# The Heat Pulse Method for Soil Physical Measurements: A Bibliometric Analysis  
[PERSON]  
1 Key Laboratory for Agricultural Environment (Ministry of Agriculture), Institute of Environment and Sustainable Development in Agriculture, CAAS, Beijing 100081, China; [EMAIL_ADDRESS]
2 College of Natural Resources and Environment, Northwest A&F University, Yangling 712100, China
3  
Key Laboratory of Agricultural Soil and Water Engineering in Arid and Semiarid Areas, Ministry of Education, Northwest A&F University, Yangling 712100, China
4  
Department of Renewable Resources, University of Alberta, Edmonton, AB T6G 2H1, Canada; [EMAIL_ADDRESS]
5  
Key Laboratory of Plant Nutrition and the Agri-Environment in Northwest China (Ministry of Agriculture), Northwest A&F University, Yangling 712100, China  
5  
Key Laboratory of Plant Nutrition and the Agri-Environment in Northwest China (Ministry of Agriculture), Northwest A&F University, Yangling 712100, China
and accurate method that can be used to continuously measure soil thermal conductivity, thermal diffusivity and heat capacity [4; 5; 6]. The heat pulse method is a transient/non steady-state method that estimates soil thermal properties by characterizing the radial transport of short-duration line-source heat applied to soils [7; 8; 9]. Compared to the steady-state methods (e.g., guarded hot plate, heat flux meter, and divided bar method), the heat pulse method is a more amenable approach with respect to accuracy, measurement time, cost, and portability [10; 11]. Therefore, the heat pulse method has been widely used to measure soil's physical properties in disciplines such as earth and environmental sciences, planetary science, and engineering [12; 13; 14; 15; 16; 17; 18; 19]. The heat pulse method can be divided into single-probe heat pulse (SPHP) and dual-probe heat pulse (DPHP) methods [20; 21]. Probes of SPHP and DPHP are composed of a heater that functions as a heat source and one or more temperature sensors that are used to record temperature change. SPHP differs from the DPHP probe mainly in two aspects: (1) Probe design: the temperature sensor and heater of the SPHP probe are mounted together within the same probe needle instead of in separate needles. The sensor developed by [PERSON] [10] to measure soil thermal conductivity was considered as the prototype of the SPHP probe, while the probe developed by [PERSON] et al. [9] was considered as the prototype of the DPHP probes. (2) Heating strategies and analytical solutions: the SPHP method generally applies a longer period of heat pulse than that of the DPHP method (e.g., 8-15 s for DPHP [9; 21; 22; 23; 24] vs. \(\geq\) 1 min for SPHP [10; 25; 26]). The DPHP is not affected by probe characteristics as SPHP and all three soil thermal properties can be simultaneously estimated using DPHP. [PERSON] et al. [6] reviewed the theory and solutions of SPHP and DPHP methods for determining soil thermal properties. Recent developments have also coupled the heat pulse method with other probes (e.g., time domain reflectometry (TDR), frequency domain reflectometry (FDR) and porous media) to measure a wide range of other soil physical properties (e.g., water content, ice content, bulk density, water flux, heat flow and evaporation) in addition to soil thermal properties in laboratory and in situ. [PERSON] et al. [4] reviewed the development and application of the heat pulse method for soil physical measurements over a period from 1888 to 2018, with a focus on probe design, construction, calibration, applications, and limitations and perspectives on future studies. Other studies reviewed the advances of heat pulse from different aspects [5; 27; 28]. However, it is noticeable that all the current studies focus on the scientific aspects of the heat pulse method using selected publications. There is a lack of investigating the research status of heat method with all the related publications.