## 3 Results and Discussion
### Differences in Output and Use of Tiers 1 and 2 to Assess FFU and Determine WQRs
#### 3.2.2 WQR Determinations for Tiers 1 and 2  
The range of irrigation water salinities presented in Table 6 indicates that when irrigating according to Tier 1 assumptions, an irrigation water EC of up to 106 mS/m will produce a soil profile salinity within the ideal FFU category (i.e., having a soil saturation extract salinity of less than 200 mS/m). This implies that if water quality managers wish to provide irrigation water that would not lead to soil profile salinities exceeding 200 mS/m, they should aim to supply irrigators with water not exceeding 106 mS/m. This corresponds with the FFU calculation, wherein irrigation with water having an EC of 100 mS/m was estimated to produce a soil profile salinity of 188 mS/m (Table 4). Conversely, irrigation water salinities exceeding 426 mS/m will produce an unacceptably high soil profile salinity, which exceeds 800 mS/m. Theoretically, it is possible to establish similar soil profile salinities while irrigating with waters of widely different ECs, provided the leaching regime  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Fitness for Use** & **EC\({}_{e}\) Interval** & **Predicted Equilibrium** \\  & & **Soil Profile Salinity** \\  & & **Acceptable** & 200-400 \\  & &  
is altered by adopting different irrigation management practices. Similarly, a range of soil profile salinities can be obtained by irrigating differently with the same EC water. While it is possible to specify these conditions for Tier 2 simulations, this is not possible for Tier 1 simulations since Tier 1 model parameters are fixed.  
Several interacting variables, including weather variability, form part of the Tier 2 simulations, ensuring a range of potential outcomes for SWB-model simulations. It is, thus, important to specify realistic site-specific conditions when employing Tier 2 assessments. Even when using the same site-specific conditions, the variability in weather conditions from year to year ensures that each year has a unique irrigation water EC that will produce a specific target soil profile salinity. It was deemed undesirable to establish WQRs that accommodate the most extreme irrigation water EC that would still produce a specific target soil profile salinity. To be pragmatic, it was decided to define WQRs in IrrigWQ as those irrigation water EC values that would achieve the desired aim for 95% of the time. The WQR values reported in Table 7, thus, represent threshold irrigation water ECs that will, for 95% of the simulation period, produce the target soil profile salinity, i.e., in this case, for 95% of the time, an irrigation water salinity of 85 mS/m will produce a soil profile salinity of 200 mS/m or less. If we wished to achieve a soil profile salinity not exceeding 200 mS/m for 100% of the time (as is assumed for Tier 1), a threshold irrigation water EC lower than 85 mS/m would be required.  
The same site-specific variables used for the Tier 2 FFU calculation were used for the Tier 2 WQR assessment. The soil profile salinities predicted for Tier 2 FFU evaluations are slightly higher than those predicted for Tier 1 (Tables 4 and 5). Consequently, Tier 2 WQR calculations (Table 7) indicate that a lower irrigation water EC than for Tier 1 (Table 6) is required to establish comparable soil profile salinities.