首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   399篇
  免费   5篇
  国内免费   5篇
测绘学   14篇
大气科学   12篇
地球物理   75篇
地质学   187篇
海洋学   27篇
天文学   76篇
综合类   2篇
自然地理   16篇
  2023年   1篇
  2021年   2篇
  2020年   2篇
  2019年   7篇
  2018年   7篇
  2017年   2篇
  2016年   11篇
  2015年   9篇
  2014年   11篇
  2013年   27篇
  2012年   17篇
  2011年   26篇
  2010年   27篇
  2009年   33篇
  2008年   25篇
  2007年   16篇
  2006年   16篇
  2005年   15篇
  2004年   19篇
  2003年   20篇
  2002年   16篇
  2001年   7篇
  2000年   6篇
  1999年   8篇
  1998年   6篇
  1997年   6篇
  1996年   6篇
  1995年   5篇
  1994年   4篇
  1993年   2篇
  1992年   1篇
  1991年   4篇
  1990年   5篇
  1989年   3篇
  1988年   3篇
  1987年   1篇
  1986年   4篇
  1985年   2篇
  1984年   3篇
  1983年   3篇
  1982年   4篇
  1981年   2篇
  1980年   5篇
  1979年   1篇
  1976年   2篇
  1975年   1篇
  1974年   2篇
  1973年   1篇
  1970年   1篇
  1969年   2篇
排序方式: 共有409条查询结果,搜索用时 671 毫秒
91.
Modelling blue and green water resources availability in Iran   总被引:7,自引:0,他引:7  
Knowledge of the internal renewable water resources of a country is strategic information which is needed for long‐term planning of a nation's water and food security, among many other needs. New modelling tools allow this quantification with high spatial and temporal resolution. In this study we used the program Soil and Water Assessment Tool (SWAT) in combination with the Sequential Uncertainty Fitting program (SUFI‐2) to calibrate and validate a hydrologic model of Iran based on river discharges and wheat yield, taking into consideration dam operations and irrigation practices. Uncertainty analyses were also performed to assess the model performance. The results were quite satisfactory for most of the rivers across the country. We quantified all components of the water balance including blue water flow (water yield plus deep aquifer recharge), green water flow (actual and potential evapotranspiration) and green water storage (soil moisture) at sub‐basin level with monthly time‐steps. The spatially aggregated water resources and simulated yield compared well with the existing data. The study period was 1990–2002 for calibration and 1980–1989 for validation. The results show that irrigation practices have a significant impact on the water balances of the provinces with irrigated agriculture. Concerning the staple food crop in the country, 55% of irrigated wheat and 57% of rain‐fed wheat are produced every year in water‐scarce regions. The vulnerable situation of water resources availability has serious implications for the country's food security, and the looming impact of climate change could only worsen the situation. This study provides a strong basis for further studies concerning the water and food security and the water resources management strategies in the country and a unified approach for the analysis of blue and green water in other arid and semi‐arid countries. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   
92.
The dynamics of sediment transport in the East Frisian Wadden Sea are important for the coastal zone and for ecosystem functioning. The tidal inlets between the East Frisian islands connect the back-barrier intertidal flats to the North Sea. Here, concentrations of suspended particulate matter (SPM) in the water column are highly variable, depending on weather conditions and tides. In order to estimate the nature and quantity of sediment transport, in situ measurements were carried out at a Time Series Station in the tidal inlet between the islands of Spiekeroog and Langeoog. This study shows the suitability of multispectral transmissometry (MST) for obtaining long-term SPM measurements with high resolution. The comparability of this technique to the standard filter method and the laser diffraction method [laser in situ scattering and transmissometry (LISST)] is demonstrated. In addition, the Junge coefficients derived from both MST and LISST measurements are compared. A time series of SPM data covering nearly 4 months is presented. As a major result, the data reveal that a single storm surge can have less impact on SPM dynamics than longer-lasting gales. This high-resolution long-term data set is very valuable for modelling suspended matter flux. It also provides background information for studying the influence of SPM dynamics on coastal sediments.  相似文献   
93.
94.
Three node-centered finite volume discretizations for multiphase porous media flow are presented and compared. By combination of these methods two additional discretization methods are generated. The ability of these schemes to describe flows at textural interfaces of different geologic formations is investigated. It was found that models with nonzero-entry pressures for the capillary pressure-saturation relationship in conjunction with the Box discretization may give rise to spurious oscillations for flows around low permeable lenses. Furthermore, the applicability and sensitivity of the discretization methods with regard to the used computational grids is discussed. The schemes are used for the numerical study of two-phase flow in porous media with zones of different material properties. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
95.
96.
We have mapped U (238U) and Th (232Th) elemental concentrations as well as U-series isotope distributions in a Neanderthal tooth from the Middle Palaeolithic site of Payre using laser ablation ICP-MS. The U-concentrations in an enamel section varied between 1 and 1500 ppb. The U-concentration maps show that U-migration through the external enamel surface is minute, the bulk of the uranium having migrated internally via the dentine into the enamel. The uranium migration and uptake is critically dependent on the mineralogical structure of the enamel. Increased U-concentrations are observed along lineaments, some of which are associated with cracks, and others may be related to intra-prismatic zones or structural weaknesses reaching from the dentine into the enamel. The uranium concentrations in the dentine vary between about 25,000 and 45,000 ppb. Our systematic mapping of U-concentration and U-series isotopes provides insight into the time domain of U-accumulation. Most of the uranium was accumulated in an early stage of burial, with some much later overprints. None of the uranium concentration and U-series profiles across the root of the tooth complied with a single stage diffusion-adsorption (D-A) model that is used for quality control in U-series dating of bones and teeth. Nevertheless, in the domains that yielded the oldest apparent U-series age estimates, U-leaching could be excluded. This means that the oldest apparent U-series ages of around 200 ka represent a minimum age for this Neanderthal specimen. This is in good agreement with independent age assessments (200-230 ka) for the archaeological layer, in which it was found.The Th elemental concentrations in the dental tissues were generally low (between about 1 and 20 ppb), and show little relationship with the nature of the tissue.  相似文献   
97.
Corrosion of steel canisters, stored in a repository for spent fuel and high-level nuclear wastes, leads to the generation and accumulation of H2 gas in the backfilled emplacement tunnels, which may significantly affect long-term repository safety. Previous studies have used H2 generation rates based on the volume of the waste or canister material and the stoichiometry of the corrosion reaction. However, Fe corrosion and H2 generation rates vary with time, depending on factors such as amount of Fe, water availability, water contact area and aqueous and solid chemistry. To account for these factors and feedback mechanisms, a chemistry model was developed related to Fe corrosion, coupled with two-phase (liquid and gas) flow phenomena that are driven by gas-pressure buildup associated with H2 generation and water consumption. Results indicate that by dynamically calculating H2 generation rates based on a simple model of corrosion chemistry, and by coupling this corrosion reaction with two-phase flow processes, the degree and extent of gas-pressure buildup could be much smaller compared to a model that neglects the coupling between flow and reactive transport mechanisms. By considering the feedback of corrosion chemistry, the gas pressure increases initially at the canister, but later decreases and eventually returns to a stabilized pressure that is slightly higher than the background pressure. The current study focuses on corrosion under anaerobic conditions for which the coupled hydrogeochemical model was used to examine the role of selected physical parameters on H2 gas generation and corresponding pressure buildup in a nuclear waste repository. The developed model can be applied to evaluate the effect of water and mineral chemistry of the buffer and host rock on the corrosion reaction for future site-specific studies.  相似文献   
98.
Mine tailings are ubiquitous in the landscapes of mined areas. Metal solubilities were compared in two chemically distinct mine tailings from the old Mining District of Cartagena-La Unión (SE Spain). One of the tailings was acidic (pH 3.0) with 5400 mg/kg Zn, 1900 mg/kg As and 7000 mg/kg Pb. The other was neutral (pH 7.4) with 9100 mg/kg Zn, 5200 mg/kg Pb and 350 mg/kg As. In samples from the acidic tailings, more than 15% of the Zn and 55% of the Cd were extractable with 0.1 M NaNO3, and distilled water. In the neutral tailings, using the same reagents, less than 1% of the metals were extractable. A sequential extraction procedure revealed that the sum of the residual and the Fe oxide fractions of Cu, Zn and Pb comprised 80–95% in the acidic tailings and 70–90% in the neutral tailings. The acidic mine tailings had a higher metal solubility, resulting in more metal leaching in the short-term, but also a higher fraction of inert metal. In contrast, in the neutral tailings, the metals were evenly distributed between, oxides and the residual fraction. This implies lower metal mobility in the short-term, but that metal mobility may increase in the long-term. When applied to mine tailings, sequential extractions may provide misleading results because the strong cation exchange capacity of some extractants may induce pH changes and thereby significantly change metal solubility.  相似文献   
99.
The content and distribution of ΣPCB (as the sum of 24 individual components), HCB and p,p′-DDE was determined in sediments taken from the North Sea during two seasons. The mass-balance of aromatic organochlorine compounds in the upper 2 cm of sediments is calculated and dicussed in relation to pollutant dispersal.  相似文献   
100.
Cr(III) sorbed at the solid/water interface of latex and hectorite was studied by ESR spectroscopy in the presence of different inorganic ligands. The ESR spectra of the surface obtained in the presence of selenite, phosphate and fluoride can be explained in terms of ternary surface complex formation. This is contrasted by the behaviour of sulfate and selenate ions which were found to have no effect on the ligand field of Cr(III), either in the adsorbed state or in solution.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号