This paper reviews observation- and model-based studies of stable isotopes in precipitation and discusses the Global Network for Isotopes in Precipitation (GNIP) and other regional networks and projects involving isotope observations. Using data from the latest GNIP/Isotope Hydrology Information System (ISOHIS) database, the annual and monthly variabilities of δD-δ¹⁸O relationships, the temperature effect, and the amount effect for δ¹⁸O were calculated as averages for the Northern Hemisphere, Southern Hemisphere, and globe. At the global scale, the global Meteoric Water Line (MWL) was δD = 8.00 × δ¹⁸O + 9.62 (R² = 0.99), the temperature effect was 0.53‰/°C (R² = 0.71), and the precipitation amount effect was -1.05‰/100 mm (R² = 0.20). Comparisons of modeled and observed temperature and amount effects indicate that most General Circulation Models (GCMs) have reproduced the present climate well. This paper also summarizes the use of stable isotopes in precipitation as proxy data for climate variability and water origins, as well as the results of recent isotope-observation studies, especially in the Asian monsoon regions.

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Stemflow has a significant influence on hydrological and ecological processes within forests. However, research on stemflow is scant as stemflow comprises a very small volume compared to throughfall. In Japan, forest cover mainly distributed in mountainous areas occupies nearly 70% of the total land area and ranges from subalpine to subtropical zones. Many hydrological and agricultural studies have been conducted but a comprehensive literature review that summarizes the generation and chemical characteristics of stemflow is lacking. The purpose of this review is to interpret the results from Japanese forest stemflow papers and to propose objectives for future studies.

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The results of the research on the circulation and the mixing of lake water were organized, and the perspective of the research was considered. The flow of lake water is divided into a horizontal flow and a vertical flow. A horizontal flow is chiefly caused by the wind shear stress. As for the research of the flow caused by the wind shear stress, a field observation, a model flume experiment, and a numerical experiment have been conducted. They provided a remarkable advancement from the 1970s to the 1980s. Hereafter, using a new technology such as GPS will be hoped in the flow observation.
A vertical circulation is caused by the density difference of water due to the cooling on the lake surface and the wind shear stress. A vertical circulation causes the mixing of lake water. The mixing of lake water is investigated by observing the water quality distribution, especially there are a lot of researches that consider a vertical mixing making use of the profiles of the dissolved oxygen. In a tropical lake, there is a close relationship between the mixing of lake water and the heat balance on the lake surface, then the depth of vertical mixing is estimated by the amount of cooling on the lake surface. Therefore, it is considered that the clarification of the lake water mixing on a tropical lake contributes to the information about the global warming phenomenon. Moreover, some natural lakes are expected as a monitor of acid precipitation and the micro pollutant.

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Increasing numbers of studies have recently used stable isotopes of nitrogen (δ¹⁵N) and carbon (δ¹³C) to clarify the source of nutrients in lake water and sediments, the structure and energy flow of food webs, the bioaccumulation and biomagnification of hazardous chemicals in aquatic animals, and temporal trends in lake environments. In lake research in particular, the analysis of δ¹⁵N and δ¹³C is a powerful tool in analyzing the structure of the food web and energy flows, as well as the bioaccumulation and biomagnification of toxic chemicals in aquatic animals. Furthermore, the integration of data on food webs, stable isotope ratios, and environmental geochemistry can be useful in evaluating the risks of exposure to environmental contaminants. It is hoped that further research on lake environments using stable isotopic methods can be performed with the aim of conserving water quality and aquatic ecosystems.

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Environmental tracers become a common tool for the groundwater study and a number of methods have been presented in order to understand groundwater flow processes, water budget, origins, chemical reaction processes and retention time. Tracers often used are selected and reviewed for their various methods and advantages as follows; 1) stable ¹⁸O, D in water, 2) stable ¹³C and radioactive ¹⁴C in DIC, 3) noble gases such as He, Ne, Ar, Kr, Xe and their isotopes, 4) radioactive ³⁶Cl in dissolved chloride and some heavier isotopes, and 5) inert gaseous species such as CFCs.
If they are less reactive species, they likely preserve information at the time of recharge or their origin. Use of D, ¹⁸O and the d-value of water is the powerful tool to determine the recharge area because recharged meteoric water have their inherent isotopic ratios correlated with the recharge elevation, distance from the coast, or the local topography. Carbon-bearing species are more reactive though, use of stable isotopes of DIC leads to identify its origin and helps to analyze the chemical reaction between minerals and water or gas addition processes during the groundwater flow in aquifers. Radioactive ¹⁴C has been used to estimate groundwater age however special attention should be paid for, i.e., the origin of DIC, before applying the method. Noble gas tracers are the useful species to presume recharge temperature from their concentrations in water using their temperature dependence of solubilities. Radiogenic ⁴He concentration can be used for the very long-term groundwater dating since the ⁴He is produced in the crust and is accumulated in the deep aquifers, if the local accumulation rate of ⁴He is known. Radioactive ³⁶Cl has been used to determine the age of very old saline waters up to million years. This isotope will also be convenient for the dating of very younger waters, by the use of bomb-produced ³⁶Cl resulted from surface nuclear experiments near the seawater in the 1950s. Chlorofluorocarbons (CFCs) are the gas species produced by the recent human activity and dissolve in water during the recharge, therefore, the affected younger groundwater will have equivalent CFCs concentrations with the atmospheric CFCs concentrations at the time of the recharge. As these species are easy to detect with very high sensitivity, this tracer has now been applied not only for the age determination but for the mixing or contamination of shallow young water to a deep old groundwater. As an individual method listed above is valid only for the very simple flow system, appropriate assumptions or coupling of using different tracers is necessary to understand natural complex groundwater flow system where mixing of groundwaters of different origin or age occurs. Combination of tracers helps us simulating the complex system in detail and is being a growing trend in groundwater study.

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Subsurface thermal regime is affected not only by thermal conduction but also by advection owing to groundwater flow. The effect of thermal advection is especially large in shallow sedimentary layer with active groundwater flow. Subsurface temperature distributions in some basins and plains in Japan are affected by regional groundwater flow systems. Both field observation and simulation results in the study area show that thermal transport conducted by regional groundwater flow system, driven by topographic effect, distorts subsurface temperature distributions. Moreover, subsurface temperature distributions in the Yonezawa Basin and Nagaoka Plain show effect of groundwater pumping. Subsurface temperatures in shallow layers in these areas have seasonal change due to groundwater pumping for snow melting in winter season.
In recent years, subsurface temperature data have been used for reconstruction of ground surface temperature (GST) history as well. The effects of temperature change at the ground surface in the past have conducted into subsurface and have been recorded as transient temperature perturbations onto the background thermal field. Many studies on GST history in North America and Europe are inferred from inversion of borehole temperature profiles in log data.
On the other hand, direct utilization of low-temperature geothermal resources, such as geothermal heat pump system, is important from the viewpoint of reduction of CO₂ that causes global warming. Such systems are gradually increasing in Japan. To evaluate the influence of direct utilization onto subsurface temperature, an areal simulation, which numerical model was cut out from regional 3D groundwater flow and heat transport model of the Sendai plains, was conducted.
Subsurface temperature observed with comparative ease includes important clues to understand global environment problems and/or hints for development of new energy to preserve global environment.
It is hoped that hydrological knowledge contributes to understand many problems in various studies with subsurface temperature.

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Urbanization affects quantitatively and qualitatively groundwater environment in urban area. Especially, groundwater environment in large urban area has been changed more than several decades. However, there are few studies about evaluation of secular change of groundwater environment with developing of urban area. In this study, long term change of groundwater environment in the Kanto plain and the Nobi plain which had the very large urban areas were summarized from the view point of regional groundwater flow.
Present groundwater flows in these plains were significantly different from the groundwater flows in the periods when the effects of urbanization were small, even though more than 30 years have passed since groundwater extraction had been regulated. Also, three dimensional distributions of chemical properties of groundwater quality and environmental isotopes have changed. These results show that the effects of human activities were still strongly remained. On the other hand, demands for groundwater for public use and private use have become increasing in recent years because of recovery of hydraulic potentials of confined aquifers that had depleted from the 1950s to 1970s by huge amount of groundwater abstraction. Therefore, it is important to keep monitoring and evaluating the groundwater environment in these urban areas for sustainable management of groundwater resources.

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Redox potential (Eh) is one of the most important physico-chemical properties of groundwater. Groundwater Eh have been studied in terms of safety of high-level radioactive waste disposals, water resource management and so on. As the results, it is getting clear that redox potential of deep groundwater in sedimentary and crystalline rocks have a remarkable variability. For example, some reported Eh values of deep groundwater decrease with increasing depth, i.e. with residence time. However, there are few reports showing data measured in situ. Only some Eh values of deep groundwater measured from research borehole drilled at crystalline rocks in Japan and Sweden have been reported. Between 200 and 1000m depth, the Eh values of the deep groundwater range from about 0 to -400mV.
This essay first briefly summarizes methodology for Eh measurement and reports Eh values of deep groundwater. Then, it discusses the remaining issues of Eh measurement that are to develop of measurement technology applicable for various geological environments and to understand the redox evolution process through the groundwater flow system.

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In this paper, the author reviews the present situation of basin artificial recharge of ground water (MAR: managed aquifer recharge) as of 2007 in Japan. Most of the artificial recharge of basin method is carried out using alluvial fans. The enhancing groundwater resources in the Rokugo alluvial aquifer has resulted in sustainability for the groundwater environment, especially in the distal fan. As a general judgment, the basin artificial recharge contributes to sustainable aquifer management in alluvium. As a result of this review, the basin artificial recharge will be utilized more in the future, not only in Japan, but in monsoon Asian countries as well.

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