## 2 Materials and Methods
### Calculating Procedures
#### 2.5.2 Effect of Water Quality Constituents on Crop Yield and Quality  
The effect irrigation water quality constituents have on crop yield and quality are both direct and indirect. The direct contact of irrigation water with a crop mostly affects crop quality, while indirect impacts mostly affect crop yield. Indirect impacts are a consequence of the accumulation and redistribution of irrigation water constituents within the root zone. IrrigWQ considers the following suitability indicators to assess the effect irrigation water constituents have on crop yield and quality:  
1. Root zone effects of irrigation water salinity, B, Cl and Na on crop yield  
The accumulation of salt, B, Cl, and Na in the root zone of crops has an indirect effect on crop yield, in that when these constituents are absorbed by plant roots, they are translocated to the above-ground parts where they influence several plant physiological processes that affect crop growth and yield. Salinity (salt content) within the root zone inhibits crop growth by reducing the ability of plant roots to absorb water from soil. The osmotic effect exerted by soluble ions reduces the availability of water to crops. In addition to EC, crop growth is also affected by the accumulation of B, Cl, and Na in the root zone. The effects of the latter three constituents are deemed to be of a toxic nature since their effect on yield reduction is more pronounced than would be expected from their contribution to soil osmotic potential alone. A large body of data that links the yield response of different crops to EC levels and the concentrations of B, Cl, and Na in the root zone of soil within which the crop is growing is available. This body of data is available as crop-specific parameters for use in IrrigWQ. The crop tolerance parameters (the threshold concentration of the constituent of concern and the slope of the yield reduction curve) are used to calculate relative crop yield, according to the procedure of [PERSON] and [PERSON] [18] from the SWB-model-calculated seasonal root zone salinity, B, Cl, and Na concentrations, as also used by the FAO [2]. Relative crop yields between 90 and 100% are considered to be ideal, while yields of less than 70% are considered unacceptable. 2. Scorching of leaves when wetted  
Crops susceptible to foliar damage caused by salts directly absorbed through their leaves display greater yield reductions than when only exposed to root zone salinity. However, practically no quantitative data are available to quantify the effect on yield, and only limited qualitative data (and then only for Cl and Na) are available to assess the relative susceptibility of crops to foliar injury [21]. Crop species for which information is available, and their qualitative susceptibility rating, form part of the IrrigWQ database. For the purposes of IrrigWQ, the degree of leaf scorching is evaluated only in qualitative terms. 3. Contribution to the NPK uptake of the crop  
N, P, and K are essential macro plant nutrients that are normally present in low concentrations in irrigation water. However, their concentrations can be relatively high in effluents and greywater that may be considered for irrigation. The presence of plant nutrients in irrigation water is mostly viewed as beneficial by irrigators since its availability represents a saving in fertiliser costs. High concentrations can, however, have undesirable side effects. High N concentrations may stimulate excessive vegetative growth and cause lodging, delayed crop maturity, and poor quality. High nutrient concentrations may further complicate fertiliser management, the timing of fertiliser applications, and the control of nitrate and phosphate leaching and runoff losses. When present in significant quantities in irrigation water, fertiliser applications need to be reduced proportionally, and it becomes increasingly difficult to manage fertiliser applications optimally. The presence of plant nutrients in irrigation water, thus, presents both advantages and disadvantages.