*3.1. Modelling Approach*

The development of vadose zone transfer functions by [22,23] has allowed for the development of a more integrated modelling method. The method, illustrated in Figure 3, includes three elements that correspond to the three domains of the conceptual model (Figure 2):


3. The saturated zone is modelled by a groundwater flow model (a modified version of the existing Loxton–Bookpurnong model [21]) to generate groundwater returns and salt load to the river as an output.

**Figure 3.** Process used in the integrated modelling method.

The method links on-ground actions directly with predicted groundwater returns and provides for two separate calibration targets: the observed data on drainage rates and groundwater levels.

#### *3.2. Agronomic Water Balance*

The AWB, as described by [26], was used to estimate the total rootzone drainage volume across each irrigation district. Unlike [26], who used the observed CDS drainage data as part of the water balance, it was removed from the calculations and set aside to use as a calibration target. The water balance was thus reduced to the following equation,

$$RZD = (P + I)(1 - Eff)\_\prime \tag{1}$$

where *RZD* is the root zone drainage volume across an entire irrigation district, *P* is the precipitation volume over the district (L<sup>3</sup>/L3), *I* is the total volume of irrigation water diverted to the district (L<sup>3</sup>/L3), and *E*ff is the irrigation efficiency factor (%), noting that other factors are included in the water balance during early time periods at Loxton to estimate seepage and transmission losses from open irrigation channels.
