Heat tracer-based sap flow methods for tree transpiration measurements: a mini review and bibliometric analysis.

Accurate measurement of plant transpiration is critical to gaining a better understanding of plant water use and exploration of the influence of plants on regional and even global climate. Heat tracer-based sap flow (HTSF) techniques are currently the dominant method to estimate plant transpiration at the individual plant level. However, the majority of current research focuses on specific applications or the evaluation of the method itself, and there is a lack of an overall analysis of HTSF methods. The objectives of this study were: (i) to briefly review the theories and categories of the various HTSF methods, and (ii) to undertake a bibliometric analysis of the use of HTSF methods in measuring plant transpiration. Each HTSF method is described mathematically and their application and pros and cons are briefly discussed. A bibliometric analysis was conducted using 3964 papers published between 1992 and 2020 archived in the Web of Science core collection. The analysis identified publication trends, the most productive authors, organizations, and countries, as well as the most utilized HTSF method (i.e., thermal dissipation) and journals in which these papers were published. In addition, world distribution maps of the use of HTSF methods and tree species measured were drawn based on 741 selected publications with in situ measurements.

Microbial community structure characteristics among different karst aquifer systems, and its potential role in modifying hydraulic properties of karst aquifers.

Little is known about how microbial activity affects the hydraulic properties of karst aquifers. To explore the potential impacts of microbial activity on the hydraulic properties of karst aquifers, microbiological analysis, heat tracer, isotope (dissolved inorganic carbon isotope, δ13CDIC) and aqueous geochemical analyses were conducted at six monitoring wells in Northern Guangdong Province, China. Greater hydraulic conductivity corresponded to a low temperature gradient to an extent; the temperature gradient in karst groundwater aquifers can reflect the degree of dissolution. Higher HCO3 - concentrations coupled with lower d-excess and pH values at B2 and B6 reflect potential microbial activity (e.g., Sulfuricurvum kujiense) causing carbonate dissolution. Microbial activity or the input of anthropogenic acids, as evidenced by significantly more positive δ13CDIC values, potentially affect carbonate dissolution in deep karst aquifers, which eventually alters hydraulic properties of karst aquifer. However, more direct evidence is needed to quantify the effects of microbial activity on carbonate dissolution in karst aquifers.

Coupling heat and chemical tracer experiments for estimating heat transfer parameters in shallow alluvial aquifers.

Geothermal energy systems, closed or open, are increasingly considered for heating and/or cooling buildings. The efficiency of such systems depends on the thermal properties of the subsurface. Therefore, feasibility and impact studies performed prior to their installation should include a field characterization of thermal properties and a heat transfer model using parameter values measured in situ. However, there is a lack of in situ experiments and methodology for performing such a field characterization, especially for open systems. This study presents an in situ experiment designed for estimating heat transfer parameters in shallow alluvial aquifers with focus on the specific heat capacity. This experiment consists in simultaneously injecting hot water and a chemical tracer into the aquifer and monitoring the evolution of groundwater temperature and concentration in the recovery well (and possibly in other piezometers located down gradient). Temperature and concentrations are then used for estimating the specific heat capacity. The first method for estimating this parameter is based on a modeling in series of the chemical tracer and temperature breakthrough curves at the recovery well. The second method is based on an energy balance. The values of specific heat capacity estimated for both methods (2.30 and 2.54MJ/m(3)/K) for the experimental site in the alluvial aquifer of the Meuse River (Belgium) are almost identical and consistent with values found in the literature. Temperature breakthrough curves in other piezometers are not required for estimating the specific heat capacity. However, they highlight that heat transfer in the alluvial aquifer of the Meuse River is complex and contrasted with different dominant process depending on the depth leading to significant vertical heat exchange between upper and lower part of the aquifer. Furthermore, these temperature breakthrough curves could be included in the calibration of a complex heat transfer model for estimating the entire set of heat transfer parameters and their spatial distribution by inverse modeling.