A lot of sedimentary particles are known to be mixed into dense shelf water (DSW) produced in the northern part of the Sea of Okhotsk as a result of brine rejection during sea ice formation. To investigate the behavior and distribution of materials in DSW, tracer experiments with an ice-ocean coupled model have been conducted. It was shown that a tracer injected in winter over the northwestern shelf flows on the shelf until early summer with the concentration higher near the bottom. Then, it goes down along the slope to the intermediate layer along the east coast of Sakhalin; the core of tracer is settled around a depth of 400 m. These features are consistent with observations. The experiment in which tidal mixing is absent along the Kuril Islands shows a shallower core of the tracer at a depth of about 200 m owing to the density decrease of DSW.

View full abstract

Although zero-order basins (geomorphic hollows) are important components of headwater catchments, their hydrologic regime has not been thoroughly investigated. A multi-tank model approach is used to simulate flow from zero-order basins in Hitachi Ohta Experimental Watershed, Japan, and simulations are compared with six months of wet season flows. A three-tank model accurately simulated runoff for the 6-month period from basin (FA) with two zero-order basins and deep soils, whereas a two-tank model performed satisfactorily in a zero-order basin with shallower soils (ZB). Characteristics of flow paths were evaluated and the concept of “threshold response” was assessed in simulations. In FA, preferential flow from the upper outlet of Tank 1 only occurred during the two largest storms; no overland flow was simulated. Less rapid subsurface flow emitted from the side outlet of Tank 2 during large and several moderate-size storms. During small storms, no overland, preferential, or subsurface flows occurred. Water depth in Tank 3, which indicates shallow groundwater storage in FA, is highly correlated with 30-day antecedent rainfall. The concept of “threshold response” is evidenced by intermittent quick and moderate flows from Tanks 1 and 2, respectively.

View full abstract

The geographical distribution of the Monthly maximum number of Consecutive Dry Days (MCDD) over Japan during June–October in 2002–2006 is investigated from observed raingauge data. In June and July, areas with low MCDD (< 6 days) are analyzed associated with the climatological location of the Baiu front. In August, low MCDD [high MCDD (> 10 days)] is found in inland areas [coastal areas of the Seto Inland Sea, southern Kanto, and parts of Hokkaido]. In September and October, low MCDD [high MCDD] is found on the Japan Sea side of Honshu [in Kyushu and coastal areas of the Seto Inland Sea].
The reproducibility of the MCDD by a 5-km-mesh cloud-system resolving regional climate model (NHM-5km) is statistically examined in numerical experiments with regional objective analysis data of the Japan Meteorological Agency. NHM-5km performs well in reproducing the features of the local geographical distribution and seasonal march of MCDD, although MCDD is slightly overestimated. The MCDD values averaged for 11 sub-regions of Japan are compared with observations, revealing a good agreement (correlation coefficient, 0.755; root mean square error, 1.49; mean error, 1.10). Moreover, annual variations in MCDD averaged over Japan are well reproduced by the model.

View full abstract

Upscaling and the effect of slope length on the sediment production are important issues in hillslope hydrology. This study investigated the effect of slope length at a steep forested site between 2006 and 2010 using erosion plots and neighboring sediment traps. The hillslope had high litter cover but sparse understory due to overgrazing by wild deer. Sediment transport increased for short distances (5 m to 10 m), then decreased over the hillslope (> 10 m). In plots this effect was due to organic litter accumulation. Organic litter production declined progressively along the hillslope continuum. When accumulated organic litter altered the microtopography, sediment production declined more with increase in slope length. Heavy precipitation events did not change the pattern of these scale effects. These results demonstrate that the slope length effect had inconsistent controls on sediment transport due to variations in soil surface conditions of the landscape. Such variations in the slope length effect cause uncertainty in sediment flux estimations in steep forested landscapes.

View full abstract

This study aimed to assess factors that determine changes in river water level in a relatively large forested catchment. Analysis of official river flow data for the Aya-minami River in Miyazaki, Japan, revealed a significant reduction for several years after 1970 and a prolonged reduction over 36 years in 95-day flow, 185-day flow, and 275-day flow; the climatic factors and changes in water use during this period had not contributed to this reduction. Forests cover approximately 90% of the 5,884 ha catchment area, and an analysis of tree ages showed that the clear-cut in the 1960s and 1970s most probably resulted in an elevated flow level around 1970, and that the tree growth thereafter have reduced discharges from the forest area. No statistically significant reduction trend was observed in the 355-day flow. Discharges from broad-leaved forests during periods of less precipitation tended to be higher than that of coniferous forests in small catchments, though we could not confirm that larger discharges from broad-leaved forests replenish the 355-day flow of the Aya-minami River.

View full abstract

Urbanization has compromised water quality globally, especially stream temperature, by reducing shading and converting natural landscapes to impermeable surface coverage (ISC). We analyze stream-air temperature relationships in a low-order, moderate-gradient urban stream using three years of stream temperature data collected at nine monitoring sites. At the sub-catchment scale, ISC increases from 13.7% to 24.3% among sites, causing mean summer temperatures to increase 4–5°C or 0.37°C for each 1% increase in ISC. ISC at these spatial scales influences stream-air temperature relationships at daily-, weekly-, and monthly-averaged time scales. ISC at smaller spatial scales within a 25-m buffer of the stream, which ranges from 1% to 75%, does not correlate with mean stream temperatures at any temporal scale; however, buffer ISC does correlate with short-term temperature surge events, which we define as an increase of at least 1°C within 15 minutes. Mean surge amplitudes range from 1.90°C to 3.27°C in areas with low and high buffer ISC, respectively. Our results show that ISC influences stream temperatures at stream buffer and sub-catchment spatial scales and daily, weekly, and monthly temporal scales, and may render the concept of equilibrium temperature obsolete for predicting stream temperatures, especially in low-order, headwater streams.

View full abstract

To examine the cause of isotopic difference among gross rainfall, throughfall and stemflow, water sampling with high temporal resolution was conducted in a Japanese cedar forest during a typhoon event. In this event, δ¹⁸O variation of throughfall was similar to that of gross rainfall except for the beginning of the rainfall event. However, isotopic fluctuations of stemflow differed from those of gross rainfall and throughfall throughout the event, although the temporal trend of stemflow volume was similar to that of gross rainfall and throughfall volumes. Comparisons between the observed δ¹⁸O of stemflow and that estimated by a model simulation suggests that isotopic difference between stemflow and gross rainfall or throughfall is caused principally by mixing of waters within canopy and stem storage along their flow paths with secondary effects of evaporation and isotopic exchange with ambient water vapor.

View full abstract

To quantitatively evaluate how different methods for creating 5 km mesh Radar/Raingauge-Analyzed Precipitation (R/A) affected annual precipitation, we compared three kinds of 5 km mesh R/A during 1991–2009 by devoting attention to the modification of its spatial resolution. It was, respectively, 5 km until March 2001, 2.5 km from April 2001 to December 2005, and 1 km from January 2006. For creating 5 km mesh R/A, three methods were adopted, i.e., (I) spatial average of 2.5 km or 1 km mesh was calculated. (II) Maximum value of 2.5 km or 1 km mesh was selected. (III) For creating 2.5 km mesh from 1 km mesh, spatial average of 3 × 3 1 km mesh was calculated considering the representative area of each mesh. For creating 5 km mesh from 2.5 km mesh, maximum value of four 2.5 km mesh was selected. The method III is suggested by JMA. We found abrupt changes of annual precipitation were seen in 2000/2001 (method I) and 2005/2006 (method II), both of which were artificial temporally and spatially compared with AMeDAS data. The 5 km mesh R/A created by the method III was statistically homogeneous, so we recommend that we should adopt the method III.

View full abstract

Sulfur hexafluoride (SF₆) concentrations in springs and groundwater were measured in the Chubu region of central Japan to evaluate the impact of natural and local anthropogenic SF₆ and the validity of SF₆ for dating young groundwater in Japan. Sampled water showed detectable concentrations of SF₆ at 0.21–125 f mol/L. Most of the mountain springs have SF₆ concentration assumed by the dissolution of the clean ambient air in Northern hemisphere. The SF₆-based apparent ages for the mountainous springs were estimated at less than 6 years in smaller catchment areas (< 0.5 km²), and ranged from 4 to 32 years in the larger catchment areas located on the Quaternary volcanoes. The SF₆ ages for mountain springs were consistent with the scale of groundwater flow and with previously determined ³H ages for groundwater in similar settings in Japan, suggesting the loading of natural SF₆ in the groundwater is relatively small in the mountainous areas. In the plains, local industrial activities led to high concentrations of SF₆ in some of the analyzed groundwater. The results suggest SF₆ can be an effective dating tool for young groundwater in Japan, when and where the input of local anthropogenic SF₆ is negligible.

View full abstract

This study aims to evaluate nutrient flux to the coast from the inland areas of Shiretoko in order to understand the key factors controlling the ecological systems of the coast. As an external force, rainfall is considered one of the most significant components controlling nutrient supply to coastal systems in this area. Therefore, to estimate nutrient supply in the future, the bias correction was applied by using Meteorological Research Institute Global Climate Model, which shows good agreement with Automated Meteorological Data Acquisition System data. A synthetic generation technique is used to produce hourly rainfall data, which is necessary for evaluating nutrient supply in Shiretoko. The robustness of the duplicated hourly rainfall intensity was investigated, which reveals that its standard deviation controls nutrient flux when nonlinearity becomes stronger for the evaluation of nutrient supply from a river basin.

View full abstract

This study addresses the effect of uncertainty in temperature and precipitation inputs and spatial resolution on crop simulation results for Hungary and Romania. Crop yield and harvested area for maize and winter-wheat were simulated using the improved Global Agro-Ecological Zones model (iGAEZ) for the years 1990–1999 with two climate inputs (Climatic Research Units Global 0.5°C Monthly Time Series, Version 2.1 (CRU TS 2.1) and Meteorological Research Institute Global Climate Model with the 20-km mesh horizontal resolution (MRI-GCM20)). The mean, standard deviation and RMSE of the differences between constraint-free and moisture-limited crop yield demonstrate that uncertainty in temperature and precipitation is a significant cause of the considerable uncertainty on crop simulation results at 0.5-degree grid. This uncertainty decreases when simulation results are spatially aggregated to the country scale. Next, to assess the effects of spatial averaging of climate input data, we performed the crop simulations at 0.5 and 0.25-degree grid using MRI-GCM20. The results showed that the correlation of the simulation results at 0.5-degree and at 0.25-degree grid scale is very weak. It seems that within-grid variability in climatic data significantly affects the crop simulation results. Moreover, the comparison of the Food and Agriculture Organization (FAO) yield statistics with crop simulation results shows that simulation results at 0.25-degree grid are much better than that of 0.5-degree grid.

View full abstract

Major ion chemistry, oxygen isotope and multivariate statistical method were used to investigate aquifers interaction in the southwestern foot of Mt. Fuji, the largest Quaternary stratovolcano in Japan. Groundwater samples were taken from different aquifers which are in descending order: the superficial aquifer, aquifers residing in the New Fuji Lava and the Old Fuji Aquifer. The geochemical signatures of groundwaters, notably the molar Na/Cl and Na/Ca ratios, oxygen stable isotope and the nitrate contents evolution, in conjunction with the hierarchical cluster analysis allowed to highlight three groundwater systems differing by their hydrochemical facies and residence time: 1) The shallow and highly polluted groundwaters of the superficial aquifer, 2) Groundwaters of the New Fuji Lava moderately mineralized and polluted, 3) The non-polluted groundwaters of the Old Fuji Aquifer. Waters of these systems are interacting. In fact, nitrate contamination, in provenance from the superficial aquifer, was detected in the New Fuji Lava layers and the deep groundwaters of the Old Fuji. Furthermore, many samples taken from the Old Fuji Aquifer were characterized by a relatively enriched δ¹⁸O. These results prove that groundwaters of the New Fuji layers could be mixed with groundwaters of the overlaying superficial aquifer and that these groundwaters, supposed to flow only horizontally, can flow vertically through the cracks, causing the mixing of deep and old groundwater of the Old Fuji with groundwaters recently recharged and polluted.

View full abstract

This paper is a case study on the detection of forest disturbances in airborne synthetic-aperture radar (SAR) data. We investigated changes in the L-band SAR backscattering coefficient (σ⁰) following thinning and a typhoon in a larch forest in northern Japan. The thinning caused changes of intermediate magnitude in the above-ground biomass (AGB), whereas the typhoon broke or uprooted most of the trees, thereby significantly changing the forest structure without changing the biomass. Thinning led to back-scattering coefficients change of less than about 1 dB; HV polarization was most sensitive to the change in AGB, and HH was insensitive to it. In the case of the typhoon, VV was sensitive to the change in forest structure, whereas HH and HV were insensitive to it, suggesting that the tree trunks, which were felled in one direction by the typhoon, enhanced the backscattering signal of the polarisation that accorded with the trunk direction.

View full abstract

A gigantic earthquake with a magnitude of 9.0, the maximum ever recorded in Japan, struck the Tohoku region of Japan on 11 March, 2011. As a result, as many as 745 reservoirs in Fukushima prefecture were damaged. The failure of the earth-fill dam at the Fujinuma reservoir in Fukushima prefecture resulted in eight deaths in a village downstream. This was only the second such dam to fail completely in the recorded history of Japan, the first being Mannou Lake dam. The failure was caused by the Ansei Nankai earthquake in 1854. According to official records, of the 210,000 reservoirs in Japan, at least 20,000 dams are vulnerable to future earthquakes. Therefore, it is imperative that the failure mechanism of the Fujinuma reservoir be understood. As such, we developed several theories to explain what happened. Adherence to recommendations made in this report will reduce the potential for damage in future catastrophic events.

View full abstract

Eddy covariance (EC) flux measurement is the most-used technique for observation of the fluxes of sensible heat, latent heat, and carbon dioxide between the land surface and the atmospheric boundary layer. Despite the availability of plentiful EC data from numerous research projects, it is difficult to make meaningful comparisons of EC at different sites, to validate the models used, and to integrate observed data with models because the uncertainties of the method are inadequately defined. We developed a method to evaluate the uncertainties of the EC method without the need to consider individual site specifications and flux characteristics. We showed that the fractional error φ of EC (i.e., tolerance T) can be separated into random and illegitimate components. T can be used as a scale parameter for spatiotemporal stationarity, and can be defined as a rational function. We demonstrated a practical application of T analyses for two contrasting areas: a low-relief paddy field and an area of more complex land-forms where dramatic wind changes affect fluxes, and showed that T analysis provides an appropriate and effective method to determine the uncertainties in EC.

View full abstract

Air temperature rise in Tokyo is distinctively large after 1980. Reflecting this feature, water temperature at Masugatanoike, a spring in Tokyo that stands a foot of terrace scarp that the height is 14 m, rose after the end of 1980s. The water temperature began to rise 4–7 years after the air temperature rose, showing a statistically significant correlation at the 5% level. This warming water must be explained by downward thermal conduction because there are no hot springs, subways, or major sewers around Masugatanoike spring. In this study, we first solved the surface energy balance in Tokyo from 1951 to 2009 to estimate the daily mean ground surface temperature. Next, we calculated the thermal conduction from the ground surface to a depth of 16 m. The initial condition was optimized so that root mean square error of the observed and calculated temperatures at a depth of 14 m from 1976 to 2009 was minimized. We found that 4–8 years were necessary until the effects of ground surface temperature reached a depth of 14 m after 1975. Namely, we demonstrated that downward thermal conduction is responsible for the above-mentioned lag relation, which was found in the observation.

View full abstract

Evapotranspiration (ET) is not only a vital component of water budget, but also plays an important role in the energy budget of the earth-atmospheric system, ultimately driving many regional and global scale climatological processes. This paper describes the ET characteristics and factors controlling ET across the 17 forest sites in East Asia (2°S to 64°N latitude). ET was measured using the eddy covariance technique at each site. Daytime dry-canopy data for the growing season were used in this study. Growing season mean ET gradually decreased as latitude increased, with a range of 4.4 to 1.2 mm d⁻¹. The growing season mean decoupling coefficient (Ω) ranged from 0.42 to 0.11 across the studied sites. At low-latitude forest sites, Ω was close to 0.50, indicating that the bulk surface was partially decoupled from the atmosphere and ET was strongly controlled by net radiation and vapour pressure deficit. At high-latitude forest sites, Ω was low (∼0.12), indicating that the bulk surface was well coupled to the atmosphere and ET was mainly controlled by surface conductance. The value of Ω was determined mainly by the ratio of aerodynamic conductance to surface conductance across the studied forests of East Asia.

View full abstract

Evapotranspiration over forest surfaces is mainly constrained by environmental and forest structural components through their influence on surface conductance (G_s) and aerodynamic conductance (G_a). Tower based eddy covariance data from 16 forest sites in East Asia (2°S to 64°N) were used to examine the sensitivity of Matsumoto and Nakai models for predicting G_s and G_a, respectively. Daytime dry-canopy data for the growing season were used in this study. Comparisons between model predictions and observed G_s and G_a showed good agreement, suggesting that the models were suitable for predicting G_s and G_a with reasonable accuracy. However, the model for G_s was unable to predict G_s accurately when soil water content was low (∼10%). In this circumstance, effective soil water content and a more comprehensive method for modelling the soil water content function must be used. G_s in East Asia was largely depended on vapour pressure deficit and secondarily on soil water content. G_a was largely affected by leaf area index compared with stand density.

View full abstract

Decline of larch trees has been recently observed in eastern Siberia. We hypothesized that the decline might be caused by changes in soil water conditions of larch forests, especially wetting tendency due to recent climate change. Through pot experiments, we have investigated the effects of wet or dry soil water conditions on growth and abscission of roots in two larch species viz. Larix gmelinii that grows in boreal forests and Larix leptolepis that grows in temperate regions. The results showed that both the allocation of photosynthetic products to roots and the activity of roots decrease under wet conditions compared with drought conditions in L. gmelinii. However, the opposite results were obtained in L. leptolepis. The root response of L. gmelinii to wet conditions may be one of the causes of the recent decline in Siberian larch forests under elevated soil water conditions.

View full abstract