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131.
Kun‐xia Yu Lars Gottschalk Xiang Zhang Peng Li Zhanbin Li Lihua Xiong Qian Sun 《水文研究》2018,32(12):1844-1857
An approach for nonstationary low‐flow frequency analysis is developed and demonstrated on a dataset from the rivers on the Loess Plateau of China. Nonstationary low‐flow frequency analysis has drawn significant attention in recent years by establishing relationships between low‐flow series and explanatory variables series, but few studies have tested whether the time‐varying moments of low flow can be fully described by the time‐varying moments of the explanatory variables. In this research, the low‐flow distributions are analytically derived from the 2 basic explanatory variables—the recession duration and the recession coefficient—with the assumption that the recession duration and recession coefficient variables follow exponential and gamma distributions, respectively; the derived low‐flow distributions are applied to test whether the time‐varying moments of explanatory variables can explain the nonstationarities found in the low‐flow variable. The effects of ecosystem construction measures, that is, check dam, terrace, forest, and grassland, on the recession duration and recession coefficient are further discussed. Daily flow series from 11 hydrological stations from the Loess Plateau are used and processed with a moving average technique. Low‐flow data are extracted following the pit under threshold approach. Six of the 11 low‐flow series show significant nonstationarities at the 5% significance level, and the trend curves of the moments of low flow are in close agreement with the curves estimated from the derived distribution with time‐dependent moments of the recession duration and time‐constant moments of the recession coefficient. It is indicated that the nonstationarity in the low‐flow distribution results from the nonstationarity in the recession duration in all 6 cases, and the increase in the recession duration is resulted from large‐scale ecosystem constructions rather than climate change. The large‐scale ecosystem constructions are found to have more influence on the decrease in streamflow than on the increase in watershed storage, thus resulting in the reduction of low flow. A high return period for the initial fixed design value decreases dramatically with an increasing recession duration. 相似文献
132.
Simulation of multigaussian stochastic fields can be made after a Karhunen-Loéve expansion of a given covariance function.
This method is also called simulation by Empirical Orthogonal Functions. The simulations are made by drawing stochastic coefficients
from a random generator. These numbers are multiplied with eigenfunctions and eigenvalues derived from the predefined covariance
model. The number of eigenfunctions necessary to reproduce the stochastic process within a predefined variance error, turns
out to be a cardinal question. Some ordinary analytical covariance functions are used to evaluate how quickly the series of
eigenfunctions can be truncated. This analysis demonstrates extremely quick convergence to 99.5% of total variance for the
2nd order exponential (‘gaussian’) covariance function, while the opposite is true for the 1st order exponential covariance
function. Due to these convergence characteristics, the Karhunen-Loéve method is most suitable for simulating smooth fields
with ‘gaussian’ shaped covariance functions. Practical applications of Karhunen-Loéve simulations can be improved by spatial
interpolation of the eigenfunctions. In this paper, we suggest interpolation by kriging and limits for reproduction of the
predefined covariance functions are evaluated. 相似文献
133.
Haoyue Wang Zhengjiu Xu Harald Behrens Youxue Zhang 《Contributions to Mineralogy and Petrology》2009,158(4):471-484
Diffusion couple experiments with wet half (up to 4.6 wt%) and dry half were carried out at 789–1,516 K and 0.47–1.42 GPa to investigate water diffusion in a peralkaline rhyolitic melt with major oxide concentrations matching Mount Changbai rhyolite. Combining data from this work and a related study, total water diffusivity in peralkaline rhyolitic melt can be expressed as: where D is in m2 s?1, T is the temperature in K, P is the pressure in GPa, and X is the mole fraction of water and calculated as X = (C/18.015)/(C/18.015 + (100 ? C)/33.14), where C is water content in wt%. We recommend this equation in modeling bubble growth and volcanic eruption dynamics in peralkaline rhyolitic eruptions, such as the ~1,000-ad eruption of Mount Changbai in North East China. Water diffusivities in peralkaline and metaluminous rhyolitic melts are comparable within a factor of 2, in contrast with the 1.0–2.6 orders of magnitude difference in viscosities. The decoupling of diffusivity of neutral molecular species from melt viscosity, i.e., the deviation from the inversely proportional relationship predicted by the Stokes–Einstein equation, might be attributed to the small size of H2O molecules. With distinct viscosities but similar diffusivity, bubble growth controlled by diffusion in peralkaline and metaluminous rhyolitic melts follows similar parabolic curves. However, at low confining pressure or low water content, viscosity plays a larger role and bubble growth rate in peralkaline rhyolitic melt is much faster than that in metaluminous rhyolite.
相似文献
$ D_{{{\text{H}}_{ 2} {\text{O}}_{\text{t}} }} = D_{{{\text{H}}_{ 2} {\text{O}}_{\text{m}} }} \left( {1 - \frac{0.5 - X}{{\sqrt {[4\exp (3110/T - 1.876) - 1](X - X^{2} ) + 0.25} }}} \right), $
$ {\text{with}}\;D_{{{\text{H}}_{ 2} {\text{O}}_{\text{m}} }} = \exp \left[ { - 1 2. 7 8 9- \frac{13939}{T} - 1229.6\frac{P}{T} + ( - 27.867 + \frac{60559}{T})X} \right], $
134.
Stochastic Structural Modeling 总被引:3,自引:0,他引:3
Lars Holden Petter Mostad Bjørn Fredrik Nielsen Jon Gjerde Chris Townsend Signe Ottesen 《Mathematical Geology》2003,35(8):899-914
A consistent stochastic model for faults and horizons is described. The faults are represented as a parametric invertible deformation operator. The faults may truncate each other. The horizons are modeled as correlated Gaussian fields and are represented in a grid. Petrophysical variables may be modeled in a reservoir before faulting in order to describe the juxtaposition effect of the faulting. It is possible to condition the realization on petrophysics, horizons, and fault plane observations in wells in addition to seismic data. The transmissibility in the fault plane may also be included in the model. Four different methods to integrate the fault and horizon models in a common model is described. The method is illustrated on an example from a real petroleum field with 18 interpreted faults that are handled stochastically. 相似文献
135.
Lasse Sander Mikkel U. Hede Mikkel Fruergaard Lars Nielsen Lars B. Clemmensen Aart Kroon Peter N. Johannessen Lars H. Nielsen Morten Pejrup 《地学学报》2016,28(1):43-49
Coastal lagoons and beach ridges are genetically independent, though non‐continuous, sedimentary archives. We here combine the results from two recently published studies in order to produce an 8000‐year‐long record of Holocene relative sea‐level changes on the island of Samsø, southern Kattegat, Denmark. The reconstruction of the initial mid‐Holocene sea‐level rise is based on the sedimentary infill from topography‐confined coastal lagoons (Sander et al., Boreas, 2015b). Sea‐level index points over the mid‐ to late Holocene period of sea‐level stability and fall are retrieved from the internal structures of a wide beach‐ridge system (Hede et al., The Holocene, 2015). Data from sediment coring, georadar and absolute dating are thus combined in an inter‐disciplinary approach that is highly reproducible in micro‐tidal environments characterised by high sediment supply. We show here that the commonly proximate occurrence of coastal lagoons and beach ridges allows us to produce seamless time series of relative sea‐level changes from field sites in SW Scandinavia and in similar coastal environments. 相似文献
136.
Javier Castro-Larragoitia Utz Kramar Harald Puchelt 《Journal of Geochemical Exploration》1997,58(1):81-91
The Ag---Pb---Zn---Cu---Au mining district of Santa María de La Paz has been extensively exploited for approximately 200 years. Consequences of these activities are several deposits of tailings with high As and heavy metal concentrations, which are completely unstable. The climate is semiarid and as the dumps have no protective cover, material from the dumps is dispersed by strong winds. It is also washed out during seasonally heavy rainfalls. By these processes approximately 100 km2 of surrounding have been contaminated by dump material. The As and heavy metal content of the soils was determined as well as their level in crops (Zea Maize) from agricultural lands in the vicinity of the dumps. In the direction of prevailing winds concentrations up to 1000 ppm Zn, 400 ppm Pb, 16 ppm Cd, 550 ppm Cu and 300 As have been detected in top soils. Using fuzzy cluster analysis the different contamination sources could be identified. Grains of corn from contaminated sites showed no critical concentrations, but leaves which are also used tor fodder, have As-concentrations up to 20 ppm. 相似文献
137.
138.
Lars E. Sjöberg 《Journal of Geodesy》2009,83(10):967-972
The topographic bias is defined as the error/bias committed by continuing the external gravity field inside the topographic
masses by a harmonic function. We study the topographic bias given by a digital terrain model defined by a spherical template,
and we show that the topographic bias is given only by the potential of an inner-zone cap, and it equals the bias of the Bouguer
shell, independent of the size of the cap. Then we study the effect on the real Earth by decomposing its topography into a
template, and we show also in this case that the topographic bias is that of the Bouguer shell, independent of the shape of
the terrain. Finally, we show that the topographic potential of the terrain at the geoid can be determined to any precision
by a Taylor expansion outside the Earth’s surface. The last statement is demonstrated by a Taylor expansion to fourth order. 相似文献
139.
H2O diffusion in dacitic melt was investigated at 0.48-0.95 GPa and 786-893 K in a piston-cylinder apparatus. The diffusion couple design was used, in which a nominally dry dacitic glass makes one half and is juxtaposed with a hydrous dacitic glass containing up to ∼8 wt.% total water (H2Ot). H2O concentration profiles were measured on quenched glasses with infrared microspectroscopy. The H2O diffusivity in dacite increases rapidly with water content under experimental conditions, similar to previous measurements at the same temperature but at pressure <0.15 GPa. However, compared with the low-pressure data, H2O diffusion at high pressure is systematically slower. H2O diffusion profiles in dacite can be modeled by assuming molecular H2O (H2Om) is the diffusing species. Total H2O diffusivity DH2Ot within 786-1798 K, 0-1 GPa, and 0-8 wt.% H2Ot can be expressed as: where DH2Ot is in m2/s, T is temperature in K, P is pressure in GPa, K = exp(1.49 − 2634/T) is the equilibrium constant of speciation reaction (H2Om+O?2OH) in the melt, X = C/18.015/[C/18.015 + (100 − C)/33.82], C is wt.% of H2Ot, and 18.015 and 33.82 g/mol correspond to the molar masses of H2O and anhydrous dacite on a single oxygen basis. Compared to H2O diffusion in rhyolite, diffusivity in dacite is lower at intermediate temperatures but higher at superliquidus temperatures. This general H2O diffusivity expression can be applied to a broad range of geological conditions, including both magma chamber processes and volcanic eruption dynamics from conduit to the surface. 相似文献
140.
Modeling path delays in the neutral atmosphere for the analysis of Very Long Baseline Interferometry (VLBI) observations has
been improved significantly in recent years by the use of elevation-dependent mapping functions based on data from numerical
weather models. In this paper, we present a fast way of extracting both, hydrostatic and wet, linear horizontal gradients
for the troposphere from data of the European Centre for Medium-range Weather Forecasts (ECMWF) model, as it is realized at
the Vienna University of Technology on a routine basis for all stations of the International GNSS (Global Navigation Satellite
Systems) Service (IGS) and International VLBI Service for Geodesy and Astrometry (IVS) stations. This approach only uses information
about the refractivity gradients at the site vertical, but no information from the line-of-sight. VLBI analysis of the CONT02
and CONT05 campaigns, as well as all IVS-R1 and IVS-R4 sessions in the first half of 2006, shows that fixing these a priori
gradients improves the repeatability for 74% (40 out of 54) of the VLBI baseline lengths compared to fixing zero or constant
a priori gradients, and improves the repeatability for the majority of baselines compared to estimating 24-h offsets for the
gradients. Only if 6-h offsets are estimated, the baseline length repeatabilities significantly improve, no matter which a
priori gradients are used. 相似文献