別紙様式５(Attached Form 5)
学位論文要旨 Abstract of Thesis
所属専攻Field: 複合新領域科学専攻(Field)
氏 名 Name: DONG, Linyao （董林垚）
Title of Thesis
Analytical study to understand groundwater flow system and surface warming effect using subsurface thermal regimes—A case study in Kumamoto area, Japan
地下水温を用いた地下水流動系および地表面温度上昇効果の解析 -熊本平野を例として-
Abstract（within 1600 words）
Subsurface temperature-depth profile is disturbed by the combined influences of surface warming and subsurface groundwater flowing. Consequently, the imparted signatures can be interpreted inversely to detect groundwater flow system or estimate the surface warming rate. The pertinent literature review indicates that the heat transferred by groundwater serves as an effective tracer to identify groundwater recharge/discharge, depict groundwater flow system, estimate the hydrogeological parameters, reconstruct the paleoclimate, and detect the surface warming effects.
Numerous studies have been investigated in the metropolitan cities (such as Tokyo, Osaka, Taipei and Seoul) or enclosed groundwater basins (such as Kumamoto plain and Nagaoka plain) in Asia. These studies emphasize the analytical interpretation on observed temperature-depth profiles to detect surface warming effects or depict groundwater flow system in the study area. Based on the theory and methodology in these investigations, an integrated study on the subsurface thermal distribution in Kumamoto plain was conducted in this study.
The study area is a relatively enclosed groundwater flowing basin consisted of clay, deposits and volcanic rocks. The field measurements on the temperature-depth profiles were conducted around 1988, 2000 and 2012 in the observation wells distributed in the study area, and the corresponding hydraulic head data were also measured. Then the one-dimensional (1-D), two-dimensional (2-D), three-dimensional (3-D) analytical and numerical implements, coupled with the groundwater and heat-flow model
(GETFLOWS) were employed in the interpretation, and the following points were concluded:
(1) The 1-D implement identifies the groundwater recharges/discharges in the study area. The heat tracer reveals that the aquifer recharges commonly occurs in the mountainous regions, whereas the groundwater discharges mainly exists in the coastal area, indicating the groundwater flows from the northeast to the southwest in the study area. The interpreted groundwater flow patterns fit favorably with the observed hydraulic heads and the previous estimates from the perspective of heat.
(2) The least-square solver was introduced to estimate the groundwater velocity. The estimate velocity ranges from -5.900 to 1.223 m/year in the study area. The employed analytical solution didn’t perform well in the fitting of
temperature-depth profiles in some specific observations, which was associated with the multi-layer system in the aquifer. The observed temperature-depth profiles act as an effective implement to detect the geological differences in the stratum.
(3) The temperature increasing was observed in the subsurface temperature-depth profiles, which is associated with the surface warming events. The 1-D analytical implement was employed to estimate the surface warming rate in Kumamoto plain.
The estimated surface temperature increase in rural areas from 1987 to 2012 is about 0.7 degree, whereas that estimated in urban areas is about 1.5 degree. And the estimated surface warming rates keep accordance with the measured air
temperature records in Kumamoto area. It can be inferred that the surface warming in rural areas was solely caused by the climate change, while the surface warming events near urban areas was caused by both climate change and heat islands effect associated with urbanization.
(4) The developed 2-D numerical model interprets the disturbances of subsurface groundwater flowing and surface warming on temperature-depth profiles. The surface warming influence commonly occurs in the shallow aquifer, and the influence propagates deeper in the recharge sites. And the spatial and temporal features in the temperature-depth profiles were reflected by the numerical model.
This model can also provides an excellent interpretation to the temperature depth profiles observed in Kumamoto plain.
(5) The coupled 3-D GETFLOWS model provides an integrated simulation of the subsurface temperature in Kumamoto plain. The simulate temperature profiles fitted favorably with the observed data. And the detailed groundwater flow system can be depicted based on the simulated temperature. The thermal wave
propagation towards downward was also evaluated based on the simulated data.
And the quick thermal wave amplitude attenuation indicates groundwater discharges, which occurs in the coastal regions.
