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851.
Theoretical and Applied Climatology - Atmospheric rivers (ARs) are channels of high water vapor flux that transport moisture from low to higher latitudes on synoptic timescales. In areas of... 相似文献
852.
853.
The contribution of the International VLBI Service for Geodesy and Astrometry (IVS) to the ITRF2005 (International Terrestrial
Reference Frame 2005) has been computed by the IVS Analysis Coordinator’s office at the Geodetic Institute of the University
of Bonn, Germany. For this purpose the IVS Analysis Centres (ACs) provided datum-free normal equation matrices in Solution
INdependent EXchange (SINEX) format for each 24 h observing session to be combined on a session-by-session basis by a stacking
procedure. In this process, common sets of parameters, transformed to identical reference epochs and a prioris, and especially representative relative weights have been taken into account for each session. In order to assess the quality
of the combined IVS files, Earth orientation parameters (EOPs) and scaling factors have been derived from the combined normal
equation matrices. The agreement of the EOPs of the combined normal equation matrices with those of the individual ACs in
terms of weighted root mean square (WRMS) is in the range of 50–60 μas for the two polar motion components and about 3 μs
for UT1−UTC. External comparisons with International GNSS Serive (IGS) polar motion components is at the level of 130–170 μas
and 21 μs/day for length of day (LOD). The scale of the terrestrial reference frame realized through the IVS SINEX files agrees
with ITRF2000 at the level of 0.2 ppb. 相似文献
854.
855.
Critical Issues in Participatory GIS: Deconstructions, Reconstructions, and New Research Directions 总被引:5,自引:0,他引:5
Sarah Elwood 《Transactions in GIS》2006,10(5):693-708
In the mid‐1990s, several critical texts raised concerns about the social, political, and epistemological implications of GIS. Subsequent responses to these critiques have fundamentally altered the technological, political, and intellectual practices of GIScience. Participatory GIS, for instance, has intervened in multiple ways to try to ameliorate uneven access to GIS and digital spatial data and diversify the forms of spatial knowledge and spatial logic that may be incorporated in a GIS. While directly addressing core elements of the ‘GIS & Society’ critique, these reconstructions of a critical GIScience introduce their own ambiguities with respect to access, equity, digital representation of spatial knowledge, and epistemologies of new GIS research practices. In this paper, I examine some of the new and persistent ambiguities of participatory GIS that bear inclusion in future critical GIScience research. 相似文献
856.
The pattern and influence of low dissolved oxygen in the Patuxent River,a seasonally hypoxic estuary
Denise?L.?BreitburgEmail author Aaron?Adamack Kenneth?A.?Rose Sarah?E.?Kolesar Beth?Decker Jennifer?E.?Purcell Julie?E.?Keister James?H.?Cowan 《Estuaries and Coasts》2003,26(2):280-297
Increased nutrient loadings have resulted in low dissolved oxygen (DO) concentrations in bottom waters of the Patuxent River,
a tributary of Chesapeake Bay. We synthesize existing and newly collected data to examine spatial and temporal variation in
bottom DO, the prevalence of hypoxia-induced mortality of fishes, the tolerance of Patuxent River biota to low DO, and the
influence of bottom DO on the vertical distributions and spatial overlap of larval fish and fish eggs with their gelatinous
predators and zooplankton prey. We use this information, as well as output from watershed-quality and water-quality models,
to configure a spatially-explicit individual-based model to predict how changing land use within the Patuxent watershed may
affect survival of early life stages of summer breeding fishes through its effect on DO. Bottom waters in much of the mesohaline
Patuxent River are below 50% DO saturation during summer. The system is characterized by high spatial and temporal variation
in DO concentrations, and the current severity and extent of hypoxia are sufficient to alter distributions of organisms and
trophic interactions in the river. Gelatinous zooplankton are among the most tolerant species of hypoxia, while several of
the ecologically and economically important finfish are among the most sensitive. This variation in DO tolerances may make
the Patuxent River, and similar estuaries, particularly susceptible to hypoxia-induced alterations in food web dynamics. Model
simulations consistently predict high mortality of planktonic bay anchovy eggs (Anchoa mitchilli) under current DO, and increasing survival of fish eggs with increasing DO. Changes in land use that reduce nutrient loadings
may either increase or decrease predation mortality of larval fish depending on the baseline DO conditions at any point in
space and time. A precautionary approach towards fisheries and ecosystem management would recommend reducing nutrients to
levels at which low oxygen effects on estuarine habitat are reduced and, where possible, eliminated. 相似文献
857.
Fatemeh Jalayer Raffaele De Risi Francesco De Paola Maurizio Giugni Gaetano Manfredi Paolo Gasparini Maria Elena Topa Nebyou Yonas Kumelachew Yeshitela Alemu Nebebe Gina Cavan Sarah Lindley Andreas Printz Florian Renner 《Natural Hazards》2014,73(2):975-1001
Identifying urban flooding risk hotspots is one of the first steps in an integrated methodology for urban flood risk assessment and mitigation. This work employs three GIS-based frameworks for identifying urban flooding risk hotspots for residential buildings and urban corridors. This is done by overlaying a map of potentially flood-prone areas [estimated through the topographic wetness index (TWI)], a map of residential areas and urban corridors [extracted from a city-wide assessment of urban morphology types (UMT)], and a geo-spatial census dataset. A maximum likelihood method (MLE) is employed for estimating the threshold used for identifying the flood-prone areas (the TWI threshold) based on the inundation profiles calculated for various return periods within a given spatial window. Furthermore, Bayesian parameter estimation is employed in order to estimate the TWI threshold based on inundation profiles calculated for more than one spatial window. For different statistics of the TWI threshold (e.g. MLE estimate, 16th percentile, 50th percentile), the map of the potentially flood-prone areas is overlaid with the map of urban morphology units, identified as residential and urban corridors, in order to delineate the urban hotspots for both UMT. Moreover, information related to population density is integrated by overlaying geo-spatial census datasets in order to estimate the number of people affected by flooding. Differences in exposure characteristics have been assessed for a range of different residential types. As a demonstration, urban flooding risk hotspots are delineated for different percentiles of the TWI value for the city of Addis Ababa, Ethiopia. 相似文献
858.
Maria Boni Rosario Terracciano Giuseppina Balassone Sarah A. Gleeson Alexander Matthews 《Mineralium Deposita》2011,46(7):707-729
Zambian willemite (Zn2SiO4) deposits occur in the metasedimentary carbonate rocks of the Proterozoic Katangan Supergroup. The most important orebodies
are located around Kabwe and contain both sulphides and willemite in dolomites of low metamorphic grade. The Star Zinc and
Excelsior prospects (Lusaka area), discovered in the early 1920s, occur in the metamorphic lithotypes of the late Proterozoic
Zambezi Supracrustal sequence, which were deposited in a transtensional basin formed during the oblique collision of the Kalahari
and Congo cratons. The deposits are hosted by the limestone and dolomitic marbles of the Cheta and Lusaka Formations. Structural
analysis indicates that several fracture sets host the deposits, which may be genetically related to the Pan-African Mwembeshi
dislocation zone (a major geotectonic boundary between the Lufilian Arc and the Zambezi Belt). In both prospects, willemite
replaces the marbles and is found along joints and fissures with open-space filling textures and locally may develop colloform
and vuggy fabrics as well. Silver as well as traces of germanium and cadmium have been detected within the willemite ore,
and lead or zinc sulphides are scarce or absent. Calcite locally replaces willemite. Willemite is associated with specular
hematite and franklinite and post-dates the Zn-spinel gahnite in the paragenesis. Genthelvite [Zn4Be3(SiO4)3S] occurs as a minor phase in irregular aggregates. The willemites from the Lusaka area, though Mn-poor, show green cathodoluminescence
colours and bright green fluorescence in short-wave UV (as the high-temperature willemites in USA). Thermometric analyses
of primary fluid inclusions in willemite yield homogenization temperatures that range from 160°C to 240°C and salinities of
8–16 wt.% equiv. NaCl. The homogenization temperatures suggest a hypogene–hydrothermal origin for the willemite concentrations.
The geochemistry of fluid inclusion leachates suggests that the hydrothermal fluids were brines derived from highly evaporated
seawater. Precise age constraints are currently lacking for the Lusaka area deposits, though the deposits are not deformed,
indicating that they post-date the Lufilian orogeny (~520 Ma). The possibility of precursor ores exists; the gahnite–franklinite–willemite
deposits could have been derived from a metamorphosed primary sulphide (or even nonsulphide) concentration that has subsequently
been completely destroyed. However, there is no real evidence of such a primary source for the willemite mineral association.
The Lusaka zinc ores may have been produced by an extensive hydrothermal system, with fluids discharging along basinal fracture
zones controlled by the pre-Pan-African rifting stage. A paragenesis similar to that of the Lusaka prospects has been proposed
to be a vector towards massive sulphide ores in several parts of the world; therefore, it is possible that these small willemite
showings in Zambia may be part of a much bigger, and still unexplored, zinc province. 相似文献
859.
Zhaohui Li Hanlie Hong Jiin-Shuh Jean Andria J. Koski Chia-Chuan Liu Selim Reza Jeffrey J. Randolph Stephan R. Kurdas Jonathan H. Friend Sarah J. Antinucci 《Environmental Earth Sciences》2011,64(3):823-831
Higher levels of arsenic in the aquifers of Chia-Nan Plain in southwestern Taiwan were attributed to the cause of Blackfoot
disease in the area half a century ago. Although extensive studies were conducted on the occurrence, speciation, mobility,
and transport of arsenic in the region, the relationship between arsenic adsorption by different aquifer materials and the
concentration and speciation of arsenic in these aquifers was poorly delineated. This study focused on characterization of
sediments in the region, as well as determination of relations between arsenic adsorption and other geochemical and clay mineralogical
properties of the sediments. The arsenic adsorption capacity was positively related to the clay minerals, Fe, and Mn contents
in the sediments. The higher arsenic adsorption capacity of the sediments served as a sink for the arsenic during its transport
from the central mountains to the coast and as a source for its release under reduced environment and in the presence of humic
substances. 相似文献
860.
Influence of rural land use on streamwater nutrients and their ecological significance 总被引:1,自引:0,他引:1
Helen P. Jarvie Paul J.A. Withers Robin Hodgkinson Adam Bates Margaret Neal Heather D. Wickham Sarah A. Harman Linda Armstrong 《Journal of Hydrology》2008,350(3-4):166-186
Concentrations and loads of N and P fractions were examined for lowland rivers, the Wye and Avon, draining a range of representative agricultural land-use types in two major UK river basins. Data collected over a 2-year period demonstrated important diffuse agricultural source contributions to N and P loads in these rivers. Ground water provided a major source of total dissolved nitrogen (TDN) loads, whereas near-surface sources provided a major contribution to total phosphorus (TP) loads. In terms of aquatic ecology, concentrations of nutrients, at times of eutrophication risk (spring and summer low flows) were of key environmental and management significance. Agricultural diffuse sources provided the major source of long-term P loads across the two basins. However, the results demonstrated the dominance of point-source contributions to TP and SRP concentrations at times of ecological risk. Point sources typically ‘tip the balance’ of dissolved inorganic P (soluble reactive P, SRP) above the 100 μg l−1 guideline value indicative of eutrophication risk. The significance of point sources for TP and SRP concentrations was shown by (a) the strong correlations between TP, SRP and B concentrations, using B as a tracer of sewage effluent, (b) the dominant contribution of SRP to TP concentrations and (c) the predominant pattern of dilution of SRP and B with flow. The clean Chalk streams draining low intensity grassland in areas of the Avon with sparse human settlement were oligotrophic and P limited with low SRP concentrations under spring and summer baseflows attributable to groundwater sources. The data provide important insights into the ecological functioning of different lowland stream systems. There was evidence of greater SRP losses and N-limitation in a stream which drains a pond system, demonstrating the importance of longer water residence times for biological nutrient uptake. 相似文献