Estimation of Temperature Gradient and Soil Heat Flux from Fourier Analysis of Soil Temperature

Field-measured values of soil temperature were used to estimate the soil temperature gradient and heat flux, an important component in the energy balance of the surface using Fourier analysis of soil temperature. The temperature gradient on both days under study was negative from about 0800 to 1600 h and positive for the remaining period. The temperature gradient at 5 cm depth on both the days reversed the direction and became negative from about 0800 to 1600 h and thereafter changed its direction and was positive. The soil heat flux at 5 cm depth was positive (downward) from about 0800 to 1600 h and negative (upward) for the remaining period. The instantaneous heat flux density attained its maximum value at about 1200 h whereas maximum soil temperature occurred at about 1400 h. The average downward heat flux into the soil was 95.2 watt m⁻² on day 1 and 118.2 watt m⁻² on day 2. The corresponding average upward heat flux for these days was 58.4 watt m⁻² on day 1 and 78.3 watt m⁻² on day 2. The method though requires hourly measurement of soil temperature provides more reliable estimates of soil heat flux since errors due to assumption of sinusoidal temperature wave at the soil surface can be reduced by using Fourier series to accurately describe the temporal variation in soil temperature. The method does not require separate independent measurement of thermal diffusivity and conductivity. However, moisture content and some physical properties need to be known to evaluate volumetric heat capacity.