Governing Equations

The transport of solutes on the ground surface is governed by the two-dimen­sional advection-dispersion equation

(33.28)   

where c is the concentration of the solute, R_c is sum of sources and sinks, D_ij is the dispersion tensor and v_i is the velocity tensor.

The velocity of water is determined from the water flux and water depth calcu­lated during the WM simulation.

For overland transport the longitudinal and transverse dispersion coefficients (D_L and D) are specified directly and the dispersion coefficients applied in Eq. (33.28) are determined as for isotropic conditions in groundwater as

(33.29)   

The water depth on the ground surface varies in space and time, due to vari­ations in topography, as well as variations in precipitation, evaporation, infil­tration etc. Since evaporation can concentrate a solute beyond its solubility, a mass balance of precipitated solute is maintained, where the solute will re-dissolve if additional water becomes available. The precipitation and dissolu­tion of the solute is controlled by its solubility.