MODELING THE EFFECT OF MULCH OPTICAL-PROPERTIES AND MULCH SOIL CONTACT RESISTANCE ON SOIL HEATING UNDER PLASTIC MULCH CULTURE

A numerical model was developed to simulate the effect of plastic mulches on the field energy balance and soil temperature regime. Newton-Raphson methods were used to solve the energy balances of the soil and mulch simultaneously. The resultant temperature of the soil surface was used as a boundary condition for an implicit finite difference model of soil heat flow. The model contained a detailed radiation section to account for mulch optical properties in shortwave and longwave bands. Heat transfer between the mulch and soil surface was modeled using a thermal contact resistance, and transport in the boundary layer was quantified using flux profile theory. The effect of evaporation and condensation on the underside of the mulch was not considered. The accuracy of the model was verified by comparing simulated soil temperatures with field data collected under five commercially available mulches. Simulated average daily temperatures were within +/- 1.3-degrees-C of the observed values and root mean square errors were less than 2.5-degrees-C in all cases. The model showed that mulch optical properties strongly influenced the way energy was partitioned at the surface. Complete characterization of the mulches in the longwave spectrum was required for realistic simulation. Changing longwave reflectance by only 0.3, for example, caused maximum soil temperature to change by 5-degrees-C under some mulches. Thermal contact resistance between the mulch and soil affected temperatures by up to 20-degrees-C beneath mulches that strongly transmitted or absorbed radiation. However, contact resistance was not an important factor for mulches that had nearly equal shortwave transmittance and absorptance. Results demonstrate that a complex relationship exists between mulch optical properties and thermal contact resistance. Modification of mulch longwave properties during manufacturing and control of thermal contact resistance during mulch installation appear to be two promising areas for research. An alternative numerical scheme was evaluated that utilized linear forms of the energy balance equations. The linearized model produced results almost identical to the original model, but required less computing time and could be adapted easily to simulate multiple mulch layers.