Heat and mass transfer in layered seedbed
Agricultural cultivation and seeding practices often leave a "layered" seedbed structure. Layering is assumed to reduce evaporative losses from the soil surface. In this study, the effect of layering on temperature and water content variations in a clay soil was investigated. Soil columns 150 mm in height and 200 mm diameter were prepared with four different layering configurations, and three different initial water contents, and allowed to dry for a week in a controlled environment. Settings in the growth chamber were 15°C and 44% relative humidity for 16 hours and 5°C and 85% relative humidity for 8 hours. Temperature and water content in different layers of the soil column were monitored continuously. Statistical analysis of the data revealed that layering had a significant effect on moisture retention in soil columns with low initial water contents, while no effect or even increased moisture losses were found in soils with high initial water contents. Layering had no effect on temperature variations in the soil columns. A pressure-based coupled heat and mass transfer model was developed to simulate temperature and soil water pressure (suction) in a layered soil profile. A finite difference formulation of the model was solved using a FORTRAN 77 computer code. Model predictions were compared with experimental data. The model predicted the water content (MRPD values ranged from 2.1% to 7.7%) and temperature (MRPD values ranged from 0.60% to 0.77%) with reasonably low deviations from the measured values. The model showed very high sensitivity to the proposed hydraulic conductivity function, while lower sensitivity was found for the proposed thermal conductivity function. For practical purposes in the clay soil tested, layering may be desirable for low initial water content conditions, while in higher water contents, stratification may have an adverse effect on moisture retention. Verification of the model for field conditions is left for future studies.