The model simulates salt transport downward and upward. Downward movement involves two parts - (1) modeling movement in the top layer of 10 cm thickness; and (2) modeling the movement under the layers below the top layer (Fig. 22.1). In the top layer, total water flow leaving the surface layer consists of rainfall, lateral subsurface flow, and vertical percolation (Fig. 22.2). In other soil layers, the total water flow consists of only lateral subsurface flow and vertical percolation (Fig. 22.3).
The downward salt movement can be formulated as follows :
Fig. 22.1 Schematic representation of layers in a soil column m layer evaporation lateral subsurface flow rainfall lateral subsurface flow percolation to 2nd soil layer evaporation from 2nd soil layer
Fig. 22.2 Schematic representation of flow and evaporation mass transport in the top soil layer evaporation to upper soil layer lateral subsurface flow percolation from upper soil layer lateral subsurface flow evaporation to upper soil layer percolation from upper soil layer lateral subsurface flow lateral subsurface flow
percolation to lower soil layer evaporation from lower soil layer
Fig. 22.3 Schematic representation of flow and evaporation mass transport in an inner soil layer percolation to lower soil layer evaporation from lower soil layer
Fig. 22.3 Schematic representation of flow and evaporation mass transport in an inner soil layer where S is salt mass in total water flow, S. is initial salt mass in the soil layer; Wt is total water flow, n is soil porosity, and 6w is the wilting point water content. The final salt mass contained in the soil layer is expressed as Sf= S. - S and the average salt concentration is expressed as C = S/W where Cs is the average salt concentration associated with the total water flow. Hence, salt mass contained in runoff, lateral flow, and percolation is found by the product of corresponding water flow and salt concentration.
The upward salt movement is due to water evaporation from the spoil. When water is evaporated from the soil surface, salt is moved upwards into the top soil layer by mass flow. The equation for estimating this salt transport is expressed as :
where Svl is salt mass moved from lower layers to top layer by soil water evaporation and Evl stands for soil water evaporation amount in the contributing layers. Subscription l refers to soil layer and m represents the number of layers contributing to soil water evaporation.
Other major source of salt in a soil column comes from gypsum dissolution. The time dependent gypsum dissolution is defined by Kemper et al.  as:
where C is solution concentration at any time, C is solution concentration at g J gs gypsum saturation which is taken as 4% (g of gypsum/g of soil) or 2.63 g/L in soil solution  and Kd is the dissolution coefficient.
Integrating Eq. 22.3 from t = 0 (water enters the soil layer) to t = tc (water leaves the soil layer) yields Kdtc = ln(Cg/C -1). Keren and O'Connor  conducted a gypsum dissolution study using soil samples amended with 2% and 4% gypsum under different water flow velocities and concluded that Kjc = atc05 + b where a and b are coefficients. According to Kemper et al. , tc=L/W where L is thickness of soil layer and W is actual flow velocity which is the Darcy velocity divided by the porosity. Assuming that soil porosity is equal to the saturated soil moisture content (0) , the dissolution coefficient can be expressed as:
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