KiWb WJ224

Hence, gypsum dissolution at any time can be computed as [12]

where t and (i-1) stand for present and previous time steps, respectively, and 0 is soil moisture content. The dissolved gypsum mass is the product of Cg(t) and water flux, W. The values of the a and b coefficients obtained by Keren and O'Connor [11] are as follows:

Gypsum precipitation due to the soil water evaporation can be expressed as [8]:

where Gp is the mass of calcium and sulfate ions that are evapoconcentrated and precipitated back to gypsum as a result of the soil water evaporation from lower layers towards top layer.

The total salt mass balance at any soil layer (l) can then be expressed as [8]:

where Tsl and Sil are final and initial salt mass in a soil layer l, respectively. Sl is salt mass lost from soil layer l due to total water flow leaving the layer. Gdl is mass of dissolved components (calcium and sulfate ions) of gypsum lost from soil layer l due to the total water flow leaving the layer. Sevl is salt mass moved to the layer l from contributing lower layers due to soil water evaporation. Gpl is mass of precipitated components (calcium and sulfate ions) of gypsum moved to the soil layer l due to the soil water evaporation. The total salt mass, Tsl (t/ha) and soil saturation extract Ece (dS/m) in a soil layer l can be expressed as [8]:

Tsl (t / ha)= ECe • 640 (g / m3)-6 -L(m) • lxl04(m2 / ha)-lxl0-6(g /1) (22.8)

where L is the thickness of a soil layer l (see Fig. 22.1).

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