*3.2. Soil Salinity Calculation*

AquaCrop adopts the calculation procedure presented in BUDGET [36] to simulate the movement and retention of salt in the soil profile. The salts enter the soil profile as solutes after irrigation with saline water or through capillary rise from a shallow groundwater table (vertical downward and upward salt movement). The average ECe in the compartments of the effective rooting depth determines the effects of soil salinity on biomass production.

To explain the movement and retention of soil water and salts in the soil profile, AquaCrop divides the soil profile into 2 to 11 soil compartments called "cells", depending on the type of soil in each horizon (clay, sandy horizon) and its saturated hydraulic conductivity (Ksat in mm/day). The salt diffusion between two adjacent cells (cell j and cell j+1) is determined by the differences in salt concentration and expressed by the electrical conductivity (EC) of soil water.

AquaCrop determines the vertical salt movement in response to soil evaporation, considering the amount of water extracted from the soil profile by evaporation and the wetness of the upper soil layer. The relative soil water content of the topsoil layer determines the fraction of the dissolved salts that moves with the evaporating water.

AquaCrop determines the vertical salt movement because of the capillary rise. Finally, the salt content of a cell is determined by

$$\text{Salt}\_{\text{cell}} = 0.64 \,\text{W}\_{\text{cell}} \text{EC}\_{\text{cell}} \tag{7}$$

Saltcell is the salt content expressed in grams salts per m<sup>2</sup> soil surface, Wcell its volume expressed in liter per m2 (1 mm = 1 L/m2), and 0.64 a global conversion factor used in AquaCrop to convert dS/m to g/L. The electrical conductivity of the soil water (ECsw) and of the electrical conductivity of saturation soil-past extract (ECe) at a particular soil depth (soil compartment) is calculated as

$$\text{EC}\_{\text{sw}} = \frac{\sum\_{\text{j=1}}^{n} \text{Salt}\_{\text{cell.j}}}{0.64 \left(1000 \,\theta\text{A}\_{\text{x}}\right) \Big| 1 - \frac{\text{Volt}\%\_{\text{gravel}}}{100}} \tag{8}$$

$$\text{EC}\_{\text{e}} = \frac{\sum\_{j=1}^{n} \text{Salt}\_{\text{cell},j}}{0.64 \left(1000 \,\text{\(\theta\_{\text{sat}}\,\text{\(\theta\_{\text{sat}}\,\text{\(\theta\_{\text{c}}}\)}\text{)} - \frac{\text{Volt}\_{\text{granvel}}}{100}\right)}} \tag{9}$$

where n is the number of cells in each soil compartment; θ is the soil water content (m3/m3); θsat is the soil water content (m3/m3) at saturation; Δz (m) is the thickness of the soil compartment and Vol% gravel is the volume percentage of the gravel in the soil horizon of each compartment.

### **4. Model Calibration**
