Effect of canopy openness and meteorological factors on spatial variability of throughfall isotopic composition in a Japanese cypress plantation

Spatial variability of throughfall (TF) isotopic composition, used as tracer input, influences isotope hydrological applications in forested watersheds. Notwithstanding, identification of the dominant canopy factors and processes that affect the patterns of TF isotopic variability remains ambiguous. Here, we examined the spatio‐temporal variability of TF isotopic composition in a Japanese cypress plantation, in which intensive strip thinning was performed and investigated whether canopy structure at a fine resolution of canopy effect analysis is related to TF isotopic composition and how this is affected by meteorological factors. Canopy openness, as an index of canopy structure, was calculated from hemispherical photographs at different zenith angles. TF samples were collected in a 10 × 10 m experimental plot in both pre‐thinning (from July to November 2010) and post‐thinning (from May 2012 to March 2013) periods. Our results show that thinning resulted in a smaller alteration of input δ¹⁸O of gross precipitation, whereas the changes in deuterium excess varied in both directions. Despite the temporal stability of spatial patterns in TF amount, the spatial variability of TF isotopic composition was not temporally stable in both pre‐ and post‐thinning periods. Additionally, after thinning, the isotopic composition of TF was best related to canopy openness calculated at the zenith angle of 7°, exhibiting three different relationships, that is, significantly negative, significantly positive, and nonsignificant. Changes in meteorological factors (wind speed, rainfall intensity, and temperature) were found to affect the relationships between TF δ¹⁸O and canopy openness. The observed shifts in the relationships reveal different dominant factors (partial evaporation and the selection), and canopy water flowpaths control such differences. This study provides useful insights into the spatial variability of TF isotopic composition and improves our understanding of the physical processes of interception through canopy passage.     

The mechanism of bottom water DO variation in summer at the northern mouth of Isahaya Bay,Japan

Recently, bivalves have been massively killed by anoxia or hypoxia in summer at the northern part of Isahaya Bay, Japan, which constituted a major problem for fisheries. However, the mechanism behind the occurrence of hypoxic water masses is unclear. It is known that the bottom water dissolved oxygen (DO) in this area is affected by the inflow of seawater into the northern mouth of Isahaya Bay. To understand the mechanism of hypoxia, it is necessary to determine the physical processes that cause changes in the bottom DO concentrations in this area. This study shows that there is a neap-spring tidal variation in bottom DO due to a change in vertical tidal mixing, and it also suggests that the decrease in bottom DO was generated by a baroclinic flow, which is due to the internal tide, and a shear flow, which is induced by the external tide in the bottom boundary layer. In addition, our study suggests that the source of cold and hypoxic water that appears in the bottom layer at low tide is the inner area of the Ariake Sea.     

Constraining sediment provenance for tsunami deposits using distributions of grain size and foraminifera from the Kujukuri coastline and shelf,Japan

Tsunami deposits preserved in the geological record provide a more comprehensive understanding of their patterns of frequency and intensity over longer timescales; but recognizing tsunami deposits can prove challenging due to post-depositional changes, lack of contrast between the deposits and surrounding sedimentary layers, and differentiating between tsunami and storm deposition. Modern baseline studies address these challenges by providing insight into modern spatial distributions that can be compared with palaeotsunami deposits. This study documents the spatial fingerprint of grain size and foraminifera from Hasunuma Beach and the Kujukuri shelf to provide a basis from which tsunami deposits can be interpreted. At Hasunuma Beach, approximately 50 km east of Tokyo, the spatial distribution of three common proxies (foraminiferal taxonomy, foraminiferal taphonomy and sediment grain size) for tsunami identification were mapped and clustered using Partitioning Around Medoids cluster analysis. Partitioning Around Medoids cluster analysis objectively discriminated two coastal zones corresponding to onshore and offshore sample locations. Results show that onshore samples are characterized by coarser grain sizes (medium to coarse sand) and higher abundances of Pararotalia nipponica (27 to 63%) than offshore samples, which are characterized by finer grain sizes (fine to medium sand), lower abundances of Pararotalia nipponica (2 to 19%) and Ammonia parkinsoniana (0 to 10%), higher abundances of planktonics (15 to 58%) and species with fragile tests including Uvigerinella glabra. When compared to grain-size and foraminiferal taxonomy, foraminiferal taphonomy; i.e. surface condition of foraminifera, a proxy not commonly used to identify tsunami deposits, was most effective in discriminating modern coastal zones (identified supratidal, intertidal and offshore environments) and determining sediment provenance for tsunami deposits at Kujukuri. This modern baseline study assists the interpretation of tsunami deposits in the geological record because it provides a basis for sediment provenance to be determined.     

Medium-scale traveling ionospheric disturbances observed by GPS receiver network in Japan: a short review

Medium-scale traveling ionospheric disturbances (MSTID) are wave-like perturbations of the ionospheric plasma with wavelengths of several hundred kilometres and velocities of several hundred metres per second. MSTID is one of the most common ionospheric phenomena that generally induce the perturbations of ionospheric total electron content (TEC) by ∼10¹⁶ electron/m², which corresponds to ∼54 ns (16.2 cm) delay at GPS L1 signal. In the past decade, several new characteristics on MSTIDs have been revealed by the TEC observations using the dense GPS receiver network in Japan. In this paper, we provide a short review of these new observations and summarize the morphological characteristics of MSTIDs in Japan.     

Tsunami deposit associated with the 2011 Tohoku‐oki tsunami in the Hasunuma site of the Kujukuri coastal plain,Japan

We describe the detailed sedimentary characteristics of a tsunami deposit associated with the 2011 Tohoku‐oki tsunami in Hasunuma, a site on the Kujukuri coastal plain, Japan. The thick tsunami deposit was limited to within 350 m from the coastline whereas the inundation area extended about 1 km from the coastline. The tsunami deposit was sampled by excavation at 29 locations along three transects and studied using peels, soft‐X imaging and grain‐size analysis. The deposit covers the pre‐existing soil and reached a maximum measured thickness of 35 cm. It consists mainly of well‐sorted medium to fine sand. On the basis of sedimentary structures and changes in grain size, we divided the tsunami deposit into several sedimentary units, which may correspond to multiple inundation flows. The numbers of units and their sedimentary features vary among the three transects, despite the similar topography. This variation implies a considerable influence of local effects such as elevation, vegetation, microtopography, and distance from footpaths, on the tsunami‐related sedimentation.     

Ionospheric and geomagnetic disturbances during the 2005 Sumatran earthquakes

This paper investigates the ionospheric and geomagnetic responses during the 28 March 2005 and 14 May 2005 Sumatran earthquakes using GPS and magnetometer stations located in the near zone of the epicenters. These events occurred during low solar and geomagnetic activity. TEC oscillations with periods of 5–10 min were observed about 10–24 min after the earthquakes and have horizontal propagation velocities of 922–1259 m/s. Ionospheric disturbances were observed at GPS stations located to the northeast of the epicenters, while no significant disturbances were seen relatively east and south of the epicenters. The magnetic field measurements show rapid fluctuations of 4–5 s shortly after the earthquake, followed by a Pc5 pulsation of 4.8 min about 11 min after the event. The correlation between the ionospheric and geomagnetic responses shows a good agreement in the period and time lag of the peak disturbance arrival, i.e. about 11–13 min after the earthquake.     

Factors influencing the recession rate of Niagara Falls since the 19th century

The rate of recession of Niagara Falls (Horseshoe and American Falls) in northeastern North America has been documented since the 19th century; it shows a decreasing trend from ca. 1 m y^− 1 a century ago to ca. 0.1 m y^− 1 at present. Reduction of the flow volume in the Niagara River due to diversion into bypassing hydroelectric schemes has often been taken to be the factor responsible, but other factors such as changes in the waterfall shape could play a role and call for a quantitative study. Here, we examine the effect of physical factors on the historically varying recession rates of Niagara Falls, using an empirical equation which has previously been proposed based on a non-dimensional multiparametric model which incorporates flow volume, waterfall shape and bedrock strength. The changes in recession rates of Niagara Falls in the last century are successfully modeled by this empirical equation; these changes are caused by variations in flow volume and lip length. This result supports the validity of the empirical equation for waterfalls in rivers carrying little transported sediment. Our analysis also suggests that the decrease in the recession rate of Horseshoe Falls is related to both artificial reduction in river discharge and natural increase in waterfall lip length, whereas that of American Falls is solely due to the reduction in flow volume.     

In situ Measurements of Tide Gauge Response and Corrections of Tsunami Waveforms from the Niigataken Chuetsu-oki Earthquake in 2007

Linear and nonlinear responses of ten well-type tide gauge stations on the Japan Sea coast of central Japan were estimated by in situ measurements. We poured water into the well or drained water from the well by using a pump to make an artificial water level difference between the outer sea and the well, then measured the recovery of water level in the well. At three tide gauge stations, Awashima, Iwafune, and Himekawa, the sea-level change of the outer sea is transmitted to the tide well instantaneously. However, at seven tide gauge stations, Nezugaseki, Ryotsu, Ogi, Teradomari, Banjin, Kujiranami, and Naoetsu, the sea-level change of the outer sea is not always transmitted to the tide well instantaneously. At these stations, the recorded tsunami waveforms are not assured to follow the actual tsunami waveforms. Tsunami waveforms from the Niigataken Chuetsu-oki Earthquake in 2007 recorded at these stations were corrected by using the measured tide gauge responses. The corrected amplitudes of the first and second waves were larger than the uncorrected ones, and the corrected peaks are a few minutes earlier than the uncorrected ones at Banjin, Kujiranami, and Ogi. At Banjin, the correction was significant; the corrected amplitudes of the first and second upward motion are +103 cm and +114 cm, respectively, while the uncorrected amplitudes were +96 cm and +88 cm. At other tide gauge stations, the differences between the uncorrected and corrected tsunami waveforms were insignificant.     

Properties of winter mixed layer variability on the shelf-slope region facing the Kuroshio—study of Tosa Bay, southern Japan

To examine the properties of winter mixed layer (ML) variability in the shelf-slope waters facing the Kuroshio, we analyzed historical temperature records and the simulated results of a triply nested high-resolution numerical model. As a candidate of the shelf-slope waters, we focused on Tosa Bay, off the southern Japan. A time series of observed monthly mean ML temperatures and depths in the bay exhibits a remarkable seasonal variation. The period when the ML develops can be divided into two regimes: from September to November, when the sea surface cooling is gradually enhanced, the ML temperature and depth decreases and increases, respectively; from January to March, the ML temperature and depth are kept nearly constant, while the sea surface cooling in January reaches its annual maximum. In the latter regime, variance for the monthly mean ML depth is the largest of the year. To further study the ML properties in the latter regime corresponding to winter, we examined simulated results. It was found that the largest variance for ML depth is attributed to a dominant intramonthly variation. This is related to a submesoscale variation with typical spatial scales of 10–20 km, induced by the Kuroshio and its frontal disturbances. Simulated monthly mean heat balance within the ML showed that heat advection balances with heat flux at the sea surface and entrainment through the ML bottom. Moreover, the monthly mean heat advection is determined mainly by the intramonthly eddy heat advection, suggesting that the high-frequency intramonthly variation related to submesoscale variations contributes significantly to the low-frequency monthly variations of the ML in winter.