Predicting Soil Temperature and Soil Heat Flux under Different Tillage-Surface Residue Conditions

Model descriptions for soil temperature utilizing readily available information are needed to project management effects of tillage. In many cases these model descriptions must cover periods of a month or more during the growing season. A procedure was developed to utilize daily maximum and minimum air temperature along with normalized root zone temperatures. Root zone temperatures were much more sensitive to the surface residue than the surface roughness (or other tilled layer property) characteristic of tillage-residue cover combinations. However, root zone temperatures normalized with respect to daily maximum and minimum temperatures were of a similar shape for all tillage-residue cover combinations. These normalized soil temperatures also afforded easy comparison among treatments and locations. A Fourier series analysis of daily normalized soil temperatures averaged over part of a growing season showed that the average value around which average normalized soil-temperature curves (diurnal wave) oscillate varies with depth and season. Amplitudes and phase angles of the first two harmonics of the Fourier series were approximately the same for all treatments over four seasons. A model was developed to estimate soil temperatures for various tillage-residue cover treatments with and without a crop cover for the northern Corn Belt states. The model utilizes the first two harmonics of the Fourier series and the relationships between the daily maximum and minimum soil surface temperatures vs. maximum and minimum air temperatures. Input needed for the model are: daily maximum and minimum air temperatures, thermal diffusivity of soil, and coefficients of the Fourier series fitted to average normalized soil temperatures. At one location in southern Minnesota the model predicted soil temperatures at 0.05 and 0.10-m depths over 45 d during the spring of 1980 within an average difference of 2.0°C and a deviation of no more than 3.8°C 67% of the time. A procedure to estimate hourly soil surface heat flux under different tillage and residue conditions is also presented. The procedure is based on the Fourier series soil temperature model. Inputs needed for the model are: (i) daily maximum and minimum air temperatures, (ii) thermal diffusivity, and (iii) volumetric heat capacity. Soil surface heat flux is often needed in the calculation of evaporation from the energy budget methods.