## 3 Results and Discussion
### Demonstration of Effect of Climate on Expected Crop Yield
A distinction is made in Table 8 between the water balance components during the crop growing season (identified as seasonal components) and annual components, which are calculated as the sums of the components over the crop growth and fallow periods over a year. There is a clear increase in the annual and seasonal precipitation, the amount of drainage, and the leaching fraction, from the driest (Upington) to the wettest (Pretoria) summer rainfall sites. The opposite is the case for irrigation, evaporation, and transpiration. The increase in the yield of maize (see Table 9) that is calculated by IrigiWQ to occur between the driest and wettest sites is, thus, to be expected and readily explained by lower root zone salinities caused by higher leaching fractions and increased dilution of irrigation water by higher precipitation levels. The calculated yields illustrate how IrigiWQ is able to predict and quantify how climate is expected to affect the FFU of particular water and highlight the potential benefit of using water that would be unfit for use in traditional arid irrigation areas for supplemental irrigation in areas with higher rainfall. Further insight that IrigiWQ output can give the user into processes operating in the soil-plant-atmosphere continuum is highlighted by the obvious differences in yield between the Bloemfontein and Riebeek Wes sites, which receive the same annual precipitation (see Tables 8 and 9). An obvious difference is that Riebeek Wes receives winter rainfall, while Bloemfontein falls in a summer rainfall area. Note that the seasonal precipitation (i.e., precipitation during the summer, crop growing season) for Riebeek Wes is only 165 mm compared to 485 mm for Bloemfontein. Precipitation during the fallow period would, thus, be 390 mm for Riebeek Wes and 80 mm for Bloemfontein, resulting in much better leaching of the Riebeek Wes soil profile during the fallow period, as also evidenced by the differences in seasonal and annual leaching fractions and the amount of drainage simulated to occur over the 40-year period (Figures 4 and 5). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Mean Water Balance Components**} & \multicolumn{5}{c}{**Weather Station**} \\ \cline{2-6}  & **Upington** & **Vaalharts** & **Bloem-Fontein** & **Pretoria** & **Riebeek Wes** \\ \hline Rainfall season & Summer & Summer & Summer & Summer & Winter \\ Annual precipitation (mm) & 163 & 454 & 565 & 700 & 555 \\ Seasonal precipitation (mm) & 134 & 393 & 485 & 633 & 165 \\ Seasonal irrigation (mm) & 827 & 558 & 484 & 340 & 567 \\ Seasonal evaporation (mm) & 347 & 335 & 331 & 301 & 241 \\ Seasonal transpiration & 599 & 539 & 513 & 468 & 478 \\ Seasonal evapotranspiration (mm) & 945 & 874 & 844 & 769 & 719 \\ Seasonal drainage (mm) & 16 & 80 & 129 & 207 & 32 \\ Seasonal leaching fraction (\%) & 1.7 & 8.4 & 13.3 & 21.2 & 4.3 \\ Annual leaching fraction (\%) & 2.2 & 9.5 & 14.0 & 21.9 & 17.0 \\ Lowest seasonal root zone salinity & 275 & 110 & 72 & 47 & 91 \\ (mS/m in saturation extract) & & & & & \\ Highest seasonal root zone salinity & 2557 & 540 & 376 & 189 & 249 \\ (mS/m in saturation extract) & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Mean water balance components for a maize crop grown at four Summer rainfall sites with increasing mean annual rainfall and a winter rainfall site over a period of 40 years. Figure 4: IrrigWQ simulated drainage during the maize cropping (red bars) and falllow periods (white bars) over a 40-year period in Bloemfontein. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Relative Yield (\%)**} & \multicolumn{3}{c}{**\% of Time Yield Is Predicted to Fall within a Corresponding Yield Range**} \\ \cline{2-6}  & **Upington** & **Vaalharts** & **Bloemfontein** & **Pretoria** & **Riebeek Wes** \\ \hline
9-10 & 1 & 32 & 78 & 10 & 100 \\
80-90 & 2 & 35 & 18 & 0 & 0 \\ \hline
70-80 & 5 & 22 & 5 & 0 & 0 \\ \hline
40 & 90 & 10 & 4 & 0 & 4 \\ \hline \hline \end{tabular}
\end{table}