## 3 Results and Discussion
### Differences in Output and Use of Tiers 1 and 2 to Assess FFU and Determine WQRs  
For both FFU assessments and WQR determinations, IrrigWQ displays a separate output page to report on how soil quality, crop yield and quality, and irrigation infrastructure are affected, as indicated by their respective suitability indicators. Soil profile salinity, one of the suitability indicators of soil quality, is used here 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. For Tier 2 output to be compared with that of Tier 1, the site-specific conditions used for Tier 2 simulations were adapted to reflect those used for Tier 1. The main adaptations involved the removal of the rainfall component from the weather data and 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 EC of 100 mS/m, was used for both Tier 1 and 2 assessments of FFU. IrrigWQ output for the soil profile salinity suitability indicator is presented in Tables 4 and 5 for Tiers 1 and 2, respectively. The same display format, as produced by IrrigWQ, is used in these tables. IrrigWQ uses a similar format to display results for all individual suitability indicators for both tiers. The label used to identify the reported suitability indicator (i.e., soil profile salinity in this case), the colour-coded FFU categories, and EC\({}_{\text{e}}\) (saturation extract EC) criteria defining the FFU categories are identical for both Tiers 1 and 2. The differences in the way that Tier 1 and 2 results are reported arise from the fact that Tier 1 reports a single steady-state estimate, whereas, for Tier 2, the range and frequency of reported values are calculated from annual or seasonal results over a simulation period of 10 to 45 years. For Tier 2 assessments, IrrigWQ runs the SWB model for at least 10 years and calculates values for the different suitability indicators for each year, or, in the case of crops, for each growing season. Normal weather variability causes simulated values to differ from year to year. Table 5 reports the variability in simulated profile salinity values over a 40-year period for a case where the site-specific variables mimic those of Tier 1. As would be expected, the results, thus, bear some similarity to those of Tier 1. However, the soil profile salinities predicted for Tier 2 are slightly higher than those predicted for Tier 1. This can be explained by the fact that Tier 2 simulations provide for some lag in drainage above field capacity depending on soil texture, resulting in a lower leaching efficiency compared to Tier 1 simulations, which assume instantaneous drainage. In addition, with Tier 1, an \"ideal\" volumetric soil saturation percentage of 50% is assumed, whilst for Tier 2, a soil texture-dependent saturation percentage is used. The Tier 2 results indicate that for 60% of the time (24 years of the 40-year simulation), the SWB model calculated a profile salinity of between 0 and 200 mS/m, and for 40% of the time, it predicted a value of between 200 and 400 mS/m (in fact, the values are all clustered around 200 mS/m). Although the Tier 1 and 2 outputs would, in this case, be interpreted to have similar implications, Tier 2 results, and the way they are reported, provide the user with more information about the range (as opposed to a mean value) of profile salinities that can be expected over time. It also provides the user with an indication of the risk or frequency with which higher or lower values can be expected to occur over an extended time period. The main advantage of Tier 2 simulations is, however, that its output is not limited by a predetermined set of assumptions but that the user can select the values of several variables to portray actual site-specific conditions. The above assumptions can, for example, be amended to investigate the effect of a lower leaching fraction.