|Horton, Robert - IOWA STATE UNIVERSITY|
Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 6, 2006
Publication Date: January 1, 2007
Citation: Ochsner, T.E., Sauer, T.J., Horton, R. 2007. Soil heat storage measurements in energy balance studies. Agronomy Journal. 99:311-319. Interpretive Summary: Environmental and agricultural scientists often need to measure rates of heat flow in soil. To accomplish this, they typically must account for heat storage in the soil near the surface. To calculate this heat storage they must first determine the soil heat capacity (C), which is a measure of the energy required to raise the soil temperature. The objective of this paper is to describe the effects of using different methods to determine C. We found that soil sampling three times per week to determine C was not frequent enough to consistently record the sometimes rapid changes in C with time. With sampling intervals of 2-3 days, the rate of change of C is unknown and neglected when calculating heat storage. Neglecting the rate of change of C introduced large errors during infiltration and persistent but much smaller errors during periods of soil drying. Automated hourly measurements with heat pulse sensors avoided these errors and provided the most accurate data. Scientists who measure heat flow in soil will benefit from these findings. By implementing the recommendations in this paper, these scientists will reduce one source of error in soil heat flow measurements.
Technical Abstract: Energy balance studies generally require knowledge of the heat flux at the soil surface (G0). Typically, the rate of change of heat storage in the soil (S) is used together with an estimate of the reference soil heat flux (Gr) at some depth to determine G0. The near-surface soil volumetric heat capacity (C) must be determined in order to calculate S. The objective of this paper is to quantify the effects of the method of C determination on S. The methods evaluated were estimation of C by soil sampling every 2-3 days, estimation of C based on Theta Probe water content measurement every 2-3 days, and hourly measurement of C using heat pulse sensors. Measurements were performed under a bare soil surface, a soybean [Glycine max (L.) Merr.] canopy, and a corn (Zea mays L.) canopy. When C was determined using all three methods simultaneously, the three C estimates agreed to within 6% on average. However, temporal variability of C was best recorded with the heat pulse sensors. Sampling three times per week was not frequent enough to consistently record the rate of change of C (dC/dt) for the near-surface soil. With sampling intervals of 2-3 days, dC/dt is unknown and neglected when calculating S. Neglecting dC/dt introduced errors occasionally exceeding 200 W m-2 during infiltration and persistent but much smaller errors during periods of soil drying. Hourly measurements with the heat pulse sensors avoided these errors and provided the most accurate S data.