Assessing the role of forests in mitigating eutrophication downstream of pasture during spring snowmelt          下载免费PDF全文

Masaaki Chiwa  Sachiko Inoue  Naoaki Tashiro  Daisuke Ohgi  Yoshitoshi Uehara  Hideaki Shibata  Atsushi Kume 《水文研究》2015,29(4):615-623

Little research has examined whether forests reduce stream water eutrophication in agricultural areas during spring snowmelt periods. This study evaluated the role of forests in ameliorating deteriorated stream water quality in agricultural areas, including pasture, during snowmelt periods. Temporal variation in stream water quality at a mixed land‐use basin (565 ha: pasture 13%, forestry 87%), northern Japan, was monitored for 7 years. Synoptic stream water sampling was also conducted at 16 sites across a wide range of forest and agricultural areas in a basin (18.3 km²) in spring, summer and fall. Atmospheric nitrogen (N) and phosphorus (P) deposition were measured for 4 years. The results showed that concentration pulses of nitrate, organic N and total P in stream water were observed when discharge increased during spring snowmelt. Their concentrations were high when silicate concentrations were low, suggesting surface water exported from pasture largely contributed to stream water pollution during snowmelt. Atmospheric N and P deposition (4.1 kg N ha^?1 y^?1; 0.09 kg P ha^?1 y^?1, respectively) was too low to affect the background concentrations of N and P in streams from forested areas. Reduction of eutrophication caused by nutrients from pasture was mainly due to dilution by water containing low concentrations of N and P exported from forested areas, whereas in‐stream reduction was not a dominant process. Results indicate that forests have a limited capacity to reduce the concentration pulses of N and P in stream water during snowmelt in this study basin. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

Bed morphological changes around a finite patch of vegetation          下载免费PDF全文

Hyung Suk Kim  Ichiro Kimura  Yasuyuki Shimizu 《地球表面变化过程与地形》2015,40(3):375-388

For the appropriate management and restoration of rivers, isolated vegetation is often a practical means for improving stream habitat and ecology. The effect of a finite vegetation patch on flow and bed morphology in an open channel was investigated using laboratory experiments. The patch containing emergent and submerged vegetation was modeled using circular cylinders and located mid‐channel along a side wall. Several configurations of the patch and submergence ratio (i.e. water depth to the height of vegetation), and two flow conditions (i.e. below and above the sediment motion threshold) were considered. For flows below the sediment motion threshold, erosion occurred primarily on the opposite side of the patch and near the leading edge of the patch. The degree of scouring depth observed in both these regions was affected by the submergence ratio and it increased with the non‐dimensional flow blockage (i.e. the product of the patch density and width). In contrast, for flows above the sediment motion threshold, sediment accumulated within and around the patch due to a reduction in bed shear stress, which was strongly influenced by the flow blockage and the obstruction ratio (i.e. the ratio of patch width to channel width). The eroded area observed within the patch was consistent with the interior adjustment region where the deceleration and diversion of flow occurred through the patch. As the flow blockage increased or as the obstruction ratio decreased, the deposition rate within and behind the patch decreased. Furthermore, the deposition rate increased with an increase in the ratio of flow rate through the patch to total flow rate regardless of the submergence ratio. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

Ice core record of Be over the past millennium from Dome Fuji, Antarctica: A new proxy record of past solar activity and a powerful tool for stratigraphic dating     

Kazuho Horiuchi  Tomoko Uchida  Yuko Sakamoto  Aoi Ohta  Hiroyuki Matsuzaki  Yasuyuki Shibata  Hideaki Motoyama   《Quaternary Geochronology》2008,3(3):253-261

The cosmogenic nuclide ¹⁰Be was analyzed by using accelerator mass spectrometry on an ice core drilled at the Dome Fuji station, inland Antarctica, for 700–1900 yr CE. The measured concentration of ¹⁰Be in the Dome Fuji ice core and the derived ¹⁰Be flux show similar fluctuations, with both increasing at known solar-activity minima over the last millennium in agreement with earlier observations of ¹⁰Be and ¹⁴C. Based on the similar nature of the ¹⁰Be flux to the reconstructed ¹⁴C production rate patterns, a ¹⁰Be–¹⁴C correlation age model for the Dome Fuji ice core was successfully constructed. This age model agrees well with the initial version of the tephrochronology of the core. The ¹⁰Be-flux record contains information on variability in the amount of cosmic radiation incident on the atmosphere, which is mainly attributable to high-frequency change in solar activity and low-frequency background intensity adjustment of the geomagnetic field. High-resolution ¹⁰Be analyses of the Dome Fuji ice cores promise to provide potentially important information on the history of cosmic radiation intensity over the past several hundred thousand years.  相似文献   

浅水型河道中扩散器垂向排放的三维数值模拟   总被引：4，自引：1，他引：3       下载免费PDF全文

槐文信  柴田敏彦 《水科学进展》1999,10(1):1-6

采用作者给出的三维紊流模型,结合混合有限分析法及交错网格对浅水型河道中扩散器垂向排放的流动进行了数值计算。重点对该类流动的两个流动特征进行了分析:一是水面的隆起高度,二是在横流情况下的流动转变条件。进而给出了这类排放水面的最小稀释度。这些特征的探讨对废水排入江河的水力设计有重要的参考意义。  相似文献   

Overview and current status of the VSOP mission     

J. E. J. Lovell  H. Hirabayashi  H. Kobayashi  Y. Murata  P. G. Edwards  G. A. Moellenbrock  S. Horiuchi  E. B. Fomalont  S. Kameno  K. M. Shibata 《New Astronomy Reviews》1999,43(8-10)

An overview and status report of the VLBI Space Observatory Programme is presented.  相似文献   

One-dimensional electric field structure of an outer gap accelerator – I. γ-ray production resulting from curvature radiation     

K. Hirotani  S. Shibata 《Monthly notices of the Royal Astronomical Society》1999,308(1):54-66

We study the structure of a stationary and axisymmetric charge-deficient region (or potential gap) in the outer magnetosphere of a spinning neutron star. Assuming the existence of global current flow patterns in the magnetosphere, the charge depletion causes a large electric field along the magnetic field lines. This longitudinal electric field accelerates migratory electrons and/or positrons to ultrarelativistic energies. These relativistic electrons/positrons radiate γ -ray photons by curvature radiation. These γ -rays, in turn, produce yet more radiating particles by colliding with ambient X-ray photons, leading to a pair production cascade in the gap. The replenished charges partially screen the longitudinal electric field, which is self-consistently solved together with the distribution of e^± and γ -ray photons. We find the voltage drop in the gap as a function of the soft photon luminosity. It is demonstrated that the voltage drop is less than 3×10¹³ V when the background X-ray radiation is as luminous as Vela . However, this value increases with decreasing X-ray luminosity and attains 3×10¹⁵ V when the X-ray luminosity is as low as L _X=10³¹ erg s⁻¹.  相似文献   

Tephra dispersal from Myojinsho, Japan, during its shallow submarine eruption of 1952–1953     

Richard S. Fiske  Katharine V. Cashman  Atsushi Shibata  Kazuki Watanabe 《Bulletin of Volcanology》1998,59(4):262-275

 A new and detailed bathymetric map of the Myojinsho shallow submarine volcano provides a framework to interpret the physical volcanology of its 1952–1953 eruption, especially how the silicic pyroclasts, both primary and reworked, enlarged the volcano and were dispersed into the surrounding marine environment. Myojinsho, 420 km south of Tokyo along the Izu–Ogasawara arc, was the site of approximately 1000 phreatomagmatic explosions during the 12.5-month eruption. These explosions shattered growing dacite domes, producing dense clasts that immediately sank into the sea; minor amounts of pumice floated on the sea surface after some of these events. The Myojinsho cone has slopes of almost precisely 21° in the depth range 300–700 m.We interpret this to be the result of angle-of-repose deposition of submarine pyroclastic gravity flows that traveled downslope in all directions. Many of these gravity flows resulted from explosions and associated dome collapse, but others were likely triggered by the remobilization of debris temporarily deposited on the summit and steep upper slopes of the cone. Tephra was repeatedly carried into air in subaerial eruption columns and fell into the sea within 1–2 km of the volcano's summit, entering water as deep as 400 m. Because the fall velocity of single particles decreased by a factor of ∼30 in passing from air into the sea, we expect that the upper part of the water column was repeatedly choked with hyperconcentrations of fallout tephra. Gravitational instabilities within these tephra-choked regions could have formed vertical density currents that descended at velocities greater than those of the individual particles they contained. Upon reaching the sea floor, many of these currents probably continued to move downslope along Myojinsho's submarine slopes. Fine tephra was elutriated from the rubbly summit of the volcano by upwelling plumes of heated seawater that persisted for the entire duration of the eruption. Ocean currents carried this tephra to distal areas, where it presumably forms a pyroclastic component of deep-sea sediment. Received: 5 December 1996 / Accepted: 17 September 1997  相似文献   

Iodine (and Other Halogens) in Twenty Six Geological Reference Materials by ICP-MS and Ion Chromatography     

Bernhard Schnetger  Yasuyuki Muramatsu  Satoshi Yoshida 《Geostandards and Geoanalytical Research》1998,22(2):181-186

Analytical results for iodine in twenty six geological reference materials (RM) are presented. Bromine, fluorine and chlorine were also determined in some samples. Pyrohydrolysis in a heated quartz tube under a wet oxygen flow was used for the separation of the halogens from the matrix and the evolved gas was absorbed in a trap solution. The halogens were measured by ICP-MS and ion chromatography (IC). All four halogens can be collected in the trap solution from one combustion procedure. The analysed samples range in type from igneous rocks to terrestrial and marine sediments. Precision, detection limits, and accuracy are also presented.  相似文献   

On the electric field screening by electron–positron pairs in a pulsar magnetosphere     

S. Shibata  J. Miyazaki  & F. Takahara 《Monthly notices of the Royal Astronomical Society》1998,295(4):L53-L58

We present a steady one-dimensional model for a pulsar polar cap accelerator, where the field-aligned electric field and flow are solved self-consistently with a given current density. It is assumed that no particles return to the star. It is known that the space-charge-limited flow is accelerated to energies high enough to create electron–positron pairs if the assumed current density is high enough. We find that when pairs are created in such a space-charge-limited flow, the accelerating electric field is screened out within a short distance after pair creation, if the pair particle flux is larger than a critical value. We also find that a space charge density wave is excited in the screening region.
We find that a pair flux larger than the critical value M _c=10³–10⁵ must be reached in a layer with thickness equal to the braking distance for the decelerating component. Therefore, the required multiplicity – the number of pairs created by one primary particle – is too large to be realized in the actual pulsar magnetosphere. We suggest that in order to obtain a localized potential drop along the polar cap magnetic flux, one needs to take into account additional effects such as wave–particle interaction or quasi-periodic pair creation.  相似文献   

On the origin of strong downdrafts associated with the birth of sunspots     

Kazunari Shibata 《Solar physics》1980,66(1):61-70

During the emerging phase of sunspot pores strong downdrafts are observed in photospheric lines, whereas the upward flow is not detected within the observational accuracy. For the study of the origin of these downdrafts, we present the numerical solutions of non-steady hydrodynamic equations in one dimension along the rising magnetic flux tubes. In these solutions, it is assumed that the solar gas inside the tube is initially in the hydrostatic HSRA and then the tube rises with a given velocity and configuration. The results reproduce well the observed radial velocity distributions, hence it is concluded that the observed strong downdrafts originate from the sliding matter along the rising magnetic flux tubes.  相似文献