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981.
Reactive barriers are passive and in situ ground water treatment systems. Heterogeneities in hydraulic conductivity (K) within the aquifer-reactive barrier system will result in higher flux rates, and reduced residence times, through portions of the barrier. These spatial variations in residence time will affect the treatment capacity of the barrier. A numerical flow model was used to evaluate the effects of spatial variations in K on preferential flow through barriers. The simulations indicate that the impact of heterogeneities in K will be a function of their location and distribution; the more localized the high K zone, the greater the preferential flow. The geometry of the reactive barrier will also strongly influence flow distribution. Aquifer heterogeneities will produce greater preferential flow in thinner barriers compared to thicker barriers. If the barrier K is heterogeneous, greater preferential flow will occur in thicker barriers. The K of the barrier will affect the flow distribution; decreasing the K of the barrier can result in more even distribution of flow. Results indicate that less variable flow will be attained utilizing thicker, homogeneous barriers. The addition of homogeneous zones to thinner barriers will be effective at redistributing flow only if installed immediately adjacent to both the up- and downgradient faces of the barrier. 相似文献
982.
The aim of this paper is to evaluate current knowledge and uncertainties associated with the impact of increasing greenhouse
gas and aerosol concentrations on the West African monsoon. For this purpose, coupled and time-slice simulations are used.
A global measure of the monsoon changes is defined in order to avoid regional biases and to try and obtain significant results.
The position and width of the monsoon in latitude are the main focuses. There is almost no agreement between the Coupled General
Circulation models from the Coupled models Inter-Comparison project—Phase II in regard to the impact of climate change on
the monsoon. Moreover, very simple discriminations between the models seem inappropriate to get a better signal. The role
of the different forcings in time-slice simulations is then investigated. The sea surface temperature (SST) and particularly
the pattern of the SST are shown to be the most important forcing. This accounts for the diversity of the results either from
the coupled or the forced simulations with different SST changes. With a fixed SST, but of a smaller magnitude in AMJ, there
are still uncertainties, coming first from the Atmospheric General Circulation models and the way they balance greenhouse
gas and global SST increase. Finally the uncertainty due to the Land Surface models (LSMs) is not negligible. The greenhouse
gas and the LSMs are shown to have more impact in August, when the monsoon is at its highest latitude on the continent. 相似文献
983.
S. G. Dobrovolski 《Water Resources》2014,41(6):728-737
A number of uncertainties of forecasts of changes in the annual runoff depths at global scale, obtained using information on results of integration of 21 IPCC climate models is studied. Following possible errors of these forecasts are calculated: errors of models; differences between main (IPCC) scenarios of emission of greenhouse gases in the atmosphere and resultant changes of global temperatures; mistakes in estimates of average long-term observed values of the runoff depths for the “control” period. Global maps of a “significance index” of forecasted changes in the runoff depths (estimations of changes in the annual runoff depths divided by mean square root values of errors of these estimations) for 2025, 2050, 2075 and 2100 are presented. It is shown that the most significant global changes of the runoff depths (growth in the north of Eastern Siberia, of the Russian Far East, of North America, falling in the “Greater Mediterranean Region”) are predicted for the second quarter of 21st century. Further changes of the runoff amplify only in the Amazon basin (reduction, by 2075). Almost everywhere else (including almost all European territory of Russia, Western Siberia, south of Eastern Siberia and of the Far East) the significance of changes in the runoff depths during 21st century is negligible. 相似文献
984.
We investigate the effect of a variable, i.e. time-dependent, background on the standing acoustic (i.e. longitudinal) modes generated in a hot coronal loop. A theoretical model of 1D geometry describing the coronal loop is applied. The background temperature is allowed to change as a function of time and undergoes an exponential decay with characteristic cooling times typical for coronal loops. The magnetic field is assumed to be uniform. Thermal conduction is assumed to be the dominant mechanism for damping hot coronal oscillations in the presence of a physically unspecified thermodynamic source that maintains the initial equilibrium. The influence of the rapidly cooling background plasma on the behaviour of standing acoustic (longitudinal) waves is investigated analytically. The temporally evolving dispersion relation and wave amplitude are derived by using the Wenzel–Kramers–Brillouin theory. An analytic solution for the time-dependent amplitude that describes the influence of thermal conduction on the standing longitudinal (acoustic) wave is obtained by exploiting the properties of Sturm–Liouville problems. Next, numerical evaluations further illustrate the behaviour of the standing acoustic waves in a system with a variable, time-dependent background. The results are applied to a number of detected loop oscillations. We find a remarkable agreement between the theoretical predictions and the observations. Despite the emergence of the cooling background plasma in the medium, thermal conduction is found to cause a strong damping for the slow standing magneto–acoustic waves in hot coronal loops in general. In addition to this, the increase in the value of thermal conductivity leads to a strong decay in the amplitude of the longitudinal standing slow MHD waves. 相似文献
985.
Bipolar active regions (ARs) are thought to be formed by twisted flux tubes, as the presence of such twist is theoretically
required for a cohesive rise through the whole convective zone. We use longitudinal magnetograms to demonstrate that a clear
signature of a global magnetic twist is present, particularly, during the emergence phase when the AR is forming in a much
weaker pre-existing magnetic field environment. The twist is characterised by the presence of elongated polarities, called
“magnetic tongues”, which originate from the azimuthal magnetic field component. The tongues first extend in size before retracting
when the maximum magnetic flux is reached. This implies an apparent rotation of the magnetic bipole. Using a simple half-torus
model of an emerging twisted flux tube having a uniform twist profile, we derive how the direction of the polarity inversion
line and the elongation of the tongues depend on the global twist in the flux rope. Using a sample of 40 ARs, we verify that
the helicity sign, determined from the magnetic polarity distribution pattern, is consistent with the sign derived from the
photospheric helicity flux computed from magnetogram time series, as well as from other proxies such as sheared coronal loops,
sigmoids, flare ribbons and/or the associated magnetic cloud observed in situ at 1 AU. The evolution of the tongues observed in emerging ARs is also closely similar to the evolution found in recent MHD
numerical simulations. We also found that the elongation of the tongue formed by the leading magnetic polarity is significantly
larger than that of the following polarity. This newly discovered asymmetry is consistent with an asymmetric Ω-loop emergence,
trailing the solar rotation, which was proposed earlier to explain other asymmetries in bipolar ARs. 相似文献
986.
I. S. Shklovskii 《Journal of Astrophysics and Astronomy》1984,5(1):13-18
It is argued that the iron nucleosynthesis rate in the universe due to SNI outbursts is dependent on the mass function of
star formation. Since the mass function depends on the chemical composition and since the masses of SNI precursors have upper
limits, the iron nucleosynthesis rate was low at an earlier evolutionary epoch of the universe when mainly massive stars were
formed. The iron nucleosynthesis rate should reach a maximum near z ∼ 0.5. At such or similar value of z the well-known ‘step’
in the cosmic γ-ray background spectrum may be explained by the presence of γ-gray quanta accompanying the radioactive56Co →56Fe decay. An argument is presented against the identification of the hidden mass of the universe with black-hole remnants
of ‘type III’ stars. 相似文献
987.
H. Got A. Monaco J. Vittori A. Brambati G. Catani M. Masoli N. Pugliese M. Zucchi-Stolfa A. Belfiore F. Gallo G. Mezzadri L. Vernia A. Vinci G. Bonaduce 《Sedimentary Geology》1981,28(4):243-272
Lithological, sedimentological, mineralogical and faunal analyses of twenty cores from the western Peloponnesus margin and adjacent Matapan Trench permit the origin of the sediments and the processes of sedimentation to be defined in some detail.Except for the ash layers derived from the Ischian Province, the major part of the sediment is local in origin (Peloponnesus). Analyses of heavy, light and clay minerals enable three main provinces to be distinguished. Each of these provinces includes one or more slope basins and a portion of trench, without lateral connection between them and mixing of inputs.The diversity and complexity of sedimentary structures, the granulometric characters, the mixed fauna (molluscs and ostracods) and the variability of rates of sedimentation all testify to the prevalence of reworking phenomena and gravitative processes.These data confirm and elaborate on the model of sedimentation deduced from seismic studies in which sediments are successively trapped, released by the slope basins and transferred via channels and canyons towards the underlying trench basins. This “cascade feeding” appears characteristics of this subductive sedimentary environment. 相似文献
988.
989.
S. M. Moorthi Nitant Dube D. Dhar B. Kartikeyan R. Ramakrishnan 《Journal of the Indian Society of Remote Sensing》2005,33(2):277-283
Remote sensing data products need to meet stringent geodetic and geometric accuracy specifications irrespective of intended
user applications. Georeferencing is the basic processing step towards achieving this goal. Having known the imaging geometry
and mechanism, the mathematical models built with the use of orbit and attitude information of the spacecraft can correct
the remote sensing data for its geometric degradations only up to system level accuracy (IRS-P6 DP Team, 2000). The uncertainties
in the orbit and attitude information will not allow the geometric correction model to generate products of accuracy that
can meet user requirements unless Ground Control Points (GCP) are used as reference geo-location landmarks. IRS-P6 data processing
team has been entrusted with developing a software system to generate data products that will have desired geodetic and geometric
accuracies with known limitations. The intended software system is called the Value Added Data Products System (VADS). Precision
corrected, Template Registered, Merged and Ortho Rectified products are the value added products planned with VADS. 相似文献
990.
A field experiment was conducted on wheat at New Delhi with five treatments of Nitrogen (N) fertilizer application (0, 30,
60, 90 and 120 kgha-1). Relationship has been established between observed leaf area index (LAI) and remotely sensed vegetation indices. These
relationships are inverted and used for predicting LAI from vegetation indices on different days after sowing. The “re-initialization”
strategy is implemented in model WTGROWS in which initial conditions of model are changed so that the model simulated LAI
match remote sensing predicted LAI. The model performance with re-initialization has been evaluated by comparing the simulated
grain yield and total above-ground dry matter (TDM) values with the actual observations. The results show that in-season re-initialization
is effective in model course correction by improving the simulated results of yield and TDM for different N treatments even
though the model was run with no N stress condition. Model re-initialization at different days shows that the closer is the
day of re-initialization to crop anthesis the more effective is model course correction. Also, the treatment showing maximum
error in yield simulation without re-initialization shows maximum reduction in error by re-initialization. The approach shows
that the remote sensing inputs can substitute for some of the inputs or errors in inputs required by crop models for yield
prediction. 相似文献