TRACE GAS EXCHANGES IN MIDWEST CROPPING SYSTEMS
Location: Soil, Water, and Air Resources Research Unit
Title: Latent Heat in Soil Heat Flux Measurements
| Heiman, Josh - |
| Horton, Robert - |
| Ren, Tusheng - |
| Xiao, Xinhua - |
Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 25, 2010
Publication Date: July 15, 2010
Citation: Heiman, J.L., Horton, R., Sauer, T.J., Ren, T., Xiao, X. 2010. Latent Heat in Soil Heat Flux Measurements. Agricultural and Forest Meteorology. 150(7-8):1147-1153.
Interpretive Summary: Some of the sun's energy that reaches the earth warms the soil. This warming, and cooling at night when the sun isn't shining, involves heat flow among soil particles and heat flow in the air and water within the soil pores. It is difficult to measure all three of these processes accurately. Some energy is also used to evaporate water and some energy is released when dew forms. An experiment was conducted to test new techniques for measuring soil heat flow by focussing on the energy used in evaporating water near the soil surface. Measurements of soil heat flow at different depths and changes in energy stored in the soil layers were used to estimate evaporation. These measurements were compared to evaporation estimated by weighing soil cores in plastic tubes in the soil during the day as the loss in weight equals the amount of water evaporated as the soil dried. The results showed that evapration near the soil surface could be measured and was an important part of the overall heat flow. This research is important for scientists and practitioners interested in understanding and improving water management and water-use efficiency for crop growth.
The surface energy balance includes a term for soil heat flux. Soil heat flux is difficult to measure because it includes conduction and convection heat transfer processes. Accurate representation of soil heat flux is an important consideration in many modeling and measurement applications. Yet, there remains uncertainty about what comprises soil heat flux and how surface and subsurface heat fluxes are linked in energy balance closure. The objective of this study is to demonstrate the presence of a subsurface latent heat sink, which must be considered in order to accurately link subsurface heat fluxes between depths near and at the soil surface. Measurements were conducted under bare surface conditions in a silty clay loam soil near Ames, IA. Soil heat flux was measured with heat-pulse sensors using the gradient heat flux approach at 1-, 3-, and 6-cm soil depths. Independent estimates of the daily latent heat sink were obtained by measuring the change of mass of microlysimeters. Heat flux measurements at the 1-cm depth deviated from heat fluxes at other depths, even after calorimetric correction was made. This deviation was most pronounced shortly after rainfall, where the 1-cm soil heat flux exceeded 400 W m-2. Cumulative soil heat flux at 1-cm depth exceeded measurements at the 3-cm depth by > 75% over a 7-d rain-free period, whereas calorimetric correction allowed 3- and 6-cm depth measurements to converge. Latent heat sink estimates from the microlysimeters accounted for nearly all of the difference between the 1- and 3-cm depth heat flux measurements, indicating that the latent heat sink was distributed between the 1- and 3-cm depths shortly after the rainfall event. Results demonstrate the importance of a latent heat sink when attempts are made to link or extrapolate subsurface soil heat flux measurements to the surface soil heat flux.