The Heat Pulse Method for Soil Physical Measurements: A Bibliometric Analysis

: Heat pulse method is a transient method that estimates soil thermal properties by characterizing the radial transport of short-duration line-source heat applied to soils. It has been widely used to measure a wide range of soil physical properties including soil thermal conductivity, thermal di ﬀ usivity, heat capacity, water content, ice content, bulk density, water ﬂux and evaporation in laboratory and ﬁeld environments. Previous studies generally focus on the scientiﬁc aspects of heat pulse method based on selected publications, and there is a lack of study investigating the heat pulse publication as a whole. The objective of this study was to give an overall view of the use of heat pulse method for soil physical measurements from the bibliometric perspectives. The analyses were based on the Web of Science Core Collection data between 1992 and 2019 using HistCite Pro and VOSviewer. The results showed an increasing trend in the volume of publications on this ﬁeld and Dr. Robert Horton was the most productive researcher coauthoring papers on the heat pulse method. The co-authorship analysis revealed that researchers from soil science are closely collaborated, but this is not true for researchers in other ﬁelds. There is a lack of new young scientists committing to this ﬁeld while the older generation of researchers are retiring. The United States Department of Agriculture Agricultural Research Servics (USDA-ARS), the China Agriculture University and the Chinese Academy of Science were the top three organizations applying the heat pulse method, while the USA, China and Canada were the top three countries. The Soil Science Society of America Journal, Water Resources Research and Agricultural and Forestry Meteorology were the most widely used journals. The con-occurrence and citation analysis could be used to map the development of the ﬁeld and identify the most inﬂuential publications. The study showed that the bibliometric analysis is a useful tool to visualize research status as well as to provide the general information of novices and experts alike on the heat pulse method for soil physical measurements.


Introduction
Soil physical properties are critical parameters determining soil's physical, biological and chemical processes, including seed germination and plant growth, decomposition of organic matter, nitrification and denitrification, and surface energy balance [1][2][3]. At present, heat pulse is the only automated

Co-authorship of Authors, Organizations and Countries
A total of 52 authors met the threshold of a minimum of five publications per author in the coauthorship network map ( Figure 2). There are 16 clusters, which indicate 16 closely worked groups highlighted in different colors. The middle five clusters have close connections between each other but the remaining 11 clusters did not show collaboration with the others. It is noted that Drs. Robert Horton, Tusheng Ren, Joshua L. Heitman Gerard J. Kluitenberg and Tyson E. Ochsner are the top five most productive researchers on the heat pulse method for soil physical measurements (Table 1). Their research was highly cited by both the heat pulse method community, as indicated by the high TLCS values, and researchers outside of the community, as indicated by the high TGCS/C values (accounting for ~16% of the total citation). It is interesting to note that the top 2-5 researchers were members of Dr. Horton's research group in Iowa State University. Some of them still collaborate closely, as indicated by the thicker link between them ( Figure 2 and Table 1).
However, there is a desire to attract more researchers to improve or apply the heat pulse method, because some of the researchers are going to retire or switch their research interests. For example, Dr. Keith Bristow moved his research interests to different fields after his many pioneering works on the heat pulse method [8,41,42]. Dr. Jan Hopmans recently retired from the University of California, Daivs. However, it is good to see that some talented young scientists including Drs. Sen Lu, Yili Lu, Zhengchao Tian, Yuki Kojima and Minmin Wen are rising in this field. It is also noteworthy that researchers from Engineering such as Drs. Nan Zhang and Xinbao Yu (University of Texas at Arlington, USA) and Devendra N. Singh (Indian Institute of Technology, India) also applied the heat pulse method for the measurement of the thermal conductivity/resistivity of soil and other materials. However, other engineers, including Drs. Kathleen Smits and Benjamin M. Wallen (Colorado School of Mines, USA), were not detected by the VOSviewer [43][44][45].
The top productive authors, organizations and countries are closely related (Table 1). Five institutes in USA and three institutes in China were among the top 10 organizations committing to publications on the heat pulse method for soil physical measurement (accounting for ~34% of the total publications). Therefore, the USA and China were ranked as the top two countries on the number of publications, citations as well as links, and total link strength. This is followed by Canada, with contributions mainly from University of Saskatchewan, Saint Mary's University and University of Alberta. Australia and Germany ranked as fourth and fifth on the number of publications.
It is interesting to notice that the majority of the soil scientists (e.g., Robert Horton, Keith Bristow, Gerard Kluitenberg, Jan Hopmans, Tusheng Ren, Joshua Heitman and Bingcheng Si) focused on the development and application of dual-probe heat pulse (DPHP) method to measure a wide range of

Co-Authorship of Authors, Organizations and Countries
A total of 52 authors met the threshold of a minimum of five publications per author in the co-authorship network map ( Figure 2). There are 16 clusters, which indicate 16 closely worked groups highlighted in different colors. The middle five clusters have close connections between each other but the remaining 11 clusters did not show collaboration with the others. It is noted that Drs. Robert Horton, Tusheng Ren, Joshua L. Heitman Gerard J. Kluitenberg and Tyson E. Ochsner are the top five most productive researchers on the heat pulse method for soil physical measurements (Table 1). Their research was highly cited by both the heat pulse method community, as indicated by the high TLCS values, and researchers outside of the community, as indicated by the high TGCS/C values (accounting for~16% of the total citation). It is interesting to note that the top 2-5 researchers were members of Dr. Horton's research group in Iowa State University. Some of them still collaborate closely, as indicated by the thicker link between them ( Figure 2 and Table 1).
However, there is a desire to attract more researchers to improve or apply the heat pulse method, because some of the researchers are going to retire or switch their research interests. For example, Dr. Keith Bristow moved his research interests to different fields after his many pioneering works on the heat pulse method [8,41,42]. Dr. Jan Hopmans recently retired from the University of California, Daivs. However, it is good to see that some talented young scientists including Drs. Sen Lu, Yili Lu, Zhengchao Tian, Yuki Kojima and Minmin Wen are rising in this field. It is also noteworthy that researchers from Engineering such as Drs. Nan Zhang and Xinbao Yu (University of Texas at Arlington, USA) and Devendra N. Singh (Indian Institute of Technology, India) also applied the heat pulse method for the measurement of the thermal conductivity/resistivity of soil and other materials. However, other engineers, including Drs. Kathleen Smits and Benjamin M. Wallen (Colorado School of Mines, USA), were not detected by the VOSviewer [43][44][45].
The top productive authors, organizations and countries are closely related (Table 1). Five institutes in USA and three institutes in China were among the top 10 organizations committing to publications on the heat pulse method for soil physical measurement (accounting for~34% of the total publications). Therefore, the USA and China were ranked as the top two countries on the number of publications, citations as well as links, and total link strength. This is followed by Canada, with contributions mainly from University of Saskatchewan, Saint Mary's University and University of Alberta. Australia and Germany ranked as fourth and fifth on the number of publications. flux, evaporation, water content and bulk density. Sheng et al. [53] proposed a new design by coupling a heat pulse probe and FDR to make a thermo-FDR that can get a better estimation of soil water content and thermal properties at a cheaper price. Reece [54] combined a heat pulse probe with porous media (e.g., gypsum or ceramic cup) to construct a line heat dissipation sensor that can be used to determine matric potential. Studies [55][56][57] have also extended the heat pulse method to measure soil water content at the middle scale by using actively heated fiber optics.  It is interesting to notice that the majority of the soil scientists (e.g., Robert Horton, Keith Bristow, Gerard Kluitenberg, Jan Hopmans, Tusheng Ren, Joshua Heitman and Bingcheng Si) focused on the development and application of dual-probe heat pulse (DPHP) method to measure a wide range of soil physical properties [46,47], while the engineers (e.g., Devendra Singh) applied the single-probe heat-pulse (SPHP) method to measure soil thermal conductivity or thermal resistivity, which is the inverse of thermal conductivity [48][49][50]. However, more and more engineers are applying the DPHP method nowadays [17,51], which is probably because the DPHP method can be used to estimate all three soil thermal properties (i.e., thermal conductivity, heat capacity and diffusivity) and water content. In addition, researchers have also combined the heat pulse method with other probes (e.g., time domain reflectometry (TDR), frequency domain reflectometry (FDR) and porous media). For instance, Noborio et al. [52] first combined heat pulse probe and TDR (hereafter thermo-TDR) to measure soil thermal properties and water content. Ren et al. [19] advanced the thermo-TDR to measure a wide range of soil physical properties, including soil thermal properties, heat flux, water flux, evaporation, water content and bulk density. Sheng et al. [53] proposed a new design by coupling a heat pulse probe and FDR to make a thermo-FDR that can get a better estimation of soil water content and thermal properties Appl. Sci. 2020, 10, 6171 6 of 15 at a cheaper price. Reece [54] combined a heat pulse probe with porous media (e.g., gypsum or ceramic cup) to construct a line heat dissipation sensor that can be used to determine matric potential. Studies [55][56][57] have also extended the heat pulse method to measure soil water content at the middle scale by using actively heated fiber optics. Table 1. Top 10 authors, organizations and countries commits to publications on heat pulse method for soil physical measurement with a threshold of minimum five publications per author. The HistCite indices are number of publications (N), total local citation score (TLCS, times to be cited by the 1319 papers) and total global citation score (TGCS, times to be cited by the Web of Science Core Collection). The VOSviewer indices are links (L, the number of collaborations or lines between investigated author/country/organization), total link strength (TLS) and citations (C). Value of TGCS is equal to C (TGCS/C).

No.
Items N TLCS TGCS/C L TLC

The Most Recognized Journals
There were 291 journals that publish 1319 studies on the heat pulse method for soil physical measurements. The top 10 journals publishing this topic are generally in the categories of Soil Science, Hydrology and Agriculture, with one in Engineering (i.e., Geotechnical Testing journal), as shown in Figure 3. This is understandable because the main applications of this method are for soil physical measurements (e.g., thermal properties and water content). The journals also cited each other intensively compared to other journals (data not shown), which indicate that these journals are highly recognized by researchers in the community of the heat pulse method.

History of the Heat Pulse Method and the Highly Impacted Studies
The citation analysis with HistCite Pro showed that the papers numbered 20 [8], 25 [58] and 42 [59] are the most influential pieces of research in the area of the heat pulse method (Figure 4). These studies established the widely applied approaches for data interpretation and error analysis of soil thermal properties. The papers of 97 [60], 151 [61] and 278 [62] highlighted the extended use of the heat pulse method for the measurement of soil water content and electrical conductivity. The papers of 173 [46] and 279 [19] extended the heat pulse for soil water flux measurements. The papers of 280 [63] and 474 [64] extended the use of heat pulse method to estimate soil evaporation. The papers of 216 [65] and 421 [66] were marked as the modelling of soil thermal properties. While the papers of 82 [67], 142 [68] and 316 [69] only highlighted the significance of this method and were only highly cited by the researcher outside of the field, as indicated by no lines between these three papers and the other 27 papers in Figure 4. Because only data from 1992 up to date are available for SCI-EXPANDED of the WoSCC, some of the pioneer works [9,10,[70][71][72][73][74][75][76][77] were not included. However, it should be noted that de Vries [10] and Campbell et al. [9] were among the pioneers applying the single-probe heat-pulse and dual-probe heat-pulse method, respectively, to determine soil thermal properties. A detailed history of the development and evolution of heat pulse method is referred to by He et al. [4].

Co-Occurrence Analysis of Keywords
There were 111 keywords provided by the authors, and it is not surprising that thermal conductivity is the most highly used term, because the heat pulse method is widely used to measure soil thermal conductivity ( Figure 5). The terms "soil thermal properties", "thermal properties", "thermal diffusivity", "heat capacity", "volumetric heat capacity" were also widely used because the heat pulse can be used to simultaneously estimate thermal diffusivity, volumetric heat capacity and thermal conductivity [4,59]. However, it should be noted that the VOSviewer was not capable of detecting the total co-occurrences of soil thermal properties because of the lack of a uniform use of keywords. This makes the yellow color background (or high co-occurrences) of these terms not as strong as it should be, which is also true for other related terms, discussed below.

Co-occurrence Analysis of Keywords
There were 111 keywords provided by the authors, and it is not surprising that thermal conductivity is the most highly used term, because the heat pulse method is widely used to measure soil thermal conductivity ( Figure 5). The terms "soil thermal properties", "thermal properties", "thermal diffusivity", "heat capacity", "volumetric heat capacity" were also widely used because the heat pulse can be used to simultaneously estimate thermal diffusivity, volumetric heat capacity and thermal conductivity [4,59]. However, it should be noted that the VOSviewer was not capable of detecting the total co-occurrences of soil thermal properties because of the lack of a uniform use of keywords. This makes the yellow color background (or high co-occurrences) of these terms not as strong as it should be, which is also true for other related terms, discussed below. The terms of "soil water content", "soil water", "soil moisture", "water content", "moisture content", "moisture", "dry density", "infiltration" and "solute transport" revealed the wide applications of heat pulse method, including thermo-TDR, to measure a wide range of soil physical and hydrological properties in the vadose zone [18,28,90]. It also should be noted that the growing application of the heat pulse method in frozen soils was observed [91][92][93][94][95][96]. The "transpiration", "sap flow", "soil evaporation", "heat transfer" and "soil heat flux" were also popular terms, because the heat pulse method enables the measurement of both evaporation and transpiration (converted from sap flow). Therefore, the heat pulse method is among the key methods partitioning evapotranspiration [27,97,98] and it can be combined with the "remote sensing" or "eddy covariance" methods [99]. The measurement of thermal properties for geotechnical or construction applications (e.g., "ground heat exchanger", "ground source heat pump") has also been widely reported. However, the use of the heat pulse to estimate thermal properties of extra-terrestrial bodies [12][13][14][15] was not detected in Figure 5, which could be due to the comparably small number of applications. The terms of "soil water content", "soil water", "soil moisture", "water content", "moisture content", "moisture", "dry density", "infiltration" and "solute transport" revealed the wide applications of heat pulse method, including thermo-TDR, to measure a wide range of soil physical and hydrological properties in the vadose zone [18,28,90]. It also should be noted that the growing application of the heat pulse method in frozen soils was observed [91][92][93][94][95][96]. The "transpiration", "sap flow", "soil evaporation", "heat transfer" and "soil heat flux" were also popular terms, because the heat pulse method enables the measurement of both evaporation and transpiration (converted from sap flow). Therefore, the heat pulse method is among the key methods partitioning evapotranspiration [27,97,98] and it can be combined with the "remote sensing" or "eddy covariance" methods [99]. The measurement of thermal properties for geotechnical or construction applications (e.g., "ground heat exchanger", "ground source heat pump") has also been widely reported. However, the use of the heat pulse to estimate thermal properties of extra-terrestrial bodies [12][13][14][15] was not detected in Figure 5, which could be due to the comparably small number of applications.

Conclusions and Perspectives
We analyzed the publications of the heat pulse method over the period of 1992 and 2019 with bibliometric methods. The result showed an increasing trend in the number of publications in this field. The co-authorship analysis showed that Dr. Robert Horton from Iowa State University is the most productive researchers and he has a close collaboration with other top ranked authors. The USDA-ARS, the China Agriculture University and the Chinese Academy of Science were the top three organizations applying the heat pulse method. The USA, China and Canada were the top three countries and the Soil Science Society of America Journal, Water Resources Research and Agricultural and Forestry Meteorology were the most widely used journals. The con-occurrence and citation analysis could be used to map the research topics and landmark publications. We concluded that the bibliometric analysis is a useful tool to visualize the research status of the heat pulse method for soil physical measurements.
Although great progress in the development and application of heat pulse method has been made in recent decades, future studies should focus on the following aspects: (1) A new design of heat pulse probes with good performance at a lower cost and higher energy efficiency. The cost of the currently available heat pulse sensors impedes the wide application of the heat pulse method and the high energy consumption hinders the continuous measurement of soil thermal properties at remote locations with limited access to a power grid. This may explain why none of the meteorological stations install heat pulse sensors, but sensors of soil water/moisture and temperature. In addition, the thermo-TDR is limited by the short length of the probe needle that affects the accuracy of estimating soil water content [91,100]. Because FDR is less likely to be affected by the probe length [53,101] and Acclima Inc. developed the TDR 305 with TDR needles of 5-cm length, operating at 2 GHz, to accurately estimate soil water content. Possible solutions to this are to combine the heat pulse probe with FDR operating at dual frequencies (e.g., KHz and MHz) or TDR operating at a high frequency (e.g.,~2 GHz); (2) The development of a database on soil thermal properties. Soil thermal properties affect agricultural microclimates and therefore influence seed germination, seedling development and the subsequent establishment of stand, soil thermal regime, and the development and calibration of soil thermal conductivity models [102][103][104][105][106][107]. They also play a key role in the mass and energy exchange through porous media and interactions between the ground and atmosphere, influencing climate at regional and global scales [108][109][110][111][112][113][114][115][116]. Unlike the hydraulic properties [117][118][119], there is no database on experimental measurements of soil thermal properties. There is an urgent call to establish such a database.