## 2 Materials and Methods
### Calculating Procedures  
Both FFU and WQRs are assessed based on the effect water constituents have on indicators describing their effect on the suitability of soil quality and crop yield and quality, as well as irrigation infrastructure. The effect water constituents have on suitability indicators can be either direct (such as when water with a high chloride concentration causes scorching of wetted leaves) or indirect (when the suitability indicator is affected by the net effect of processes taking place within the soil-plant-atmosphere continuum). Direct effects are usually quite easy to  
Figure 1: Simplified schematic representation of IrigWQ’s structure. predict and evaluate from cause-effect relationships. Indirect effects are more difficult to predict since the cause-effect relationships are more complex. When irrigation is applied, the soil acts as a temporary store of water from which plant roots extract water as needed in between irrigation applications. The dissolved water constituents are transported into the soil and interact with soil constituents, thereby changing its composition. Since most irrigation water constituents are actively excluded from uptake by plants, they tend to accumulate in soil and further change the soil composition. The net effect of these processes on soil composition has to be estimated to assess the indirect effects of water composition on the selected suitability indicators of soil quality and crop yield. Because of the involved nature of these interactions, they are modelled in IrrigWQ using a simplified dynamic soil water balance model for Tier 2 applications, while a steady-state calculating procedure that relies on simplifying assumptions is used for Tier 1 assessments. Tier 1 calculations of soil-crop-water interactions assume an idealised four-layered soil profile, from which crops withdraw 40% of their water requirement from the top layer, 30% from the second, 20% from the third, and 10% from the bottom layer [13]. The steady-state (or equilibrium) concentration of soluble constituents in each layer is calculated from the concentration of constituents in the irrigation water, assuming a conservative 10% leaching fraction for the profile as a whole, with immediate percolation and leaching of salts in solution when the profile exceeds field capacity. The same approach is used by others for developing their water quality guidelines [2; 6; 7; 8]. Tier 2 calculations make use of a simplified version of the dynamic, daily time-step, soil water balance (SWB) model, using a cascading or \"tipping-bucket\" soil water balance approach [14] that is run for a minimum of 10 years, using daily temperature and precipitation data from an appropriate user-selectable weather station to calculate the water requirements and uptake of a user-selected crop or crops. Selectable irrigation management rules specify the timing (irrigation interval in days, amount in mm, or percentage depletion) and amount (refill to field capacity, apply a fixed amount, apply a specified leaching fraction, or leave a specified amount as room for rain) to be refilled. The ability of the SWB model to predict interactions in the crop-soil-atmosphere continuum has been verified extensively, which provides confidence in using it for calculating the parameters that are used to infer the effect selected water constituents to be refilled have on soil quality and crop yield and quality [14]. Reference evapotranspiration (ETo) is calculated in SWB according to the Penman-Monteith grass reference method, as recommended by the FAO [15]. It also simulates transient salt transport and simplified soil chemical interactions [16; 17]. Model output is used to calculate the value of variables that determine the magnitude of several soil quality and crop yield indicators. For example, variability in weather conditions gives rise to differences in the seasonal water balance and the seasonal root-profile-weighted soil salinity, from which crop yields are derived using the threshold and slope approach [18].