Heat-pulse method for soil water content measurement: Influence of the specific heat of the soil solids

The heat-pulse method measures soil volumetric water content (theta) based on the linear relationship between soil volumetric heat capacity (C) and theta. Previous work suggested that this method was more appropriate for determining change of theta rather than its absolute value. In this study, we demonstrate that the heat-pulse method can give accurate theta estimation when values for the specific heat of the soil solids (c(s)) are determined using the heat-pulse method. Heat-pulse measurements were performed on packed columns of three soils with a wide range of bulk density (rho(b)) and theta. When the commonly used c(s) value (0.725 kJ kg(-3) K-1) was used, the heat-pulse method overestimated theta by 0.052 m(3) m(-3) on average. However, when c(s) values determined from heat-pulse measurements on oven-dried soil were used, the heat-pulse method provided accurate theta results (-0.006 m(3) m(-3) average error). Using soil-specific, heat-pulse determined c(s) values also reduced the average root mean square error (RMSE) in theta for the three soils from 0.061 to 0.039 m(3) m(-3).