This article summarizes the nature and the behavior of waters in Japan from the viewpoint of physical geography. Because Japan is located in a humid temperate zone lacking a severe dry season, a water deficit is not detected in normal years in terms of water balance analysis. Mean annual specific discharge is between 3 m3/s/100 km2 and 5 m3/s/100 km2 in many river basins, although it is more than 5 m3/s/100 km2 in rainy or snowy regions. Snow is an important source of rivers in Japan. Lakes are relatively abundant. Basin profiles are characterized by steep slopes, reflecting the general landform of Japan. Lakes of volcanic origins and coastal lakes are widely distributed. Strong thermal stratification is also a characteristic of lakes. There are about 210, 000 irrigation ponds, which is one of the special features in Japan. Heavy snowfalls in mountain ranges in northern Japan create perennial snow patches. As summer is very hot, the occurrence of snow patches needs an accumulation of snow of more than 20 m. Groundwater is also abundant, especially at flood plains, alluvial fans, and bases of large volcanoes. On plains where rice paddies are distributed, the groundwater level rises during the irrigation period due to seepage. Groundwater is also influenced by human activities as rivers and lakes.
The Rokugo alluvial fan lies around 39° 25' N and 140° 34' E in northern Japan. The distance between the proximal fan at 90 meters above sea level and the distal fan at 45 meters is about four kilometers. The unconfined aquifer of the fan consists mainly of gravel and sand. Four artificial recharge basins were constructed on the central part of the fan. Observation in struments are wells and piezometers installed in depths of 20 m and 50 m. In this paper, the author discusses the effects of artificial recharge using the basin No.2 at the center of the fan. The recharge experience was done during the period when ground water level was affected by melting snow, from April 8 to May 2, 1998. Results obtained are as follows : first, artificial recharging caused a rise in the hydraulic head, which was related to the Rokugo town area situated on the distal fan, and second, because the author supplied the basin No.2 with water with a low water temperature after snow melted, the characteristic phenomenon of ground water temperature was recorded by piezometers in a depth of 20 m at both sites of central and distal fan. Consideration of this point is a theme for future studies. As a synthetic judgment, the artificial recharge of ground water contributes to sustainable aquifer management in the alluvial fan.
Water is a naturally circulating resource that is constantly recharged. Therefore, even though stocks of water in natural and artificial reservoirs help increase water resources available to human societies, the flow of water should be the main focus of water resources assessments. The climate system puts an upper limit on the circulation rate of available renewable freshwater resources (RFWR). Although current global withdrawals are well below the upper limit, more than two billion people live in highly water-stressed areas because of the uneven distribution of RFWR in time and space. Climate change is expected to accelerate water cycles, and thereby increase the available RFWR. This would slow down the increase of people living under water stress ; however, changes in seasonal patterns and increasing probability of extreme events might offset this effect. Reducing current vulnerability is the first step to prepare for such anticipated changes.
An international water body has territories of more than one nation in its basin. Centuries-long conflicts among basin countries of some international water bodies have been recorded. As water resources become more precious resources for basin countries in the future, conflicts among basin countries are expected to intensify. Such renowned people as Former Secretary General of the United Nations, Mr. Gahli and King Hussein I of Jordan once hinted at the possibility of observing “water wars” in this century. This mainly stems from the fact that water resources are closely connected with the security of nations in the Middle East. Existing conflicts among basin countries in semi-arid and arid regions are likely to intensify. Traditionally, basin countries downstream of an international water body accuse basin countries in upstream areas of abusing shared water resources. While upstream countries seldom compromise with those downstream, traditional upstream-downstream relations are changing, with increasing interactions among basin countries in non-water sectors. This implies that agreements among upstream and downstream countries might be established in some international water bodies, in which relations among basin countries have been “conflictive” in the past with little hope of having an agreement. International organizations are expected to be instrumental in abating or resolving conflicts among basin countries in international water bodies, while international organizations might be more instrumental in elaborating a collaborative mechanism for riparian countries rather than serving as a mediator to solve conflicts among basin countries.
Changes in the components of hydroclimatological characteristics including precipitation, evapotranspiration, and runoff over the last 100 years were investigated in the Kiso, Nagara, Ibi, and Kumozu River Basins as a case study. Annual precipitation in the study basin has tended to decrease since the first half of the 1970s. On the other hand, the smooth trend curve indicates that annual evapotranspiration has increased over the long term, especially since the 1980s. Smoothed secular changes in the difference between annual values of precipitation and potential evapotranspiration are analogous to those of annual precipitation. The average annual runoff ratio for each year has also shown a tendency to decrease during the last few decades. It is pointed out that the notable characteristics of current frequency-magnitude distributions in anrange in each year. Under conditions of extreme meteorological events that increased air temperature in the summer of 1994, river water quality showed a remarkable change compared to its average value in a normal year. The most striking feature due to high temperature was a very low value for the concentration of dissolved oxygen, especially in July and August. The facts identified in the present study provide a meaningful perspective of the possible consequences of global warming for hydrological processes, and are also useful basic data for evaluating the effects of future climate change on the aquatic environment.
The results of hydro-environment researches into marine and lacustrine sediments revealed climatic variations which seem to have some relationships between turns in world history and climatic changes. In the Japan Sea and northeast Pacific off Kashima from the Late Glacial to the Holocene, diatom temperature, Td' = (frequency of warm-water species) - (frequency of warm-and cold-water species) -1?100, values show rhythmic fluctuations with durations of 1 kyr and 400-500 yrs (Koizumi et al., 2004, 2006). This indicates a strong and regular inflow of the warm Tsushima Current into the Japan Sea as a branch of the warm Kuroshio Current. The decreases of Td' values correspond to a period of climatic deterioration under which cultural changes occurred in Japan. The paleoclimatic variations and the triple events of high 14C in the atmospheric residual 14C record denoted by Stuiver et al. (1991) are recognized in the diatom variations. A similar periodicity is also recognized in North Atlantic drift ice (Bond et al., 2001), concentrations of sea salt and terrestrial dust in the GISP2 ice core (O'Brien et al., 1995), in lake deposits from the Jura in France (Magny, 1995), Tibet (Gasse et al., 1991; Ji et al., 2005), and Alaska (Hu et al., 2003), and oxygen isotope variations from stalagmites in southern China (Wang et al., 2005) and Oman (Neff et al., 2001). Climatic deteriorations are synchronous with periods when serious changes occurred in human intellectual achievements during the Holocene. The Agricultural Revolution (11, 000-10, 000yr BC), with the cultivation of wild plants and domestication of wild animals to produce and secure food, seems to coincide with the Younger Dryas event. Increased agricultural production made it possible to afford a population not engaged in agriculture, for example, people engaged in handicrafts, festivals, and politics. This seems to occur in the period coinciding with T2 event in the T1-T4 triplets, which have Maunder-and Sporer-type patterns occurring in sets of three, denoted by Stuiver and Braziunas (1993). The religious courtesy belief in the recycling and the reinforcing of life was converted into a rational and systematic consideration of life based on universal theory. This conversion of thinking seems to coincide with the T3 event. Modern science was established in Europe in the 17th century. And now human beings are destroying natural environments using highly developed scientific technologies of their own. This is a negative revolution that human beings have never previously experienced. In the south-to-north transect of the cores from both the Pacific and Japan Sea sides of Honshu, Td' values indicate that the Kuroshio Current and Tsushima Warm Current are both weakening at present. The present interglacial period might end and become a cooler climatic condition like a Little Ice Age within 300 years. The insolation at 37°N among the Earth's orbital parameters is weak at present (Koizumi and Ikeda, 1997). However, the mean global temperature has risen about 0.5°C during the last century. Warming of the Earth, which is caused mainly by carbon dioxide in the atmosphere, has made natural environments unstable.
Liquid water on the surface of the Earth might have frozen entirely at least 3 times during the history of the Earth (650 Ma, 700 Ma, and 2.2 Ga). Assuming such extreme conditions, the snowball Earth hypothesis explains several unusual geological features associated with glacial deposits in the Proterozoic glaciations. Life should, however, have faced serious crises during these glaciations because liquid water is necessary for life. In particular, survival of photosynthetic algae, which are supposed to have appeared before the Neoproterozoic glaciations, might have been difficult if the surface water froze completely. There would have been refugia for life during the global glaciations. Life could have survived if the equatorial ocean was not completely frozen (soft-snowball condition), or equatorial sea ice might have been very thin (on the order of 10 meters). Even if these conditions were not achieved, life could have survived in shallow hot springs around volcanic islands. It would be much more difficult for eumetazoa to survive such severe conditions if they appeared before the Neoproterozoic glaciations as suggested by molecular clock studies. The appearance of eumetazoa after the last global glaciation (Marinoan glaciation), as suggested by the paleontological record, however, avoids this problem.
The redox state of the surface environment of early Earth is still controversial (e.g. Ohmoto, 1997). Many previous papers suggest that oxygen was free before 2.7 Ga, and then gradually increased due to oxygen-producing photosynthesis (e.g. Holland, 1999; Farquhar et al., 2000). But, a detailed and quantitative estimate is still lacking. It is well known that deposited carbonate minerals are equilibrated with ambient seawater in a microbial or abiotic environment. The composition and mineralogy allow us to estimate the physical and chemical properties of pale minerals with primary sedimentary structures in shallow and deep-sea deposits, in order to eliminate secondary carbonate and contamination of detrital materials, and to estimate the redox condition of seawater over time. We estimated the depositional environments from the field occurrence of coexisting basaltic lava and sedimentary rocks and the fabric of the carbonates themselves. The shallow marine deposits have included sedimentary carbonates with a stromatolite structure and elastic layers, and amygdaloidal and matrix carbonates of hot-spot basaltic lava since 3.5 Ga. Deep-sea carbonates have included interstitial carbonate minerals in a matrix of hyaloclastite and amygdaloidal carbonate minerals accompanied by MORB-type basalts since 3.5 Ga. In addition, we excavated at three localities in South China, and obtained the complete sequence from the Marinoan tillite to early Cambrian rocks. The carbonate rocks belong to shallow marine deposits. Deep-sea carbonates have only faint Ce and Eu anomalies between 3.5 and 1.9 Ga. The negative Ce anomaly of shallow carbonates has frequently deviated from those of deep-sea carbonate since 2.78 Ga. It fluctuated greatly, and was very large at 2.5 and 2.3 Ga. We calculated the oxygen activity of shallow and deep seawater respectively, based on Ce content and anomalies of carbonate minerals at given parameters of atmospheric carbon dioxide content (pCO2) and Ca. content of seawater. The results show that the oxygen content of the deep sea was low and constant until at least 1.9 Ga. The oxygen content of shallow seawater increased from 2.7 Ga, but fluctuated. In particular, it was at a minimum during and after Snowball Earth events. It became quite high at 2.5 and 2.3 Ga, but eventually increased after the Phanerozoic. We also calculated it under another condition of high pCO2 to show that the seawater was more oxic even in the Archean than at present. The calculation suggests a relatively low pCO2 through geologic time.
Water affects many geological and geophysical phenomena, for example, earthquake generation. Recently, anomalous earthquakes, which are strongly related to fluid have been detected in a subduction zone by densely distributed geodetic and seismic observation networks. These are called slow earthquakes and are divided into many categories of earthquakes. The long-term slow slip in Tokai or Bungo Channel, which occurs at the subducting plate interface, is a phenomenon with a very long time-constant ranging from months to years. At the deeper part of the long-term slow slip, the short-term slow slip occurs with a period of several days associated with the non-volcanic deep low-frequency tremor in the transition zone on the plate interface in southwest Japan. These slow earthquakes might be related to fluid liberated from the down-going slab by dehydration process. At the shallower part of the subducting plate interface, the very low-frequency earthquakes occur in the accretionary prism near the Nankai trough. These slow earthquakes indicate a weakening of frictional strength at the plate interface and low stress drop due to the existence of fluid.
Water has been continuously degassed from the Earth's interior by magmatism throughout evolution, and can significantly affect dynamic processes of its carrier, i.e., magmas, during their transport from the mantle to the Earth's surface. This paper summarizes the effects of water on the physical and thermodynamic properties of magmas, and their roles in magmatic processes. Magmas commonly contain at least 0.2 wt.% of water, and some magmas can have up to 6 wt. %. Despite the fact that water is a minor component in silicate liquids, the effects of dis-solved water on the properties of silicate melt are significant because it has a much lower molecular weight at 18.0 than those of the other components (SiO2. 60.1, for example). Dissolved water greatly affects the density and the viscosity of silicate melts, thereby controlling rates of dynamic processes of magmas, such as segregation of primary melts in the mantle, transport of magmas from the mantle to the crust, convections and crystal-melt separation in crustal magma chambers, and ascent of magmas in volcanic conduits. Water also influences solid-melt thermodynamic equilibrium relationships, and this affects the chemical differentiation paths of magmas, in addition to the amount of melt production in the mantle by changing solidus temperatures. The eruptive behavior of volcanoes is driven by the exsolution of magmatic water, and as such depends on the water solubility of magmas mainly as a function of pressure. Water has also played important roles in the evolution of the Earth. Magma generation has been induced by water in the Earth's interior, and magmas have carried materials and energy from the interior to the surface of the Earth. In particular, water transport beneath an island arc is important in the global water cycle, and has greatly affected the environment of the Earth's surface.
Using recent results from high-pressure experiments and thermodynamic calculations, phase diagrams of simplified hydrous peridotite in the system MgO-SiO2-H2O have been constructed to lower mantle conditions. Possible water subduction and circulation processes in the deep mantle are discussed on the basis of the constructed phase relations. For water transportation by subduction of peridotites, important phase relations are : (1) of antigorite (serpentine) at lower pressures (less than 10 GPa), and (2) of seven different high-pressure hydrous phases at higher pressures. In cold subducting slabs, water in antigorite is partially transferred to the hydrous phase A at depths greater than 160 km. With further subduction, water in phase A may be transported to the bottom of the transition zone via solid-solid reactions among seven high-pressure hydrous phases. If the slab temperature at 30 GPa is lower than 1000°C, hydrous phase D will carry water into the deep lower mantle. Along the cold slab geotherm, large fluid fluxes are predicted at shallow (300-km depth) and deep (700-km depth) levels, depending on the slab temperature. The depth distributions of dehydration reactions suggest that observed subduction zone seismicity could be related to dehydration reactions in the slab. The lower water activities in the fluid phases at deep mantle conditions imply that such fluid phases could dissolve significant amounts of silicate components. Therefore, fluid phases released by dehydration reactions at deeper levels should have different physical properties from those at shallower levels.
To understand the transportation and the circulation of water in subduction zones and the mantle, the maximum H2O content of rocks under variable pressure-temperature conditions was reviewed. The maximum H2O content is also important for assessing the potential storage of H2O in the mantle, which shows that the mantle can contain 4.6 to 12.5 times more H2O than the current ocean mass, assuming a standard geothermal gradient. Although the estimation has a large degree of uncertainty, the mantle is a significant storage, and plays an important role in global water circulation, especially due to the unexpectedly high capacities of nominally anhydrous minerals (NAMs). Based on this information, water circulation in subduction zones (especially in the Japanarcs) and the mantle has been discussed with the relevant numerical modeling and geophysical and geological observations. Consequently, the following points have been clarified : (1) breakdown depths of the major hydrous minerals within and around the subducting slab depend on the thermal structure (deeper for colder environments); (2) when a hot slab, including a spreading mid-ocean ridge, subducts, both water and heat are supplied to the forearc region, contributing to the formation of regional metamorphic belts; (3) both high-pressure-type and high-temperature-type metamorphism occur within a limited time in a single forearc domain associated with ridge subduction ; (4) arc magmatism and regional metamorphism are regarded as a series of geological events corresponding to different stages of a temporal change in the thermal state of a subduction zone; (5) although major dehydration of subducting slabs occurs at depths shallower than 200 km, triggering arc magmatism or regional metamorphism, the subducting slab and the bottom portion of the overlying mantle wedge above the slab transport several 100 to several 1000 ppm of H2O with NAMs to the transition zone ; (6) the influx of H2O into the transition zone by this mechanism is comparable to or exceeds outgassing by magmatism at mid-ocean ridges and hotspots; (7) the influx was suppressed in the past where the potential temperature was high (hot-dry regime), while it will be enhanced in the future as the Earth cools (cold-wet regime), resulting in stabilization and prolongation of mantle convection; and, (8) considering the presence of very cold subduction zones such as central Japan, where a significant amount of water is exceptionally transported to the deep mantle, the present-day Earth is probably in a transition from a hot-dry regime to a cold-wet regime. In any case, the maximum H2O content in NAMs in the upper mantle controls the influx, which needs to be constrained tightly for more accurate estimations of global water circulation.
Because of the dissolution of lighter elements such as sulfur and carbon, the Earth's outer core is about 10 percent less dense than molten iron at the relevant pressure and temperature conditions. To determine whether hydrogen can account for a major part of the density deficit, and is therefore an important constituent of the molten iron outer core, the hydrogen concentration in molten iron has been measured using a specially developed method. From these measurements, metal-magma partitioning of hydrogen in the Earth's primordial magma ocean was determined as a function of temperature and pressure where the reaction occurred. It was found. that if the pressure at the bottom of the magma ocean was above 7 to 8 GPa, most of the H2O absorbed in this ocean should have reacted with the iron to be transported into the core. This seems almost inevitable after the giant impact that formed the moon, although details of the metal-magma reaction process after the impact are not well constrained.
Although the mass of oceans on the Earth is a tiny fraction (0.023 wt%) of the bulk Earth mass, the existence of oceans is sufficient to distinguish the Earth from the other planets in the solar system. Water is essential for the origin and evolution of life, the stability of the surface environment, and the evolution of the planetary interior. At present, we have no definite answers to fundamental questions about the origins of water on the Earth. The process by which water was supplied to the Earth is thought to be closely related to the dynamics of the solar system. Because of several uncertainties about the planet formation process (the position of the snow line in the solar nebula, the lifetime of the nebula gas, the timing of the formation of Jupiter, etc.), the origins of the Earth's oceans are still much in question. In this paper, we describe several possible sources of the Earth's water in the solar system and possible processes by which water was supplied to the Earth. Within the context of the modern theory of planetary formation, we discuss several scenarios for the origins of the Earth's oceans.