Soil Heat Storage Measurements in Energy Balance Studies

Energy balance studies require knowledge of the heat flux at the soil surface. This flux is determined by summing the heat flux at a reference depth (zr) some centimeters below the surface and the rate of change of heat storage in the soil above zr. The rate of change of heat storage, or heat storage for short ((delta)S), is calculated from soil volumetric heat capacity (C) and temperature. The objectives of this study were to determine how choices regarding zr, C measurements, and (delta)S calculations all affect the accuracy of (delta)S data. Heat transfer theory and data from three field sites were used toward these ends. In some studies, shallow reference depths have been used and (delta)S neglected. Our results indicate that when zr is sufficiently deep to permit accurate heat flux measurements, (delta)S is too large to neglect. Three methods for determining C were evaluated: soil sampling, the ThetaProbe soil moisture sensor, and heat pulse sensors. When C was determined using all three methods simultaneously, the estimates agreed to within 6% on average; however, the temporal variability of C was best recorded with the automated heat pulse sensors. Three approaches for calculating (delta)S were also tested. The common approach of letting C vary in time but neglecting its time derivative caused errors when soil water content was changing. These errors exceeded 200 W m^-2 in some cases. The simple approach of assuming a constant C performed similarly. We introduce a third approach that accounts for the time derivative of C and yields the most accurate (delta)S data.