## 2 Materials and Methods
### Generating and Displaying IrrigWQ Output  
For Tier 1 assessments of FFU, only the composition of the irrigation water (Figure 2) is required to generate IrrigWQ output. To conduct a Tier 2 FFU assessment, the user must, in addition to the composition of the irrigation water, also specify the site-specific characteristics that need to be considered during the evaluation. Site-specific characteristics are selectable from pre-populated defaults, as indicated in Figure 3. \begin{table}
\begin{tabular}{c c c} \hline
**Fitness-for-Use Category** & **Description** \\ \hline Real & A water quality that would not normally impair the fitness of the water for its intended use \\ Acceptable & A water quality that would exhibit some impairment to the fitness of the water for its intended use \\ \hline Tolterable & A water quality that would exhibit increasingly unacceptable impairment to the fitness of the water for its intended use \\ \hline Imagenable & A water quality that would exhibit unacceptable impairment to the fitness of the water for its intended use \\ \hline \end{tabular}
\end{table} 
Table 1: A qualitative description reflecting the increasing risk of negative effects and associated colour coding of DWS FFU water categories, as used in IrrigWQ. \begin{table}
\begin{tabular}{c c c c c c} \hline  & **Total** & **Acceptable** & **Tolerable** & **Linearable** \\ \hline Suitability Indicator & Measure of Suitability Indicator & \multicolumn{4}{c}{Criteria for Indicators of Soil Quality} \\ \hline Soil profile salinity & EC of saturation extract (mSm) & 0.200 & 200-400 & 400-800 & 5800 \\ Soil permeability & Degree of reduced permeability & None & Slight & Moderate & Severe \\ Oxidisable carbon loading & COD load (kg/ha per month) & 0.400 & 400-1000 & 1000–1600 & -1000 \\ Trace element accumulation & Number of years to reach soil accumulation & 150-200 & 100-150 & 100-150 & -1000 \\ threshold & \multicolumn{4}{c}{2700 years} & **years** & **years** & **-1000 years** \\ \hline Root zone effects & Relative crop yield as affected by EC, B, Cl, and Na & 99-100 & 80-900 & 70-800 & -1000 \\ Leaf scorching when wetted & Degree of leaf scorching caused by Cl and Na & None & Slight & Moderate & Severe \\ Nutrient supply & Contribution to estimated NPK removal by crop & 0.200 & 10-300 & 30–500 & -1000 \\ Microbial contamination & Excess infections per 1000 persons p.a. & 61 & 1-3 & 3-10 & -100 \\ \hline Qualitative atrazine damage & Atrazine load (g/ha) & \multicolumn{4}{c}{Differentiated levels depending on crop sensitivity and soil texture} \\  & & & & & \\ \hline Corrosion & Langelier Index & 0.60 & -0.5 & -0.5 to -10 & -1.0 to -2.0 \\ Scaling & Langelier Index & 0.60 & -0.5 & +0.5 to +1.0 & +1.0 to +2.0 \\  & Suspended solids (mg/L) & +1.0 & 50-75 & 75-100 & -100 \\  & Manganese (mg/L) & +0.1 & 0.1-0.5 & 0.5-1.5 \\ Clogging of drippers & Total iron (mgL) & 0.2 & 0.2-0.5 & 0.5-1.5 \\  & pH & -7.0 & 7.0-7.5 & 7.5-8.0 \\  & E. coli (10\({}^{6}\) per 100 mL) & +1.0 & **1.0-2.0** & 2.0-5.0 \\ \hline \end{tabular}
\end{table} 
Table 2: Criteria used by IrrigWQ to determine the FFU category of suitability indicators. IrrigWQ currently has a built-in database of more than 50 South African weather stations, each with 50 years of quality checked, daily rainfall and maximum and minimum temperature data. Weather data from many more South African stations can be downloaded from the IrrigWQ website ([[https://nbsystems.co.za](https://nbsystems.co.za)]([https://nbsystems.co.za](https://nbsystems.co.za)), accessed on 21 November 2023). The weather data were obtained from a database developed by a team from the School of Bioresources Engineering and Environmental Hydrology at the University of KwaZulu-Natal, using the South African Atlas of Climatology and Agro Hydrology [24]. The IrrigWQ database also contains crop yield model parameters for 158 agricultural crops for their yield response to salinity, 60 for their yield response to boron, 54 for their yield response to chloride, and 74 crops for their yield response to sodium [2,18,20].