全文获取类型
收费全文 | 1001篇 |
免费 | 62篇 |
国内免费 | 13篇 |
专业分类
测绘学 | 50篇 |
大气科学 | 160篇 |
地球物理 | 222篇 |
地质学 | 432篇 |
海洋学 | 47篇 |
天文学 | 91篇 |
综合类 | 2篇 |
自然地理 | 72篇 |
出版年
2023年 | 6篇 |
2022年 | 7篇 |
2021年 | 23篇 |
2020年 | 29篇 |
2019年 | 24篇 |
2018年 | 54篇 |
2017年 | 30篇 |
2016年 | 54篇 |
2015年 | 35篇 |
2014年 | 51篇 |
2013年 | 67篇 |
2012年 | 54篇 |
2011年 | 64篇 |
2010年 | 54篇 |
2009年 | 81篇 |
2008年 | 60篇 |
2007年 | 41篇 |
2006年 | 50篇 |
2005年 | 39篇 |
2004年 | 39篇 |
2003年 | 33篇 |
2002年 | 16篇 |
2001年 | 18篇 |
2000年 | 16篇 |
1999年 | 13篇 |
1998年 | 18篇 |
1997年 | 13篇 |
1996年 | 7篇 |
1995年 | 9篇 |
1994年 | 5篇 |
1993年 | 8篇 |
1992年 | 5篇 |
1991年 | 6篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1987年 | 6篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1974年 | 1篇 |
1971年 | 1篇 |
1970年 | 1篇 |
1969年 | 5篇 |
1968年 | 3篇 |
1967年 | 4篇 |
1964年 | 1篇 |
排序方式: 共有1076条查询结果,搜索用时 31 毫秒
581.
582.
The objective of this study is to ascertain the relative importance of lithological controls and geomorphological processes in the development of Drakensberg basalt terraces. Various hypotheses for terrace formation are considered, including geological controls, macroscale geomorphology, and climatic–geomorphological controls. The variations in strength and relative age differences for scarp surface exposures on two slopes of varying aspect are determined, so that a comparison can be made between various scarp outcrops and relative rates of weathering. Scarp outcrops were measured for their rock mass strength, rock surface roughness, and percentage lichen cover. The ethylene glycol test was performed on prepared rock samples to determine susceptibility to tensional breakup. A satellite image depicting the distribution of late-lying contemporary snow was used to assist in the construction of topo-climatic linkages with scarp terrace localities. Findings show that terraces are most common on southeast-facing aspects, and coincide with the major joint strike direction. Lithological factors are thus considered the primary control to such terrace development. It is suggested that a different set of geomorphological processes operates on various slope-altitudinal and slope-orientational positions at any given time. 相似文献
583.
584.
Palsa mires are mire complexes that occur in the Northern Hemisphere, representing one of the most marginal permafrost features at the outer limit of the permafrost zone. A climate‐based spatial model is presented for the distribution of palsa mires in northern Europe. The model is based on an extensive spatial data of palsa mires and climatological variables from 1913 grid cells in an area of c. 240 000 km2. Generalized linear modelling (GLM) with curvilinear and interaction terms is used to derive the palsa mire–climate relationships. The ?nal model correctly classi?ed 77·6 per cent of the palsa mire presence squares. The results indicate a positive association of the distribution of palsa mires with increasing frost number and continentality, whereas precipitation and temperature showed a negative correlation with the distribution of palsa mires. Additionally, interaction of thawing degree days and summer time precipitation showed a negative association. Climatologically, the optimum areas of palsa mires occur in areas of low precipitation (<450 mm) and a mean annual temperature between ?3 °C and ?5 °C. Potential reasons for the performance of the model and the sensitivity of palsa mires to climate change are discussed. The application of a GIS‐based generalized linear modelling as used here provides a versatile method to study the distribution of different geomorphological phenomena across climatological gradients. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
585.
Tectonic map and overall architecture of the Alpine orogen 总被引:9,自引:0,他引:9
586.
The monthly, seasonal, and annual precipitation trends in the Yangtze river catchment have been detected through analysis of 51 meteorological stations' data between 1950-2002 provided by National Meteorological Administration. Results reveal that: 1) Summer precipitation in the Yangtze river catchment shows significant increasing tendency. The Poyanghu lake basin, Dongtinghu lake basin and Taihu lake basin in the middle and lower reaches are the places showing significant positive trends. Summer precipitation in the middle and lower reaches experienced an abrupt change in the year 1992; 2) The monthly precipitation in months just adjoining to summer shows decreasing tendency in the Yangtze river catchment. The upper and middle reaches in Jialingjiang river basin and Hanshui river basin are the places showing significant negative trends; 3) Extreme precipitation events show an increasing tendency in most places, especially in the middle and lower reaches of the Yangtze river catchment. 相似文献
587.
Analytical solutions of drawdown in unconfined aquifers are widely applied for determining the specific yield, Sy, and the horizontal and the vertical hydraulic conductivity Kr and Kz, respectively. In many previous studies, estimates of Sy and Kz were observed to be highly variable and physically unrealistic. This has been attributed to the conceptualization of flow above the declining water table and aquifer heterogeneity in the applied models. We present the analysis of time-drawdown data from a pumping test instrumented with depth-differentiated observation piezometers arranged in clusters. Applying homogeneous anisotropic aquifer models in combination with nonlinear least squares parameter identification techniques, the data were analyzed in different groups: analysis of data from individual piezometer clusters and simultaneous analysis of the entire data set from all piezometer clusters (global analysis). From the cluster analyses, estimates of Sy and Kz exhibit large variances and depart from a priori estimates inferred from the hydrostratigraphy. Parameter estimates from the global analysis do not fall within the parameter bounds (minimum and maximum values) defined by the cluster analyses. While heterogeneity appears to be the important reason for large parameter variances, we discuss the influence of rarely considered aquifer return flow on drawdown and the inconsistent results from the cluster and global analyses. We corroborate our findings with data on hydraulic gradients, slug test data, and results from the application of a more realistic numerical flow model. 相似文献
588.
Iron isotope fractionation during planetary differentiation 总被引:4,自引:0,他引:4
Stefan Weyer Ariel D. Anbar Carsten Münker Alan B. Woodland 《Earth and Planetary Science Letters》2005,240(2):251-264
The Fe isotope composition of samples from the Moon, Mars (SNC meteorites), HED parent body (eucrites), pallasites (metal and silicate) and the Earth's mantle were measured using high mass resolution MC-ICP-MS. These high precision measurements (δ56Fe ≈ ± 0.04‰, 2 S.D.) place tight constraints on Fe isotope fractionation during planetary differentiation.Fractionation during planetary core formation is confined to < 0.1‰ for δ56Fe by the indistinguishable Fe isotope composition of pallasite bulk metal (including sulfides and phosphides) and olivine separates. However, large isotopic variations (≈ 0.5‰) were observed among pallasite metal separates, varying systematically with the amounts of troilite, schreibersite, kamacite and taenite. Troilite generally has the lightest (δ56Fe ≈ − 0.25‰) and schreibersite the heaviest (δ56Fe ≈ + 0.2‰) Fe isotope composition. Taenite is heavier then kamacite. Therefore, these variations probably reflect Fe isotope fractionation during the late stage evolution and differentiation of the S- and P-rich metal melts, and during low-temperature kamacite exsolution, rather than fractionation during silicate-metal separation.Differentiation of the silicate portion of planets also seems to fractionate Fe isotopes. Notably, magmatic rocks (partial melts) are systematically isotopically heavier than their mantle protoliths. This is indicated by the mean of 11 terrestrial peridotite samples from different tectonic settings (δ56Fe = + 0.015 ± 0.018‰), which is significantly lighter than the mean of terrestrial basalts (δ56Fe = + 0.076 ± 0.029‰). We consider the peridotite mean to be the best estimate for the Fe isotope composition of the bulk silicate Earth, and probably also of bulk Earth. The terrestrial basaltic mean is in good agreement with the mean of the lunar samples (δ56Fe = + 0.073 ± 0.019‰), excluding the high-Ti basalts. The high-Ti basalts display the heaviest Fe isotope composition of all rocks measured here (δ56Fe ≈ + 0.2‰). This is interpreted as a fingerprint of the lunar magma ocean, which produced a very heterogeneous mantle, including the ilmenite-rich source regions of these basalts.Within uncertainties, samples from Mars (SNC meteorites), HED (eucrites) and the pallasites (average olivine + metal) have the same Fe isotope compositions as the Earth's mantle. This indicates that the solar system is very homogeneous in Fe isotopes. Its average δ56Fe is very close to that of the IRMM-014 standard. 相似文献
589.
A parametrization of the lateral waterflow for the global scale 总被引:1,自引:2,他引:1
The representation of hydrological land surface processes has, so far, been treated inadequately in global models of the
atmospheric general circulation (GCMs). In particular the lateral waterflows from the continents into the ocean have been
described unsatisfactorily. The aim of this study is to develop a model for the lateral waterflow on the global scale which
describes the translation and retention of the lateral discharge as a function of the spatially distributed land surface characteristics
that are globally available. Here, global scale refers to the resolution of 0.5° and smaller, corresponding to a typical GCM
gridbox area of about 2500 km2. Discharge models need a number of specific input variables that are usually not available from measurements such as runoff
and drainage. Therefore, these variables have to be derived from observed data such as precipitation and surface air temperature.
For the model development in this study, a new simplified land surface scheme was applied to compute these variables. The
discharge from a catchment of approximately the size of a 0.5° gridbox was simulated using several types of models. The intercomparison
of the simulated lateral flows of the different models shows that as a minimum a separation between flow processes such as
overland flow, baseflow and riverflow is required to yield good discharge simulations. As both the retention and translation
of a flow process need to be simulated, a two-parameter model is required for the representation of overland flow and riverflow.
For the baseflow, a one-parameter model is sufficient. The resulting model structure is called the hydrological discharge model or HD model. A first parametrization approach was defined using the gridbox characteristics of topography gradient and gridbox length.
The skill of the discharge simulation depends not only on the formulations of the model, but also on the precise definition
of the boundaries of the model catchments. The sizes and the positioning of the model catchments on the globe were defined
by using a river direction file as well as a modified topography dataset. In a first application, the HD model successfully
simulated river discharge using runoff and drainage from a five year atmospheric GCM integration (ECHAM4-T42) as input. The
annual cycles of the monthly means of the simulated discharge of several large rivers were compared to the observed discharges
provided by the Global Runoff Data Centre. The HD model achieves a considerable improvement of the simulated discharge compared
to the model which is currently operational at MPI. The improved volume of the discharge is directly related to the definition
of more realistic model catchments and the improved timing of the flow is mainly due to the newly introduced separation of
the flow processes.
Received: 20 November 1996 / Accepted: 8 July 1997 相似文献
590.
The present groundwater monitoring network in Bavaria consists mostly of wells and only a small number of natural groundwater
springs, all of which are analyzed for mainly the common physical and chemical constituents in groundwater. In order to develop
a long-term groundwater management plan for all the groundwater resources of Bavaria, the Bavarian State Office for Water
Management intends to establish a separate spring-monitoring network throughout the 11 groundwater provinces of the state.
As a first step, significant physicochemical parameters that show considerable annual fluctuation (after monitoring 1–3 years)
were determined at 21 springs or spring systems to create a basic data set to guide future monitoring. A selection procedure
was developed around four parameters: (1) geological units, which includes the principal aquifers; (2) rate of spring discharge;
(3) land utilization within a catchment; and (4) approximate size of the subterranean catchment. However, in the initial phase
of the study, only the first three parameters were investigated. These parameters established a matrix for evaluating each
groundwater region of Bavaria to aid in the selection of additional springs for the proposed monitoring network.
Electronic Publication 相似文献