessing the FFU of irrigation water. Crop yield was calculated at four summer rainfall sites with increasing mean annual precipitation, and at one winter rainfall site, to illustrate how IrrigWQ can predict and explain the effect of climate and, more specifically, the amount and seasonality of precipitation on crop yield. Since the SALF2 decision support tool also considers the diluting effect of precipitation, its use in the last two examples would be expected to lead to generally similar conclusions as for Tier 2 assessments of IrrigWQ.