Measuring Soil Thermal Parameters,Water Content And Bulk Density With Heat Pulse Probe Based On Probe Spacing Correction Theory

Soil water content,thermal parameters and bulk density are important soil physical parameters.They interact with each other and are widely used in various disciplines,such as soil science,agrometeorology,geotechnical engineering.Accurate determination of soil water content,thermal parameters and bulk density is of great significance for agricultural production,climate change,irrigation management,engineering application,etc.Heat pulse method is widely used in determining soil water content,thermal parameters and soil bulk density since the advantages of small disturbance to soil,low cost,meeting the requirement of continuous and simultaneous determination of various soil physical parameters.The accuracy of heat pulse method can be easily affected by the change of probe spacing,which in turn affects the accuracy of measuring soil water content and bulk density.The application range and prospect can be improved when spacing error is reduced or corrected.Based on improving the accuracy and application range and prospect of measuring soil water content,thermal parameters and soil bulk density via heat pulse method,this study takes the application of in-situ probe spacing correction theory as the core of the research.We will improve the design of dual probe heat pulse and the accuracy of expansive soil bulk density measurements using Thermo-TDR based on heat capacity method,which will be completed by using in-situ probe spacing correction theory.The main conclusions are as follows:?1?Whether coplanar or non-coplanar deflection,nonlinear model significantly improved the accuracy of measurements of soil specific capacity with extended dual probe?10 cm length?.In using the extended dual probes,the non-linear model performed better in correcting probe spacing in-situ than the linear model?relative error of the former was-8.30and of the latter 43.90%?.Therefore,the nonlinear model was suitable for extended dual probe,while the linear model had some limitations for extended-dual-probe spacing correction.?2?In the process of water loss shrinkage of expansive soil,probe spacings between Thermo-TDR probes changed during dehydration process of expansive soil.The probe spacing of the Thermo-TDR reduced 0.34?0.56 mm every 0.1 m³ m^-3decrease of expansive soil water content during dehydration process.Moreover,the growth of fractures of expansive soil during dehydration process was heterogeneous and anisotropic.?3?The error in measuring expansive soil bulk density using Thermo-TDR based on heat capacity method was reduced when probe spacings were corrected with nonlinear model of in situ probe spacing correction theory,with Relative Error and Root Mean Square Error decreasing from-40.7%?-7.3%to-18.5%?-2.0%and 2.13 g cm^-3 to 0.37 g cm^-3.These results indicated that spacing error in measuring bulk density could be reduced with nonlinear model.?4?The results of bulk density measured by the nonlinear model-heat capacity method and thermal conductivity method were compared with the theoretical values.The Root Mean Square Errors of these two methods were 0.37 g cm^-3 and 0.12 g cm^-3 respectively,indicating that the precision of measurement of thermal conductivity method is slightly higher than that of the spacing correction-heat capacity method;however,because thermal conductivity method was based on a complex empirical model of thermal conductivity which needs to obtain the soil texture or particle distribution in advance,while the spacing correction-heat capacity method was based on the theoretical formula,in practical application,the spacing correction heat capacity method will be recommended to measure the soil bulk density.