野外试验中热脉冲探针间距校正方法的应用

Application of Spacing Correction Method of Heat Pulse Probe in Field Measurements

土壤热特性是土壤的重要参数之一,热脉冲探针是目前土壤热特性最为常用的测量方法。在用热脉冲探针测量土壤热特性时,探针间距对测量结果的准确性影响非常大。而在野外试验中,由于土壤受到植物根系、土壤冻融、土壤动物活动等的影响,会造成探针间距的变化。因此,探针间距的校正在野外实地测量时十分重要。本研究团队在2013年提出热脉冲探针间距的校正方法,即在温度探针中使用两个热敏电阻,并通过理论公式校正探针初始间距后,得到间距变化后的探针真实间距。将此校正方法应用于野外试验中,分别于夏季和冬季监测土壤中三个深度处(3 cm、8 cm和13 cm)探针间距的变化。结果显示探针间距校正后得到的土壤热特性参数与未校正探针间距时的土壤热特性相比,探针间距校正后得到的土壤热特性参数更加准确可靠;但由于冬季表层土壤存在结冰现象,造成热脉冲探针无法准确测量热特性参数。

【Objective】Soil thermal characteristics,including thermal diffusivity,thermal capacity and thermal conductivity,are basic physical parameters reflecting changes in soil temperature,and maintenance and transport of heat in the research on soil physical processes.Heat pulse probing method is the most common one used nowadays to determine soil thermal properties.However in using the method,spacing between probes is a major factor affecting accuracy of the results.In measuring,especially in the field,when probes are inserted into the soil,spacing between probes may vary with penetration resistance of the soil.In addition,alternation of soil freezing and thawing,movement of soil organisms,and growth of plant roots may all cause expansion and contraction of the soil,thus forcing the tips of the probes to deviate or squint and hence change in spacing.It is therefore,essential to perform in situ calibration of probe spacing.【Method】To that end,Liu et al.proposed a method to calibrate probe spacing.They found that once the heating probe or temperature probe squinted by 1°,the thermal conductivity and thermal capacity measured would deviate by more than 10%.So Liu et al.put two thermistors on to the temperature probe in different positions,and figured out a theoretical formula to calibrate the spacing.The formula was very sensitive to the maximum temperature difference（ΔT^m）and the time when the maximum temperature difference appeared（tm）.In this study,the calibration method was applied to field measurement and monitoring of changes in probe spacing.Temperature probes were installed at 3 cm,8 cm and 13 cm deep in the soil,separately,to record heat pulses from the heat pulse probe during summer from May 19^（th） to July 3^（rd）,2015 and the winter from February 11^（th） to March 28^（th）.2016.【Result】Results show that firstly,probe spacing did not have any impact on the measurement of thermal conductivity,which suggests that the soil is homogeneous in texture,and when comparison was made of the measurements of thermal capacity,it was found that the differences between the two thermistors was determined by accuracy of the probe spacing;and secondly,the measurements of soil thermal properties with the probe spacing calibrated method were more accurate and reliable than those without any spacing calibration.However in the winter,when soil temperature was under 0℃,the soil would freeze and thaw alternately,causing the probe tips to deviate.Besides when the heating probe heated,the heat would be divided into two portions：one to melt the ice and the other to warm up the soil.Consequently,the measurements were not so accurate and could be corrected with the spacing calibration method.Besides,the freezing and thawing processes of the soil in winter also made it hard to use heat pulse probes to determine soil thermal properties.It was also found that the measurement with the probes embedded in the surface soil layer was more sensitive to variation of probe spacing.So far as of yet,it is still very hard to explain why,because of various unknown causes.Nevertheless soil thermal properties in the surface layer mean extremely significant to soil water movement,measurement of ground heat flux and so on.Therefore,more efforts should be done in studying accuracy of the heat pulse probe method and practicability of the spacing calibration method.【Conclusion】To sum up,in using the modified dual heat pulse probe method to monitor soil thermal properties in the field in summer and winter,it is found that probe spacing varies more in the surface soil layer than in the others,and the use of the probe spacing calibration method can significantly minimize deviation of the measurements of volumetric heat capacity,and the spacing calibration method is applicable to field measurement.However,due to the presence of frozen soil in the surface soil layer in winter,it is hard to determine accurately soil thermal properties with the heat pulse probe method and to use the probe spacing calibration method,too.