Submitted to: Advances in Water Resources
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/30/2012
Publication Date: 11/29/2012
Citation: Evett, S.R., Agam, N., Kustas, W.P., Colaizzi, P.D., Schwartz, R.C. 2012. Soil profile method for soil thermal diffusivity, conductivity and heat flux: Comparison to soil heat flux plates. Advances in Water Resources. 50:41-54. http://dx.doi.org/10.1016/j.advwatres.2012.04.012. Interpretive Summary: Evaporative losses from the soil can be an important loss of water from farming systems, including irrigation. Evaporative loss is important because it reduces the water available for crop production. Methods of determining evaporative loss rates depend on measurements of the energy and water balances at the soil surface. These are affected by the amount of sunshine, the wind speed, the air temperature and the humidity of the air. Warmer, drier, windier days with abundant sunshine cause increased evaporative loss, but can also cause the soil to warm, which reduces the potential evaporative loss. Knowledge of the degree of soil warming is therefore important in finding out how much evaporative loss will occur under given weather conditions. Scientists at the USDA Agricultural Research Service, Bushland, Texas, invented a new method of measuring soil warming and determining the amount of energy entering the soil and thus not available to drive evaporative loss. The new method showed that energy entering the soil was a more important component and was more variable in time and across the soil surface than previously thought. It also produced important other information such as the soil water content and temperature and the ability of the soil to conduct heat downward from the surface, which affects the evaporative loss rate. Use of the new method will allow scientists and engineers to more accurately estimate evaporative loss and soil warming and cooling. All three are important for producers to know as they make decisions about the timing of planting and irrigation.
Technical Abstract: Diffusive heat flux at the soil surface is commonly determined as a mean value over a time period using heat flux plates buried at some depth (e.g., 5 to 8 cm) below the surface with a correction to surface flux based on the change in heat storage during the corresponding time period in the soil layer above the plates. The change in heat storage is based on the soil temperature change in the layer over the time period and an estimate of the soil thermal heat capacity that is based on soil water content, bulk density and organic matter content. One-layer corrections using some measure of mean soil temperature over the layer depth are most common, although multiple layer corrections using two or more temperature measurements have been done, and in some cases the soil water content has been determined, although rarely. Several problems with the heat flux plate method limit the accuracy of soil heat flux values. An alternative method is presented and this flux gradient method is compared with soil heat flux plate measurements. The method is based on periodic (e.g., half-hourly) water content and temperature sensing at multiple depths within the soil profile and a solution of the Fourier heat flux equation. A Fourier sine series is fit to the temperature at each depth and the temperature at the next depth is simulated with a sine series solution of the differential heat flux equation using successive approximation of the best fit based on changing the thermal diffusivity value. The best fit thermal diffusivity value is converted to a thermal conductivity value using the soil heat capacity, which is based on the measured water content and bulk density; and the thermal conductivity as a function of water content and bulk density is thus determined. The soil heat flux between each pair of temperature measurement depths is computed using the thermal conductivity function and measured water contents. The thermal gradient method of heat flux calculation compared well to values determined using heat flux plates and calorimetric correction to the soil surface; and it provided better representation of the surface spatiotemporal variation of heat flux and more accurate heat flux values. The overall method resulted in additional important knowledge including the water content dynamics in the near-surface soil profile and a function relating thermal conductivity to soil water content and bulk density.