In Lake Kawaguchi, the existence of lake bottom springs have been estimated based on the patchy distribution of lake ice during winter. However, our knowledge on the detailed distribution and the source of these springs are still limited. In this study, we conducted hydrological and lake bottom survey using a Conductivity Temperature Depth (CTD) profiler, underwater camera and sonar to explore lake bottom springs in Lake Kawaguchi. We found upwelling of relatively warm water with low electrical conductivity (EC) at the 100 m east of the Unoshima Island, suggesting the existence of springs at this point. In addition, the results of lake bottom survey revealed the lack of muddy sediment on the lake bottom surface, a general feature observed in bottom springs, within a radius of 25 m around the springs. Stable isotope ratios of the lake bottom waters around the springs showed the values that are close to those in groundwater around the lake, suggesting that groundwater from the surrounding mountains likely wells out from the lake bottom at this location.
In this study, we focused on nitrate-nitrogen (NO3--N) contamination of groundwater in Omama alluvial fan in the Gunma Prefecture and examined the spatial distribution and seasonal variation of NO3--N concentrations during a periodic survey over nine months. The analysis results showed that the groundwater for more than 70% of the 27 surveyed sites was affected by human contamination. Moreover, high concentrations of NO3--N were measured in areas with numerous agricultural sites. Strong correlations observed between NO3--N and chlorine (Cl-)·sulfate ion (SO42-) suggested that NO3--N contamination of groundwater could be attributed to the use of fertilizers in these areas. Furthermore, the seasonal variation of the NO3--N concentration and groundwater level seemed to be correspond to the precipitation, the wettest periods are considered to have higher NO3--N concentrations Besides, it was clarified that the groundwater pollution is not transient, but certain extent in the long-term in fact, there was some in which the environmental standard value was exceeded during the research period.
SPECIAL ISSUE “Environmental changes and natural disasters related to hydrology”
A temperate deep lake, Lake Kuttara, Hokkaido (148 m depth at the deepest point) was completely frozen in winter in the 20th century. However, non-freezing of the lake over winter occurred four times in the 21st century, which is probably due to global warming. In order to understand how thermal regime of the lake responds to climate change, its heat storage was calculated by estimating heat budget of the lake and monitoring water temperature at the deepest point for 1 June 2014 –31 May 2016. As a result, the temporal variation of heat storage from the heat budget was very consistent with that from the direct temperature measurement (the determination coefficient R2 = 0.903). A sensitivity analysis was conducted by numerically changing main meteorological factors (air temperature, solar radiation, wind speed, precipitation) for the heat storage obtained from the heat budget estimate. The increase in air temperature and precipitation was very effective to increase the heat storage. It is noted that, considering the increasing rate of air temperature (0.024°C/yr), the lake could be permanently unfrozen in about two decades.
The Japanese Alps region is known to experience some of the heaviest snowfall in the world. In this region, precipitation brought by snowfall is more important as a water resource than rainfall. Snow functions as a ‘natural white dam’ by accumulating in watersheds during the winter and melting gradually in spring, when it becomes a valuable water resource for the region. Recent studies have reported that the amount of snowfall in Japan will decrease as a result of global warming. However, these studies used data observed at low altitudes, and so the question arises whether the same theory can be applied to high-altitude mountain areas. Observations of the amount of snow have not previously been carried out at high altitudes in Japan where the temperature is colder than the snow/rain threshold temperature even with the temperature rise observed in recent years. Therefore, we could not discuss the effect of global warming on the amount of snow in the mountainous regions of Japan based on existing observation data. In this study, we discuss the relationship between snowfall quantity and snowmelt runoff using observation data for the Japanese Alps catchment.
Hydrogeomorphic studies on relationship between sediment discharge and hillslope environments are reviewed. The relationship is affected by alteration of vegetation, volcanic eruption and wild fire, and is explained as follows. Firstly, poor vegetation lets hillslopes susceptible to shallow landslides and promotes sediment discharge due to high runoff coefficient (ratio of surface runoff to rainfall) and slope instability without binding effects of root network of trees. On the other hand, saturation with trees on hillslopes would suppress shallow landslides owing to low runoff coefficient and slope stability with binding effects of root network of trees. However, the saturation is one of primary causes of deep-seated landslides at dip slopes in geologic zone of accretionary prism. Secondly, volcanic ash fall often induces activation of sediment discharge from hillslope due to an increase in slope runoff. The increase in slope runoff is brought about by a drastic decrease in infiltration capacity and the land degradation of vegetation loss. Similarly, wild fire often induces activation of flash flood and sediment discharge from burned hillslopes. The activation is brought about by two causes: One is increase in hillslope runoff due to drastic decrease in infiltration capacity forced by newly formed hydrophobic (water repellent) soil layer, and the other is vegetation loss by wild fire. Although the altered hydrogeomorphic parameters of hillslope would recover to pre-alternation level often along an asymptote of exponential function, the recovery at post-wild fire is generally much faster than that at post-volcanic eruption. We should be aware of risk of hazards implied by hydrogeomorphic alternation above mentioned from a view point of hazard mitigation against various types of sediment discharge.
To clarify the carbon dioxide (CO2) dynamics in the atmosphere and the surface seawater in the coastal regions, CO2 has been continuously measured in Osaka Bay from August to November, 2012 and 2014. Water temperature, dissolved oxygen (DO) and pH in the surface seawater, and the CO2 concentration in the atmosphere were continually measured at each station in Osaka Bay. The values of CO2 in seawater were calculated using the “CO2SYS” by CDIAC. Air to sea CO2 flux was calculated from these data and wind speed measured at Kobe airport by Japan Meteorological Agency. Consequently, during the stratified period of summer, the CO2 absorption flux was high, and atmospheric CO2 concentration was low. CO2 flux had large diurnal change by daytime photosynthesis and nighttime respiration. In this period, mean CO2 absorption flux was 0.09 g-C m-2 d-1, and atmospheric CO2 concentration was 397.7 ppm. On the other hand, in the autumn when the stratification disappears, mean CO2 absorption flux was -0.04 g-C m-2 d-1, and atmospheric CO2 concentration was 429.9 ppm.
In summer, surface water CO2 fluctuation had large diurnal change by photosynthesis, and atmospheric CO2 is absorbed to surface seawater. In the autumn, CO2 in seawater was release to the atmosphere. Therefore, the atmospheric CO2 concentration changes largely by air to sea CO2 flux.
Rain-on-snow (ROS) events cause severe snowmelt hazards such as river flooding, snow avalanches, slope failure, landslides, etc. ROS event studies have been increasing since 2000. Snowmelt energy balance studies show that the amount of advective energy carried by rainfall is usually small, and substantial contributions are made by turbulent heat transfer. However, snow melting does not occur entirely due to turbulent transfers, as radiative or ground heat transfers are also important depending on the site and period. The principal energy for melting snow during ROS events depends on the rainfall amount, meteorological and snowpack conditions, and the watershed characteristics. Moreover, the percolating process of rain water through the snowpack and the runoff characteristics of the outflow from the snowpack bottom are poorly understood. Artificial rain sprinkling experiments have been performed at some snowy sites, but the methods used in these experiments differed. In Moshiri of northern Hokkaido, Japan, a total artificial rainfall of 120 mm sprinkled over the 1.12 m deep snowpack over 207 minutes. The water that flowed out from the bottom of the snowpack was 67% event water and 33% pre-event water. This result indicates that the rain and snowmelt water percolation process during ROS events is remarkably different from the process that occurs during typical snow melting on sunny days.
October 05, 2017 Due to the maintenance‚following linking services will not be available on Oct 18 from 10:00 to 19:00 (JST)(Oct 18‚ from 1:00 to 10:00(UTC)). We apologize for the inconvenience. a)reference linking b)cited-by linking c)linking to J-STAGE with JOI/OpenURL
May 18, 2016 We have released “J-STAGE BETA site”.
May 01, 2015 Please note the "spoofing mail" that pretends to be J-STAGE.