| Abstract: | Finite-difference and finite-element methods of approximation have been extended to solve the one-dimensional nonlinear partial differential equations that describe the simultaneous transport of heat, moisture and chemical in the unsaturated zone. Especially for chemical transport, nodal spacing criteria are required to minimize numerical dispersion and oscillatory behavior in the solution vector for chemical concentration. Conservative criteria for nodal spacing for saturated flow can be used to set nodal spacing for unsaturated zone transport. When nodal spacing criteria are satisfied, for the same set of transport and boundary conditions, chemical concentration profiles calculated by the two numerical methods will be almost the same. A situation that is simulated very well with one-dimensional models, is the application of chemicals to land surfaces. To compare and contrast the characteristics of solutions given by the two numerical methods, moisture content, temperature and chemical concentration profiles for a 75-day period after application in the unsaturated zone are calculated for two representative types of organic chemicals. In the first, the chemical is very slowly degraded in the subsurface environment but strongly sorbed to soil surfaces. In the second, the chemical is rapidly degraded but weakly sorbed to soil surfaces. Because of differences in sorption coefficients and mechanisms of degradation, for the same set of hydrodynamic properties of the subsurface, the weakly sorbed chemical is more widely distributed throughout the unsaturated zone, whereas the strongly sorbed chemical stays very close to where it is put initially with little penetration into the subsurface. Satisfying nodal spacing criteria minimizes the impact of the method of approximation on the calculated solutions of the transport equations. For better model predictive performance, however, there are needs for more fundamental information on processes governing transport in the subsurface. |
