## 3 Results and Discussion
### Demonstration of Effect of Climate on Expected Crop Yield  
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 \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Fitness for Use** & \
\begin{tabular}{c} **EC\({}_{\text{e}}\) Interval** \\ **(mS/m)** \\ \end{tabular} &
\begin{tabular}{c} **EC Range That Will Give Rise to the** \\ **(mS/m)** \\ \end{tabular} \\
**Soil Profile** & Ideal & 0-200 & -106 \\
**Salinity** & Acceptable & 200-400 & 106-213 \\  & Tolerable & 400-800 & 213-426 \\  & **Inacceptable** & **-800** & **-406** \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Tier 1-calculated irrigation water EC range (i.e., WQRs) that would give rise to the soil profile salinity in the corresponding FFU category. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Fitness for Use** & \
\begin{tabular}{c} **EC\({}_{\text{e}}\) Interval** \\ **(mS/m)** \\ \end{tabular} &
\begin{tabular}{c} **Irrigation Water EC That Will Give** \\ **Rise to the Corresponding EC\({}_{\text{e}}\)** \\ **for 95\% of the Time** \\ **(mS/m)** \\ \end{tabular} \\
**Soil Profile** & Ideal & 1-200 & 8-15 \\  & Acceptable & 200-400 & 85-179 \\  & Tolerable & 400-800 & 179-376 \\  & **Inacceptable** & **-800** & **-476** \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Tier 2-calculated EC range of irrigation water (i.e., WQRs) that would give rise to the soil profile salinity in the corresponding FFU category. 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 values of site-specific variables. In line with the approach to establishing irrigation quality guidelines [2], IrigiWQ estimates crop yield from mean root zone salinity, boron, chloride, or 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 quantifying the effect of irrigation water salinity, boron, chloride, and sodium contents on crop yield. However, a qualitative discussion of the role climate plays in modifying the effect of salinity will often form part of explanatory narratives accompanying these guidelines. The Tier 2 assessments of IrigiWQ, on the other hand, incorporate climate as an integral factor that modifies the effect irrigation water constituents have on crop yield. The modifying effect of climate is quite complex since it is described by several variables affecting irrigation management. To illustrate how IrigiWQ succeeds in incorporating the effect of climate and, more specifically, precipitation amount and seasonality on crop yield, simulations were run for four summer rainfall sites with increasing mean annual precipitation and at a winter rainfall site in South Africa. IrigiWQ simulated a period of 40 years, assuming irrigation of a maize crop, planted in early spring (1 September) on a 1 m deep sandy loam soil and irrigated with 200 mS/m water. The crop reached maturity after 120 days. A fallow period with no irrigation but with a continuation of rainfall, evaporation, and drainage followed each cropping period. The crop was irrigated to field capacity to replenish the accumulated water deficit whenever this exceeded 30% of plant available water and should, thus, not have experienced any water stress. No provision was made for water applications to facilitate leaching. The leaching predicted to occur, therefore, is a consequence of precipitation not utilised for evapotranspiration.