Gravity-driven groundwater flow at the scale of drainage basins (basinal flow) is influenced by topography, which controls the water-table position and shape, and geological framework. The theory of groundwater flow systems was developed in the 1960's, in an effort to reconcile field observation with the theory of fluid potential, supported by mathematical analysis. Groundwater flow systems have a hierarchical structure comprising local, intermediate, and regional flow regimes; each having recharge, through-flow, and discharge zones. The presence of high- or low-permeability layers at depth can cause conspicuous spatial distribution of discharge or recharge areas on the surface. Owing to the in-situ interaction of groundwater and the surrounding environment, and the transport and accumulation processes, groundwater is considered a geologic agent. The concept of gravity-driven flow systems has provided a unifying theoretical background for understanding a diverse range of physical, chemical, and ecological phenomena. This article presents a brief overview of the theory of gravity-driven groundwater flow and its roles as a geologic agent, and also describes how the theory was introduced to and adopted by groundwater hydrologists in Japan.

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Japanese groundwater hydrologists have played a leading role in thermal studies of groundwater systems. Pioneering work by Suzuki (1960) led to a method for analyzing temperature profiles to estimate vertical groundwater flux. Sakura (1978) introduced the quantitative analysis of temperature profiles to Japanese researchers and also introduced to Japan the application of an analytical solution of heat transport in two dimensions (Sakura 1993). Japanese hydrogeologists are leaders internationally in analyzing groundwater temperatures affected by surface warming caused by urbanization, including a pioneering study by Taniguchi et al. (1999a). Other contributions include innovative applications of methods for analyzing temperature profiles (e.g., to estimate the depth to the freshwater/saltwater interface in a coastal aquifer) and the development and application of three-dimensional numerical models of coupled groundwater flow and heat transport.

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The studies on coupled water-heat flow in subsurface environment are overviewed from four matrixes of steady-transient and heat-water as boundary conditions for subsurface system. The effects of groundwater flow on subsurface temperature and reconstructions of the heat or water boundary conditions including histories of surface temperature and groundwater recharge rate are shown. Three dimensional reconstruction of boundary conditions, effects of increase in subsurface temperature on the activities in subsurface biomass, and feedback process between surface warming and subsurface warming through ecosystem are presented as new directions of the studies for coupled water-heat transports in subsurface environment.

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The characteristics of seawater / freshwater interface vary locally with groundwater flow in coastal area. A repetition is needed to confirm the updated simulation result with the latest in-situ data. This study is aimed at making a model that can be modified at any point in time for the simulation of groundwater flow and seawater / freshwater interface at the coastal area of Horonobe, where geoscientific data is now being stored for future use. Preliminary simulation results indicated that the seawater / freshwater interface at the test site may be lowered into the Yuchi Formation at the depth of about 1000m. At the future survey, long term information on sea level change will be important for the chemical analysis of groundwater samples with borehole depth. The site at Horonobe is intended for the basic research development for geological disposal waste in coastal area, and the hydrogeological structure will be clarified to the depth of 1000m in several years. The result of this research will be verified at the time.

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