Modeling coupled water-heat flow and impacts upon chemical transport in mulched soil beds

Plastic-covered soil bed systems are widely used in the southeastern United States for vegetable and fruit production. Plastic-mulching results in highly variable and transient soil thermal regimes both in space and time under atmospheric influences (e.g., solar radiation, wind, etc.). Much evidence exists that water flow is affected by non-isothermal conditions and vice-versa. In this work, a two-dimensional model for simultaneously describing chemical transport and coupled water-heat flows in plastic-covered soil beds was developed with many improvements over earlier efforts. Realistic plastic-mulch boundary conditions are thoroughly described by including optical properties of the plastic mulch into the energy balance at the soil/atmospheric interface along with other atmospheric influences of weather components. This model provides opportunity to consider the effect of coupled water-heat flows upon estimates of chemical transport (both in the liquid and gaseous phases) in plastic-mulch culture production systems.; Chemical transport in soil beds in the model utilize consideration of chemical existence in solid, liquid, and gas phases. Temperature dependence of the rates of chemical reactions and volatilization is described efficiently by incorporating temperature dependence in fate and transport coefficients. The model uniquely provides opportunity to simulate coupled water-heat flow and multiple-solute transport with a complex plastic-mulch atmospheric boundary for realistic field situations.; The model was used to analyze the fate and transport of methyl bromide (MBr) during fumigation of mulched soil beds. MBr, a highly volatile pesticide, is widely used in California, Florida, and Hawaii for its broad-spectrum efficiency in controlling insects, fungi, and weeds. Unfortunately, MBr is a major environmental hazard due to its depletion effect on the atmosphere ozone layer. Model analyses revealed that non-isothermal conditions should be considered in the accurate description of MBr fate and transport. Modeling results indicate that utilizing a relatively wet soil profile, deeper injection, and water dousing on the bed-surface before installation of plastic mulch will decrease atmospheric emissions of MBr. However, deeper injection of MBr fumigant, although environmentally less hazardous, may decrease effective sterilization of crop root zones in the soil.