To design an efficient ground source heat pump (GSHP) system, it is important to accurately measure the thermophysical parameters of the geotechnical layer. In the current study, a borehole is tested in detail using a combined thermal response test system (CTRTS) based on a distributed optical fiber temperature sensor (DOFTS) and a laboratory test. Real-time monitoring of the stratum temperature according to depth and operation time and the geothermal profile and thermal conductivity of each stratum are obtained. The results show that the undisturbed ground temperature is 10.0 °C, and the formation temperature field within 130 m can be divided into variable temperature formation, constant temperature formation (9.13 °C), and warming formation (geothermal gradient is 3.0 °C/100 m). The comprehensive thermal conductivity of the region is 1.862 W/m·K. From top to bottom, the average thermal conductivity of silty clay, mudstone, argillaceous siltstone, and mudstone is 1.631 W/m·K, 1.888 W/m·K, 1.862 W/m·K, and 2.144 W/m·K, respectively. By comparing the measurement results, the accuracy and effectiveness of the CTRTS are verified. Therefore, it is recommended to use the thermal conductivity obtained by the CTRTS to optimize the design of the borehole heat exchanger (BHE). This study provides a case for establishing a standard distributed thermal response test (DTRT).

Potential evapotranspiration (PET) is a key parameter for calculating drought monitoring index that is generally difficult to obtain. In addition, PET has low spatial resolution and can only be obtained at a site-based point. Therefore, retrieving PET with high precision, high spatial resolution, and less meteorological data becomes the focus of this paper. In this paper, a high-precision and highspatial-resolution drought monitoring (HDM) model was established to accurately calculate PET by using the zenith troposphere delay (ZTD) derived from global navigation satellite system (GNSS) and temperature (T). The initial PET value was calculated by using the PET periodical model based on Penman-Monteith (PM)-derived PET. The PET difference (DET) between the PM and periodic model was then calculated, and a multiple linear regression model was established to fit DET by using the ZTD and T differences at meteorological stations. To improve the spatial resolution of the calculated PET, a spherical harmonic function was applied to fit the coefficients of these stations. The HDM-derived PET at grid points was eventually obtained by using the fitted coefficients and ZTD/T. The HDM-derived PET and standardized precipitation evapotranspiration index (SPEI) were compared with those from the Thornthwaite (TH) and revised TH (RTH) models over the loess plateau (LP) area with the PM-derived PET and SPEI as references. Comparison results highlight the excellent performance of the proposed HDM model and the Pearson's correlations of SPEI between the HDM and PM models all exceeded 0.96 under different month scales.

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Geothermal energy exhibits considerable development potential in space heating. Shallow geothermal energy stored in the soil in the form of low-grade energy is mainly extracted via the ground source heat pump (GSHP) system. GSHP systems use the subsoil as a heat source, typically involving a vertical borehole heat exchanger (BHE) to extract heat from the formation. Accurate measurement of the thermal properties of the formation is very important for the design of BHEs. At present, the most common and effective method to measure the thermal conductivity of the formation in the field is the thermal response test (TRT). However, the test conditions (heat load, test time) during the thermal response test can impact the test results. Therefore, in this study, a borehole with a depth of 130 m was evaluated in the field. The TRT module and the distributed thermal response test (DTRT) module based on distributed optical fiber temperature sensor (DOFTS) technology were used to monitor the test with different working conditions in real-time. In the field tests, geothermal conditions and the evolution of the formation temperature with time and depth were determined. Based on the test results under different heat loads and test times, the influence of the test conditions on the thermal conductivity results was analyzed and described. A constant temperature zone was located at a depth from 25 m to 50 m, and an increasing temperature zone was located at a depth from 50 m to 130 m, with a geothermal gradient of 3 °C/100 m. The results showed that the heat load slightly influenced the thermal conductivity test results. At the initial stage of the test, the temperature significantly increased from 0 to 12 h. After reaching the quasi-stable state, the test time slightly influenced the thermal conductivity test results. The characteristics of the formation thermal recovery stage after the test stage were studied. The heat load decreased, which could shorten the time for the formation to recover the initial temperature. The results could provide a basis for the optimization of thermal response test conditions.