Do southern Appalachian Mountain summer stream temperatures respond to removal of understory rhododendron thickets?     

Scott Raulerson  C. Rhett Jackson  Nathan D. Melear  Seth E. Younger  Maura Dudley  Katherine J. Elliott 《水文研究》2020,34(14):3045-3060

Dense understory thickets of the native evergreen shrub Rhododendron maximum expanded initially following elimination of American chestnut by the chestnut blight, and later in response to loss of the eastern hemlock due to hemlock woolly adelgid invasion. Rhododendron thickets often blanket streams and their riparian zones, creating cool, low-light microclimates. To determine the effect of such understory thickets on summer stream temperatures, we removed riparian rhododendron understory on 300 m reaches of two southern Appalachian Mountain headwater streams, while leaving two 300 m reference reaches undisturbed. Overhead canopy was left intact in all four streams, but all streams were selected to have a significant component of dead or dying eastern hemlock in the overstory, creating time-varying canopy gaps throughout the reach. We continuously monitored temperatures upstream, within and downstream of treatment and reference reaches. Temperatures were monitored in all four streams in the summer before treatments were imposed (2014), and for two summers following treatment (2015, 2016). Temperatures varied significantly across and within streams prior to treatment and across years for the reference streams. After rhododendron removal, increases in summer stream temperatures were observed at some locations within the treatment reaches, but these increases did not persist downstream and varied by watershed, sensor, and year. Significant increases in daily maxima in treatment reaches ranged from 0.9 to 2.6°C. Overhead canopy provided enough shade to prevent rhododendron removal from increasing summer temperatures to levels deleterious to native cold-water fauna (average summer temperatures remained below 16°C), and local temperature effects were not persistent.  相似文献   

Uncertain human consequences in asteroid risk analysis and the global catastrophe threshold     

Seth D. Baum 《Natural Hazards》2018,94(2):759-775

This paper studies the risk of collision between asteroids and Earth. It focuses on uncertainty in the human consequences of asteroid collisions, with emphasis on the possibility of global catastrophe to human civilization. A detailed survey of the asteroid risk literature shows that while human consequences are recognized as a major point of uncertainty, the studies focus mainly on physical and environmental dimensions of the risk. Some potential human consequences are omitted entirely, such as the possibility of asteroid explosions inadvertently causing nuclear war. Other human consequences are modeled with varying degrees of detail. Direct medical effects are relatively well-characterized, while human consequences of global environmental effects are more uncertain. The latter are evaluated mainly in terms of a global catastrophe threshold, but such a threshold is deeply uncertain and may not even exist. To handle threshold uncertainty in asteroid policy, this paper adapts the concept of policy boundaries from literature on anthropogenic global environmental change (i.e., planetary boundaries). The paper proposes policy boundaries of 100 m asteroid diameter for global environmental effects and 1 m for inadvertent nuclear war. Other policy implications include a more aggressive asteroid risk mitigation policy and measures to avoid inadvertent nuclear war. The paper argues that for rare events like large asteroid collisions, the absence of robust data means that a wide range of possible human consequences should be considered. This implies humility for risk analysis and erring on the side of caution in policy.  相似文献   

Circulation on the north central Chukchi Sea shelf   总被引：8，自引：0，他引：8  

Thomas Weingartner  Knut Aagaard  Rebecca Woodgate  Seth Danielson  Yasunori Sasaki  Donald Cavalieri 《Deep Sea Research Part II: Topical Studies in Oceanography》2005,52(24-26):3150

Mooring and shipboard data collected between 1992 and 1995 delineate the circulation over the north central Chukchi shelf. Previous studies indicated that Pacific waters crossed the Chukchi shelf through Herald Valley (in the west) and Barrow Canyon (in the east). We find a third branch (through the Central Channel) onto the outer shelf. The Central Channel transport varies seasonally in phase with Bering Strait transport, and is 0.2 Sv on average, although some of this might include water entrained from the outflow through Herald Valley. A portion of the Central Channel outflow moves eastward and converges with the Alaskan Coastal Current at the head of Barrow Canyon. The remainder appears to continue northeastward over the central outer shelf toward the shelfbreak, joined by outflow from Herald Valley. The mean flow opposes the prevailing winds and is primarily forced by the sea-level slope between the Pacific and Arctic oceans. Current variations are mainly wind forced, but baroclinic forcing, associated with upstream dense-water formation in coastal polynyas might occasionally be important.Winter water-mass modification depends crucially on the fall and winter winds, which control seasonal ice development. An extensive fall ice cover delays cooling, limits new ice formation, and results in little salinization. In such years, Bering shelf waters cross the Chukchi shelf with little modification. In contrast, extensive open water in fall leads to early and rapid cooling, and if accompanied by vigorous ice production within coastal polynyas, results in the production of high-salinity (>33) shelf waters. Such interannual variability likely affects slope processes and the transport of Pacific waters into the Arctic Ocean interior.  相似文献   

A Long-Term Decrease of the Mid-Size Segmentation Lengths Observed in the He <Emphasis Type="SmallCaps">ii</Emphasis> (30.4 nm) Solar EUV Emission     

Leonid Didkovsky  Seth Wieman  Elena Korogodina 《Solar physics》2017,292(2):32

Power spectra of segmentation-cell length (a dominant length scale of EUV emission in the transition region) from full-disk He?ii extreme ultraviolet (EUV) images observed by the Extreme ultraviolet Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory (SOHO) and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) during periods of quiet-Sun conditions for a time interval from 1996 to 2015 were analyzed. The spatial power as a function of the spatial frequency from about 0.04 to 0.27 (EIT) or up to 0.48 (AIA) Mm^?1 depends on the distribution of the observed segmentation-cell dimensions – a structure of the solar EUV network. The temporal variations of the spatial power reported by Didkovsky and Gurman (Solar Phys. 289, 153, 2014) were suggested as decreases at the mid-spatial frequencies for the compared spectra when the power curves at the highest spatial frequencies of 0.5 pix^?1 were adjusted to match each other. This approach has been extended in this work to compare spectral ratios at high spatial frequencies expressed in the solar spatial frequency units of Mm^?1. A model of EIT and AIA spatial responses allowed us to directly compare spatial spectral ratios at high spatial frequencies for five years of joint operation of EIT and AIA, from 2010 to 2015. Based on this approach, we represent these ratio changes as a long-term network transformation that may be interpreted as a continuous dissipation of mid-size network structures to the smaller-size structures in the transition region. In contrast to expected cycling of the segmentation-cell dimension structures and associated spatial power in the spectra with the solar cycle, the spectra demonstrate a significant and steady change of the EUV network. The temporal trend across these structural spectra is not critically sensitive to any long-term instrumental changes, e.g. degradation of sensitivity, but to the change of the segmentation-cell dimensions of the EUV network structure.  相似文献   

GPS for real-time earthquake source determination and tsunami warning systems   总被引：1，自引：0，他引：1  

Geoffrey Blewitt  William C. Hammond  Corné Kreemer  Hans-Peter Plag  Seth Stein  Emile Okal 《Journal of Geodesy》2009,83(3-4):335-343

We identify the key design aspects of a GPS-based system (and in the future, GNSS-based systems) that could contribute to real-time earthquake source determination and tsunami warning systems. Our approach is based on models of both transient and permanent displacement of GPS stations caused by large earthquakes, while considering the effect of GPS errors on inverted earthquake source parameters. Our main conclusions are that (1) the spatial pattern, magnitude, and timing of permanent displacement of GPS stations can be inverted for the earthquake source and so predict the 3D displacement field of the ocean bottom, thus providing the initial conditions for tsunami models, and (2) there are no inherently limiting factors arising from real-time orbit and positioning errors, provided sufficient near-field GPS stations are deployed. This signal could be readily exploited by GPS networks currently in place, and will be facilitated by the IGS Real-Time Project as it comes to fruition.  相似文献   

Recharge from a River of Large Width to a Shallow Water-Table Aquifer     

G. C. Mishra  S. M. Seth 《Ground water》1988,26(4):439-444

  相似文献   

Study of the structure of the slab-mantle interface using reflected and converted seismic waves     

George Helffrich  Seth Stein 《Geophysical Journal International》1993,115(1):14-40

  相似文献   

Analysis of Temporal Environmental Isotopic Variation in Ground Water of the Georgia Piedmont   总被引：2，自引：0，他引：2  

Seth Rose 《Ground water》1995,33(4):644-652

  相似文献   

Slow subduction of old lithosphere in the lesser antilles     

Seth Stein  Joseph F. Engeln  Douglas A. Wiens  Robert C. Speed  Kazuya Fujita 《Tectonophysics》1983,99(2-4)

The Lesser Antilles subduction zone is an extreme case of the subduction of old (~ 90 m.y.) lithosphere at a slow (~ 2 cm/y) convergence rate. Focal mechanisms of the largest earthquakes in the area have been obtained using body and surface wave data. During the time period (1950–1978) studied the subduction seismicity appears to represent primarily intraplate rather than interplate deformation. All three large (magnitude seven) earthquakes were from intraplate normal faults; no large thrust faulting earthquakes and few small ones occurred. These observations suggest that the plate boundary is largely decoupled, that subduction is at least partially aseismic, and that the downgoing slab is in a state of extension.  相似文献   

Photobleaching of chromophoric dissolved organic matter (CDOM) in rainwater   总被引：1，自引：0，他引：1  

Robert J. Kieber  Joan D. Willey  Robert F. Whitehead  Seth N. Reid 《Journal of Atmospheric Chemistry》2007,58(3):219-235

Significant photodegradation of chromophoric dissolved organic matter (CDOM) in rainwater was observed after exposure to simulated sunlight. Fluorescence excitation emission spectra (EEMS) of precipitation revealed the presence of four major peaks all of which degraded upon photolysis with the greatest loss in the region characteristic of marine CDOM. Photobleaching of absorbance also occurred in the wavelength region between 250 and 375 nm with the greatest loss of absorbance in the upper end of the UV-A region near 275 nm. There was a strong positive correlation between absorbance loss and total integrated fluorescence loss suggesting these optical properties and the degree to which they are photobleached in rainwater are directly related. The quantum yield of CDOM photodegradation in rainwater decreased dramatically with increasing wavelength and decreasing energy of incoming radiation with the average quantum yield at 325 nm approximately an order of magnitude greater than at 460 nm. The similarity of photolytic response between rainwater and Cape Fear estuarine CDOM indicates that some fraction of the compounds that make up rainwater CDOM may be derived from surface sources and/or that the processes that produce or modify humic-like substances in the atmosphere result in similar types of compounds as non-atmospheric processes.  相似文献