Japan Basin Research Articles

Temporal variations in metal enrichment in suspended particulate matter during rainfall events in a rural stream

We studied enrichment of heavy metals (V, Zn, Cr, Ni, Cu, Pb, As, Sb, and Cd) in a rural stream of the Kuji River basin in central Japan in suspended particulate matter, and associated transport flux during two rainfall events (in November 2003 and in April 2008). The concentration of heavy metals in suspended particulate matter (SPM) exhibited a distinctive temporal variation, wherein the concentrations decreased with increasing water discharge and then increased as the discharge decreased. Concentration of dissolved metal forms showed a slight increase with scatters around the flow rate peak. Enrichment factors for those metals in the SPM decreased sharply as the flow rate increased, making an obvious concave shaped curve (the November 2003 rainfall event). The metal enrichment factors under low flow conditions had a similarity to those found in atmospheric deposits at a foot of the Kuji River basin, suggesting atmospheric source would contribute to enriching the SPM with those metals in part. Mineralogical analyses and carbon content analysis (the April 2008 rainfall event) of the SPM suggests the SPM matrix became more lithological as the flow rate increased. The changes observed in the matrix are thought to be directly related to progressive changes in metal enrichment within the SPM. Concerning the transport phase of several heavy metals, a dynamic change in transport phase partitioning within a rainfall event was suggested. The present enrichment factor study and the SPM matrix characterization implied the partitioning change is due to an increase in lithologic solids during high flow conditions.

Impact analysis of stochastic inflow prediction with reliability and discrimination indices on long-term reservoir operation

Long-term stochastic inflow predictions can potentially improve decision making for reservoir operations. However, they are still not widely incorporated into actual reservoir management. One of the reasons may be that impacts of various types of uncertainty contained in stochastic inflow predictions have not been sufficiently clarified, thus enabling reservoir managers to recognize the advantages of their use. Impacts of uncertainties of stochastic inflow prediction on long-term reservoir operation for drought management are therefore investigated in order to analyze the kind of uncertainty that most affects improvements in the performance of reservoir operations. Two indices, namely reliability and discrimination, are introduced here to represent two major attributes of a stochastic prediction's uncertainty. Monte Carlo simulations of reservoir operations for water supply are conducted, coupling with optimization process of reservoir operations by stochastic dynamic programming (SDP) considering long-term stochastic inflow predictions, which are artificially generated with arbitrary uncertainties controlled by changing the two uncertainty indices. A case study was conducted using a simplified reservoir basin of which data were derived from the Sameura Reservoir basin in Japan with finer discretization settings for SDP. The results demonstrated the additional implication of the effect of stochastic inflow prediction's uncertainty on the authors’ previous work.

Assessment of climate change impacts on river discharge in Japan using the super‐high‐resolution MRI‐AGCM

AbstractTo evaluate the impacts of climate change on river discharge, we applied a hydrological simulation to several major river basins in Japan, using a super‐high‐resolution atmospheric general circulation model (AGCM) with a horizontal resolution of about 20 km that was developed by the Meteorological Research Institute (MRI) of the Japan Meteorological Agency. For our analysis, we used two AGCM datasets corresponding to the current climate (1980–1999) and future climate (2080–2099) based on the sea surface temperature ensemble projected by the Coupled Model Intercomparison Project phase 3, assuming the A1B scenario of the Intergovernmental Panel on Climate Change Special Report on Emission Scenarios. River discharge was estimated using a distributed hydrological model that was calibrated in advance with observed river discharge. We investigated the impact of climate change on river discharge by comparing hydrological simulations of current and future climate conditions. The results showed that the latest version of the AGCM (MRI‐AGCM3.2S) performs better hydrological simulations than the previous version (MRI‐AGCM3.1S). Even if the amount of precipitation does not change greatly in the future, river discharges will change significantly as air temperature rises, owing to increased rainfall, decreased snowmelt, and increased evapotranspiration. The impact of climate change on river discharge will become more significant in northern Japan, especially in the Tohoku and Hokuriku regions. Our results imply that the increase in air temperature will have important consequences on the hydrological cycle, particularly in regions where the water supply is currently domi\nated by snowmelt. Copyright © 2013 John Wiley & Sons, Ltd.

Evaluation of lead concentration by one-box type multimedia model in Lake Biwa-Yodo River basin of Japan

A one box type multimedia model was developed and applied for Lake Biwa-Yodo River basin in Japan to assess the distribution of lead in the environment. This model is based on mass balance and includes four environmental media; the atmosphere, the soil, the water body, and the sediment. The mass balance of lead is represented by the summation of mass transfer flux at equilibrium, emission flux, advection flux, and deposition flux or sedimentation flux. In the case of metallic compounds, dissolution rate and exchange equilibrium have also been taken into consideration. Pollutant Release and Transfer Registry (PRTR) in Japan was used as one of the major data source for this study. The emission of lead in Lake Biwa-Yodo River basin is calculated based on five sources of registered emission in PRTR, unregistered emission in PRTR, incinerators, leaded gasoline, and landfills. In this study, we estimated lead emission from 1957 to 2007 to observe the temporal accumulation of lead. Calculated lead concentrations were compared with the measured/observed concentrations. It was found out that the model could closely predict lead concentration in the soil and the water body. The concentration in the atmosphere was underestimated by the calculated concentrations. The reason was attributed to the underestimation of the amount of lead emission from incinerators.

Right-lateral strike-slip movement of the South Korea Plateau associated with the opening of the East Sea (Sea of Japan)

Multi-channel seismic profiles and swath bathymetric data were used to investigate the tectonic evolution of the South Korea Plateau (SKP) associated with the opening of the East Sea (Japan Sea). The SKP is a deformed fragment of continental crust with numerous horsts and sediment-filled grabens. Three sedimentary units in the plateau were identified, which consist of the lower sequence (Unit I) interpreted as syn-rifting deposition during the early to middle Miocene, and the middle and upper sequences (Units II and III) considered as post-rifting deposition since the late Miocene. The fault system in the SKP includes the South Korea Plateau Fault (SKPF) trending NNW-SSE and smaller en echelon normal faults oriented NE-SW. We interpreted the information to postulate that the formation of the SKPF is the result of divergent right-lateral strike-slip movement in the SKP. This study suggests that the dextral movement of the SKP was induced by WSW-ward propagation of the spreading center located in the Japan Basin from the early to middle Miocene times.

Spatial Distribution of Radiocarbon in the Southwestern Japan/East Sea Immediately After Bottom Water Renewal

In April 2001, immediately after bottom water renewal in the Japan/East Sea, 7 vertical profiles of radiocarbon were obtained from the area, including the Japan Basin and the Tsushima/Ulleung Basin. The bottom water Δ14C of the Tsushima/Ulleung Basin increased by 24% during 1979–2001 and was 10–15% higher than the values reported for the Japan Basin and Yamato Basin during 1995–2002. Within the depth interval 800–2000 m, Δ14C values at the southernmost station in the Japan Basin were much higher than at other stations, with the maximum difference in Δ14C values being more than 50%. Based on a combination of physical observations with analysis of the Δ14C data, we concluded that the bottom water, which was formed by the sinking of surface water off Vladivostok, Russia, in late January-early February of 2001, had been widely distributed to depths below 2000 m within the southernmost Japan Basin in a timespan shorter than 2 months. However, there was no evidence that the water had been transported into the Tsushima/Ulleung Basin during the same time interval.

Statistical analysis and estimation of annual suspended sediments of major rivers in Japan

We evaluate the spatiotemporal trends of recent suspended sediment conditions in Japanese rivers. Statistical and spatiotemporal trend analysis is conducted on the 92 major rivers in Japan based on water quality monitoring data from 1992 to 2005. The Mann-Kendall non-parametric method was used to investigate the spatial and temporal trends for the suspended sediment indicator. Results show that the mean concentration of suspended sediments in Japanese rivers has generally declined in recent years, although there are still water quality problems at some monitoring sites (Kanto, Chubu, Kinki and Kyushu regions). A positive relationship between observed yearly discharge and suspended sediment load was found. Land use maps with 100 meter spatial resolution were used to apply an empirical model and develop a regression model for estimating annual suspended sediment loads directly from land use and hydrologic data. Rivers were assigned to three groups according to statistical cluster analysis of suspended sediment (SS) concentration. The correlation between the simulation result from the empirical model and the observed data had R(2) values of 0.62 and 0.71 for groups 2 and 3, and the correlation between the simulation result from the regression model and the observed data had R(2) values of 0.48 and 0.34 for groups 2 and 3. Results show that the proposed simulation technique can be used to predict the pollutant loads to river basins in Japan. Results also suggest prioritization methods and strategies that policy-makers can use to address suspended sediment pollution in rivers and water quality management in general.

日本海東縁海底下に分布する砂層の孔隙特性と年代 : MD179航海

In 2010 the MD179 project was undertaken aiming at recovery of deep seated gas hydrate, methane induced carbonate, and deep sediments older than 300 ka. Sediment samples were obtained in the Umitaka Spur, Joetsu Channel, Toyama Trough, Japan Basin, Nishi Tsugaru and Okushiri Ridge areas. Small amounts of sandy sediment have been retrieved as thin intercalations in Pleistocene and Holocene muddy layers, where trace fossils and strong bioturbations are commonly observed.Those sandy sediments consist of very fine- to fine-grained sand grains, and are sometimes tuffaceous. Pore-size distribution measurements and thin-section observations of these sands were undertaken, which indicated that porosities of muddy sediments are around 50 % but those of arenites range from 42 to 52%. Mean pore sizes and permeabilities of those arenites are larger than those of mudstones. While the presence of gas hydrate in intergranular pores of sands has not been confirmed, the soupy occurrence in recovered sediments may strongly indicate the presence of hydrate filling the intergranular pore system of sands. Such arenites have been recognized till now in the Mallik, NW Territory Canada, as well as in the Nankai Trough areas, which are expected common even in the subsurface sandy sediments at the eastern margin of Japan Sea. Concentration of gas hydrate may need primary intergranular pores large enough for gas hydrate to occur within host sediments likely deposited in the sedimentary environment such as deep sea channels. Small amounts of sandy sediment were retrieved as thin intercalations in Pleistocene silty layers, because supplying sediments may not be abundant due to sea level fluctuation during Pleistocene ice age. As time of deposition of coarse-grained sediments can be recognized by the thermoluminescence (TL) dating method, sandy sediments are usually tuffaceous and contain a small amount of quartz grains, and TL dating has been completed only for seven samples, which indicate 48 to 980 ka in age. This study was performed as a part of the MH21 Research Consortium.

Cenozoic volcanism of the eastern Sikhote Alin: Petrological studies and outlooks

Based on geological and isotope geochemical data obtained during the past decade, the eastern Sikhote Alin volcanic belt can be considered as a polygenic structure with spatially superimposed magmatic complexes of different geodynamic stages. Only Late Cretaceous intermediate and silicic volcanics enriched in LILE and depleted in HFSE can be interpreted as typical subduction complexes. Cenozoic lavas of mainly basic composition were formed after the termination of active subduction under complex dynamic conditions of the rearrangement of eastern Eurasia owing to the collision with the Indian plate. The eruption of Eocene-Oligocene-early Miocene basalts corresponded to the transform continental margin environment, rupture of an ancient subducted slab, and upwelling of hot depleted oceanic asthenosphere of the Pacific MORB-type into the Asian subcontinental lithosphere with EMII-like isotopic characteristics. The late Miocene-Pliocene magmatic activity of the eastern Sikhote Alin showed an intraplate character, but the composition of erupted magmas was strongly affected by previous tectonomagmatic events: subduction of different ages and opening of the Sea of Japan Basin. The distinct EMI isotopic signature of low-potassium plateau basalts, which is not observed in the lavas of earlier stages of volcanic belt evolution, suggests that the continental asthenosphere contributed to magma formation, and the direction of mantle flows changed owing to the formation of a new subduction zone.

동해 전역에 장기간 발달하는 아표층 엽록소 최대층과 수괴의 물리 화학적 특성과의 상관관계

하계 울릉분지에서 관측된 SCM층은 울릉분지에만 국한되어 나타나는 것이 아니라 동해 전역에서 관측되었다. 각기 다른 시기에 관측된 여러 문헌 자료와 하계 동해 전역에서 관측된 자료에 의하면 SCM층이 나타나는 시기는 5월부터 10월 하순까지 나타나는 것으로 추정된다. SCM층 바로 상층부에 표층보다 높은 용존산소가 관측되었고, SCM층이 영양염의 농도가 급격하게 증가하는 수심에 나타나는 것으로 보아 침강이나 낮은 광량에 대한 식물플랑크톤의 생리적인 적응보다는 식물플랑크톤이 활발하게 성장하고 있음 시사한다. SCM층에서 식물플랑크톤이 오랜기간 동안 지속적으로 성장하기 위한 영양염 공급과정으로 수직 확산, 소형 및 중형동물플랑크톤의 재생산등이 중요한 역할을 하는 것으로 알려져 있다. 이들 과정은 SCM층에서 식물플랑크톤 질소 요구량의 약 50% 정도만 설명할 수 있어 SCM층의 질소 요구량에 대한 추가적인 공급기작으로는 난수성 소용돌이가 제시되었다. 그러나 난수성 소용돌이가 발달하지 않은 해역과 시기에 관측된 SCM층에 영양염은 고온 저염의 특성을 가지면서 영양염이 고갈된 표층수 아래에서 대한해협을 통해서 유입되는 영양염이 비교적 풍부한 해수가 울릉분지내로 유입되어 북상하면서 유광층까지 직접으로 상승하거나 수평이동 응력의 차이로 발생하는 난류혼합등에 의해서 추가적으로 공급되는 것으로 사료된다(Fig. 17). 그리고 동해 전역에서 5월에서 10월까지 존재하는 SCM층은 표층에서 심층으로의 유기물 수송에 상당히 기여할 것으로 사료된다. To understand the scales of the spatial distribution and temporal duration of the subsurface chlorophyll-a maximum (SCM) observed in the Ulleung Basin of the East Sea, we analyzed physical and chemical data collected during the East Asian Seas Time-series-I (EAST-I) program. The SCM layer occurred at several observation lines from the Korea Strait to <TEX>$37.9^{\circ}N$</TEX> in the Ulleung Basin during August of 2008 and 2011. At each observation line, the SCM layer extended from the coast to about 200 km off the coast. The SCM layer was observed between 30 and 40 m depth in the Ulleung Basin as well as in the northwestern Japan Basin along <TEX>$132.3^{\circ}E$</TEX> from <TEX>$38^{\circ}N$</TEX> to <TEX>$42.3^{\circ}N$</TEX> during July 2009, and was observed around 50 m depth in the northeastern Japan Basin (<TEX>$135-140^{\circ}E$</TEX> and <TEX>$40-45^{\circ}N$</TEX>) during July 2010. From these observed features, we hypothesize that the SCM layer observed in the Ulleung Basin may exist in most of the East Sea and may last for at least half-year (from the early May to late October). The nutrient supply mechanism for prolonged the SCM layer in the East Sea was not known, but it may be closely related to the horizontal advection of the nutrient rich and low oxygen waters observed in the Korea Strait between a 50 m depth to near the bottom. The prolonged development of the SCM layer in the Ulleung Basin may result in high primary production and would also be responsible for the high organic carbon content observed in the surface sediment of the region.

Sustainable Use of Regional Wood Biomass in Kushida River Basin, Japan

The amount and cost of wood biomass produced from the Kushida River basin in Japan were simulated under current and modified forestry scenarios by combining the forest ecosystem and cost calculation models at semi-compartment level; in addition, an analysis was carried out comparing the sustainable supply against heat demands in the basin. The forest ecosystem model (BGC-ES) simulates biomass growth, the effects of thinning and clear-cutting practices, and the amount of harvest from these practices. The cost calculation model estimates the cost of each stage of wood biomass production, which depends on the productivity of forestry machines and the forest location. The available amount was determined by an economic comparison between the energy density of wood chips and fossil fuel. The available wood chips were assumed to be converted to heat at a boiler plant. The current forestry scenario (thinning schedule, rotation length, and managed area) was not efficient for the production of wood chips. The economic available amount was <1 % of the potential production from the basin forests. Modified scenarios (arranging the thinning schedule to promote secondary growth, increasing the rotation length, and increasing the managed area) improved the economic availability of the basin forest (up to about 40 %). In these scenarios, the available amount indicated that saturation was determined by the maximum cost of the wood chips. The available amount of wood chip production that can be sustainably supplied by the basin forest was simulated and compared with the household and agricultural demands for heat in the basin.

Deep currents obtained from Argo float trajectories in the Japan/East Sea

The deep currents at 700–800m depths from Argo float trajectories (including pre-Argo floats) have been compiled from 1999 to 2010 in the Japan/East Sea (JES). More than 14,000 velocities from the dataset provides the first view of interbasin-scale dependency in deep currents of the JES, such as mean circulation and eddy variability, especially with an emphasis on their topographic steering characteristics. The observational findings in this study are (1) The Argo observation exhibits dominance of cyclonic mean circulation in the entire JES yet with a distinct interbasin variability of its magnitude. The mean circulation is strongest in the eastern Japan Basin (JB), while overall currents in the Ulleung Basin (UB) are weaker than that in the eastern Japan Basin and western Yamato Basin (YB) by a factor of two to three. (2) The eddy kinetic energy (EKE) is maximum in the western Japan Basin where strong dipole structure of wind-stress curl in winter is located. The interior of each basin (the eastern JB, UB, and YB) has local maxima of the EKE a factor of four to five smaller than that in the western JB. (3) Cyclonic bottom following characteristics prevail in the bin-averaged velocities. However, the individual deep flows presumably containing temporal variability have a preference to follow topography not only with cyclonic flows (shallow depth to the right), but also anticyclonic flows (shallow depth to the left), especially when flows are highly energetic. Occurrence of anticyclonic flows is proportional to the local EKE level relative to mean kinetic energy, implying that such flows could exist only in a transient manner such as topographic Rossby waves. The Argo observations may demonstrate an asymmetry of stable cyclonic and unstable anticyclonic circulation in the marginal seas of the northern hemisphere as the previous theoretical modeling studies suggested.

Distributional Change and Epidemic Introgression in Overlapping Areas of Japanese Pond Frog Species Over 30 Years

Pelophylax nigromaculatus, P. porosus porosus, and P. p. brevipoda are three pond frog species distributed in Japan. Their distributions overlap at two basins in central Japan (P. nigromaculatus and P. p. porosus in the Matsumoto basin, and P. nigromaculatus and P. p. brevipoda in the Ina basin), and hybrid descendants have been found in these areas. To clarify the distribution areas and hybrid zones of the frogs, and to understand the mode of introgressive hybridization and its impact on the frog populations, we conducted exhaustive sampling at each basin and performed allozyme and mtDNA analyses of 233 individuals. Analysis using genetic markers clearly detected nine F1 hybrids and 94 hybrid descendants of P. nigromaculatus and P. porosus from the overlapping areas of both basins. Allozyme and mtDNA data suggest directional hybridization between female P. p. porosus and male P. nigromaculatus in the Matsumoto basin. Over the past 30 years, the distribution of P. p. porosus has been narrowed and fragmented by the invasion of P. nigromaculatus, seemingly because of directional hybridization in the Matsumoto basin. In the Ina basin, the "pure" P. p. brevipoda (n = 8) population was extremely reduced by gene introgression from P. nigromaculatus, yet its distribution was barely changed compared to the Matsumoto basin. Consequently, this study shows that P. porosus populations are threatened by interspecific hybridization with P. nigromaculatus, and that introgressive hybridization damaged P. porosus populations by different means in each basin.

Quantitative evaluation of effects of the soft interventions or cleaner production in households and the hard interventions: A Social Experiment Programme in a large river basin in Japan

Efforts have been made to reduce municipal wastewater pollutant discharge and improve river water quality in the Yamato-gawa River basin since 2005, as part of a Social Experiment Programme. The Programme was the first one in Japan to disseminate soft interventions in households, measurements to reduce pollutant discharge, in a large river basin with more than two million population. The soft interventions in households are similar to cleaner production in industrial sector because they are not the wastewater treatment and changing of lifestyles. This paper presents several methods for quantification of the pollutant discharge reduction. Two hypothetical soft intervention combinations were defined, the effects of 16 soft interventions were estimated using their proliferation rates, and estimated pollutant discharge reduction was compared with changes in river water quality during the Programme events. For the first method, environmental accounting housekeeping (EAH) books were applied to estimate a 38–53% reduction in biological oxygen demand (BOD), 26–40% reduction in total nitrogen (TN), and 21–32% reduction in total phosphorus (TP). These estimation results were comparable with the field survey results of approximately 100 person population, which were conducted in the late 1980s. Soft interventions were found to be several times more cost effective than hard interventions, such as development of municipal wastewater treatment and river water purification systems. The BOD discharge reduction effects of soft interventions initiated during Programme events increased from 0.3–2.3% in 2005–2007 to 2.4–4.7% in 2007–2009. The questionnaire surveys found that participants in the Programme events in average newly implemented three or four soft interventions in addition to those already implemented at normal times.

Genogroup distribution of F-specific coliphages in wastewater and river water in the Kofu basin in Japan

To determine the genogroup distribution of F-specific coliphages in aquatic environments using the plaque isolation procedure combined with genogroup-specific real-time PCR. Thirty water samples were collected from a wastewater treatment plant and a river in the Kofu basin in Japan on fine weather days. F-specific coliphages were detected in all tested samples, 187 (82%) of 227 phage plaques isolated were classified into one of the 4 F-specific RNA (F-RNA) coliphage genogroups and 24 (11%) plaques were F-specific DNA coliphages. Human genogroups II and III F-RNA coliphages were more abundant in raw sewage than animal genogroups I and IV, excluding one sample that was suspected to be heavily contaminated with sporadic heavy animal faeces. The secondary-treated sewage samples were highly contaminated with genogroup I F-RNA coliphages, probably because of different behaviours among the coliphage genogroups during wastewater treatment. The river water samples were expected to be mainly contaminated with human faeces, independent of rainfall effects. A wide range of F-specific coliphage genogroups were successfully identified in wastewater and river water samples. Our results clearly show the usefulness of the genogroup-specific real-time PCR for determining the genogroups of F-specific coliphages present in aquatic environments.

Analysis of topography and vegetation distribution using a digital elevation model: case study of a snowy mountain basin in northeastern Japan

This study examines the relations between the local variations in vegetation and topography using logistic regression (LR) and GIS in a snowy basin in the Ohu Mountains of northeastern Japan. The spatial distribution of seven vegetation classes interpreted from aerial photographs—(a) large-sparse crown beech forest, (b) middle-dense crown beech forest, (c) dwarf beech scrub, (d) dwarf bamboo thicket, (e) Japanese white pine forest, (f) snow-avalanche scrub or meadow, and (g) riparian forest—was analyzed with terrain parameters derived from a 10 m digital elevation model (10 m DEM) and parameters of landslide distribution and surface geology. While large-sparse crown beech forests dominate the slopes, smaller crown beech forests or scrubs are often found on the west-facing (i.e., windward side) upper parts of slopes and crests. On the contrary, snow-avalanche scrubs or meadows are found on the leeward side of steep slopes and concave plan curvatures. Dwarf bamboo thickets are often found on east-facing (i.e., leeward side) gently sloping crests, where the snow remains until summer. Japanese white pine forests respond positively to sharp ridges, whereas riparian forests show a positive response to the gently sloping lower parts of slopes and bottomlands at low elevation. The heterogeneous vegetation distribution in the basin indicates differences in site conditions (e.g., soil–water conditions), and the existence of various disturbance regimes induced by the strong winter monsoon with heavy snowfall, snow movements (e.g., avalanches), and temporal differences in snowmelt on different topographies. The estimated vegetation maps could be used for forest management and restoration.

Preface to the Special Issue on “Watershed Management and Impacts of Climate Change in Hydrology”

Although practice of hydrology can be traced back to as early as 5000 to 6000 years ago in ancient Mesopotamia, Egypt and China, the study of hydrology emerged as a discipline in the 17th century when Pierre Perrault, Edme Mariotte and Edmond Halley conducted their experimental work on the hydrologic cycle. For quite a long period, hydrological education and research programs were offered primarily as an engineering discipline in universities in the United States and many East Asian countries. With our increasing understanding of the mechanisms of spatial and temporal distributions of water over large watersheds, and even the globe, and the capabilities of computer modeling of complicated hydrological processes, the study of hydrology has now extended to and interacted with meteorology, geophysics, environmental science, and mathematical statistics, and has established its own right as a branch of geoscience. Recognizing the geoscience nature of hydrology, a hydrology section in TAO was established in 2006 and has since received an increasing number of papers contributing to this growing field. This issue marks the first TAO special issue in hydrology. It focuses on two topics watershed management and climate change impacts in hydrology, both involving spatial scales which are characteristic of hydrology. This special issue is composed of seven papers on watershed management addressing water quality, groundwater, coastal inundation, hill-slope runoff, and surface and ground water interaction and five papers on climate change impacts on sea level rise, extreme rainfall frequency, runoff regime, river discharge, and water quality management. Case studies were carried out in different countries including Taiwan, United States, China, Japan, and Vietnam and in vastly different terrain ranging from Tampa Bay in Florida, USA, coastal lowlands in Taiwan, river basins in Japan and Central Vietnam, to the Tibetan Plateau in China. Hydrological research is multi-faceted, and, with our improved knowledge of hydrological processes and growing concern of the global influence of hydrological changes, new issues will arise continuously. It is the editors’ belief that this special issue will serve as a stepping stone in addressing hydrologic issues that not only are of global concern but also fundamental for universal applications. We are grateful to the TAO editorial office for their superb support and assistance during the review and editing process. Our sincere appreciations also extend to all contributing authors and reviewers for their enthusiasm and dedication, which have made this special issue a reality.

Estimates of Climate Change Impact on River Discharge in Japan Based on a Super-High-Resolution Climate Model

The impact of climate change on river discharge was assessed by hydrological simulations for several major river basins in Japan using the latest version of a super-high-resolution atmospheric general circulation model (AGCM) with a horizontal resolution of about 20km. Projections were made using two different datasets, one representing the present climate (1980-1999) and the other representing the end of the 21st century (2080-2099) assuming the SRES A1B scenario. River discharge was estimated by a distributed hydrological model calibrated against observed river discharge in advance. The results showed that even if the amount of precipitation does not change much in the future, river discharge will change significantly because of the increase in rainfall, decrease in snowmelt, and increase in evapotranspiration with higher air temperature. The impact of climate change on river discharge will be more significant in the northern part of Japan, especially in the Tohoku and Hokuriku regions. In these regions, the monthly average river discharge at the end of the 21st century was projected to be more than 200% higher in February and approximately 50 - 60% lower in May compared with the present flow. These results imply that the increase in air temperature has important consequences for the hydrological cycle, particularly in regions where the water supply is currently dominated by snowmelt.

Possible contourites in the eastern margin of the Japan Basin

Possible contourites in the eastern margin of the Japan Basin

Deep Nitrate Deficit Observed in the Highly Oxygenated East/Japan Sea and Its Possible Cause

We present evidence of denitrification on the continental slopes of the Ulleung Basin (UB) and the Eastern Japan Basin (EJB) near the Tatar Strait (TtS) in the East/Japan Sea (EJS), despite its high water column dissolved oxygen concentrations. Some nutrient concentration data deviate significantly from the fitted regression line of nitrate (N) vs. phosphate (P) in deep waters, indicating a loss of nitrate in the region. The EJS has a lower N/P ratio (ca. 12.4 below 300 dbar) than a traditional Redfield ratio (16). The N/P ratio and oxygen concentration are substantially lower at several locations whose depths are close to the sediment-water interface, near TtS (500-1100 dbar) and in UB (1100-2200 dbar). The decreased nitrate concentration is smaller than the expected nitrate level (a low N/P ratio of＜12.4), and a secondary nitrite peak near the bottom of these two regions: taken collectively, both indicate the presence of denitrification in the bottom layer. It is speculated that active re-mineralization and denitrification may occur simultaneously along the rich organic matter bottom layer on the slope environment. Denitrification rates are estimated at ~3-33 μmol N m^(-2) d^(-1). Current estimates do not support the previous idea of basin-wide denitrification in EJS, although the N/P ratio is low like in other hypoxic/anoxic seas. A better understanding of the denitrification process is necessary for predicting future changes of nitrogen cycle in the well-oxygenated EJS considering the decadal-scale physical and biogeochemical changes that have occurred.