全文获取类型
收费全文 | 13216篇 |
免费 | 2937篇 |
国内免费 | 3390篇 |
专业分类
测绘学 | 570篇 |
大气科学 | 2761篇 |
地球物理 | 4420篇 |
地质学 | 7052篇 |
海洋学 | 2043篇 |
天文学 | 156篇 |
综合类 | 724篇 |
自然地理 | 1817篇 |
出版年
2024年 | 45篇 |
2023年 | 156篇 |
2022年 | 381篇 |
2021年 | 482篇 |
2020年 | 541篇 |
2019年 | 652篇 |
2018年 | 576篇 |
2017年 | 601篇 |
2016年 | 581篇 |
2015年 | 644篇 |
2014年 | 926篇 |
2013年 | 1088篇 |
2012年 | 841篇 |
2011年 | 928篇 |
2010年 | 760篇 |
2009年 | 998篇 |
2008年 | 1031篇 |
2007年 | 1040篇 |
2006年 | 996篇 |
2005年 | 746篇 |
2004年 | 736篇 |
2003年 | 618篇 |
2002年 | 578篇 |
2001年 | 482篇 |
2000年 | 455篇 |
1999年 | 431篇 |
1998年 | 371篇 |
1997年 | 334篇 |
1996年 | 293篇 |
1995年 | 222篇 |
1994年 | 207篇 |
1993年 | 158篇 |
1992年 | 142篇 |
1991年 | 105篇 |
1990年 | 92篇 |
1989年 | 57篇 |
1988年 | 63篇 |
1987年 | 37篇 |
1986年 | 25篇 |
1985年 | 23篇 |
1984年 | 15篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 12篇 |
1980年 | 11篇 |
1979年 | 6篇 |
1978年 | 17篇 |
1977年 | 8篇 |
1976年 | 6篇 |
1954年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Rashid Umar M. Muqtada A. Khan Izrar Ahmed Shakeel Ahmed 《Journal of Earth System Science》2008,117(1):69-78
The Kali-Hindon inter-stream region extends over an area of 395 km2 within the Ganga-Yamuna interfluve. It is a fertile tract for sugarcane cultivation. Groundwater is a primary resource for
irrigation and industrial purposes. In recent years, over-exploitation has resulted in an adverse impact on the groundwater
regime. In this study, an attempt has been made to calculate a water balance for the Kali-Hindon inter-stream region. Various
inflows and outflows to and from the aquifer have been calculated. The recharge due to rainfall and other recharge parameters
such as horizontal inflow, irrigation return flow and canal seepage were also evaluated. Groundwater withdrawals, evaporation
from the water table, discharge from the aquifer to rivers and horizontal subsurface outflows were also estimated. The results
show that total recharge into the system is 148.72 million cubic metres (Mcum), whereas the total discharge is 161.06 Mcum,
leaving a deficit balance of −12.34 Mcum. Similarly, the groundwater balance was evaluated for the successive four years.
The result shows that the groundwater balance is highly sensitive to variation in rainfall followed by draft through pumpage.
The depths to water level are shallow in the canal-irrigated northern part of the basin and deeper in the southern part. The
pre-monsoon and post-monsoon water levels range from 4.6 to 17.7 m below ground level (bgl) and from 3.5 to 16.5 m bgl respectively.
It is concluded that the groundwater may be pumped in the canal-irrigated northern part, while withdrawals may be restricted
to the southern portion of the basin, where intense abstraction has led to rapidly falling water table levels. 相似文献
52.
华北克拉通破坏与岩石圈减薄 总被引:20,自引:2,他引:18
古太古代(约4.0 Ga)时地球上可能只有一个超级大陆, 它的岩石圈厚度高达400 km。在早元古代,这个超级大陆减薄、裂解成十几块,每块中心是太古宙岩石,边缘是元古宙岩石,且各块厚度不等(150~350km)。从元古宙之后这些被称之为稳定克拉通的大陆岩石圈就一直漂游在地幔软流圈之上。中国华北地块就是这些克拉通之一,与众不同的是它在中生代时遭受了第二次破坏,岩石圈厚度从古生代时的180~200 km 减少到现今的80~100 km。本文作者从流变学的视角出发,围绕华北克拉通破坏和岩石圈减薄这一核心问题,从 相似文献
53.
Delineation of groundwater protection zones by the backward particle tracking method: theoretical background and GIS-based stochastic analysis 总被引:1,自引:0,他引:1
Konstadinos N. Moutsopoulos Alexandra Gemitzi Vassilios A. Tsihrintzis 《Environmental Geology》2008,54(5):1081-1090
The backward particle tracking method, an effective and powerful tool that can be used to delineate groundwater protection
zones, is presented. The theoretical background and insights on the applicability of this method are provided. Moreover, the
present work enriches the backward particle tracking method with an uncertainty analysis concerning the porosity values, applying
a Monte Carlo (MC) approach, coupled with the use of geographical information systems (GIS). As an application example, a
wellfield in the Komotini area, Greece, is investigated. The present study may serve as a potential guideline for wellfield
delineation, particularly in areas like Greece where lack of data related to the hydrogeological system is often a problem. 相似文献
54.
The carbonate-rock aquifer in the Great Valley, West Virginia, USA, was evaluated using a database of 687 sinkholes and 350
specific capacity tests to assess structural, lithologic, and topographic influences on the groundwater flow system. The enhanced
permeability of the aquifer is characterized in part by the many sinkholes, springs, and solutionally enlarged fractures throughout
the valley. Yet, vertical components of subsurface flow in this highly heterogeneous aquifer are currently not well understood.
To address this problem, this study examines the apparent relation between geologic features of the aquifer and two spatial
indices of enhanced permeability attributed to aquifer karstification: (1) the distribution of sinkholes and (2) the occurrence
of wells with relatively high specific capacity. Statistical results indicate that sinkholes (funnel and collapse) occur primarily
along cleavage and bedding planes parallel to subparallel to strike where lateral or downward vertical gradients are highest.
Conversely, high specific capacity values are common along prominent joints perpendicular or oblique to strike. The similarity
of the latter distribution to that of springs suggests these fractures are areas of upward-convergent flow. These differences
between sinkhole and high specific capacity distributions suggest vertical flow components are primarily controlled by the
orientation of geologic structure and associated subsurface fracturing. 相似文献
55.
On CO<Subscript>2</Subscript> fluid flow and heat transfer behavior in the subsurface,following leakage from a geologic storage reservoir 总被引:1,自引:0,他引:1
Karsten Pruess 《Environmental Geology》2008,54(8):1677-1686
Geologic storage of CO2 is expected to produce plumes of large areal extent, and some leakage may occur along fractures, fault zones, or improperly
plugged pre-existing wellbores. A review of physical and chemical processes accompanying leakage suggests a potential for
self-enhancement. The numerical simulations presented here confirm this expectation, but reveal self-limiting features as
well. It seems unlikely that CO2 leakage could trigger a high-energy run-away discharge, a so-called “pneumatic eruption,” but present understanding is insufficient
to rule out this possibility. The most promising avenue for increasing understanding of CO2 leakage behavior is the study of natural analogues. 相似文献
56.
Marcel Hürlimann Dieter Rickenmann Vicente Medina Allen Bateman 《Engineering Geology》2008,102(3-4):152
Many different runout prediction methods can be applied to estimate the mobility of future debris flows during hazard assessment. The present article reviews the empirical, analytical, simple flow routing and numerical techniques. All these techniques were applied to back-calculate a debris flow, which occurred in 1982 at La Guingueta catchment, in the Eastern Pyrenees. A sensitivity analysis of input parameters was carried out, while special attention was paid to the influence of rheological parameters. We used the Voellmy fluid rheology for our analytical and numerical modelling, since this flow resistance law coincided best with field observations. The simulation results indicated that the “basal” friction coefficients rather affect the runout distance, while the “turbulence” terms mainly influence flow velocity. A comparison of the velocity computed on the fan showed that the analytical model calculated values similar to the numerical ones. The values of our rheological parameters calibrated at La Guingueta agree with data back-calculated for other debris flows. Empirical relationships represent another method to estimate total runout distance. The results confirmed that they contain an important uncertainty and they are strictly valid only for the conditions, which were the basis for their development. With regards to the simple flow routing algorithm, this methods could satisfactorily simulate the total area affected by the 1982 debris flow, but it was not able to directly calculate total runout distance and velocity. Finally, a suggestion on how different runout prediction methods can be applied to generate debris-flow hazard maps is presented. Taking into account the definition of hazard and intensity, the best choice would be to divide the resulting hazard maps into two types: “final hazard maps” and “preliminary hazard maps”. Only the use of numerical models provided final hazard maps, because they could incorporate different event magnitudes and they supplied output-values for intensity calculation. In contrast, empirical relationships and flow routing algorithms, or a combination of both, could be applied to create preliminary hazard maps. The present study only focussed on runout prediction methods. Other necessary tasks to complete the hazard assessment can be looked up in the “Guidelines for landslide susceptibility, hazard and risk zoning” included in this Special Issue. 相似文献
57.
Assessment of disaster resilience capacity of hillslope communities with high risk for geological hazards 总被引:2,自引:0,他引:2
Su-Chin Chen Jhy-Wei Ferng Yu-Ting Wang Ting-Yeh Wu Jieh-Jiuh Wang 《Engineering Geology》2008,98(3-4):86-101
This study presents a novel preparedness assessment method for assessing hazard mitigation and environmental planning of hillslope communities. A professional questionnaire was utilized to weight each indicator. Communities in Hsinchu, Taichung and Nantou counties with debris flow hazards were taken as study samples. Debris flow risk and landslide susceptibility for each community were determined using Geographic Information System (GIS) technology and logistic regression analysis. Thus, a novel risk assessment method for evaluating disaster resilience capacity of hillslope communities was established. This method was then applied to assess casualties caused by Typhoon Herb in 1996 and Typhoon Mindulle in 2004. Additionally, the analytical results generated by this assessment method were discussed with the aim of developing references for implementation of risk analysis, increasing the effectiveness of disaster mitigation, and reducing future loss of life and property. 相似文献
58.
S. T. G. Raghukanth 《Natural Hazards》2008,46(1):1-13
In this article, analytical methods have been used to estimate ground motion during the 8 October 2005, Kashmir earthquake.
Peak ground acceleration (PGA) values at several stations in the epicentral region have been estimated by empirical analytical
source mechanism models. As an alternate analysis, PGA estimates have also been obtained using the stochastic finite fault
seismological model. The estimated PGAs are compared with that obtained from damage values. A PGA contour map in the near-source
region is provided. It is found that very near to the epicenter, PGA would have reached more than 1 g. It is demonstrated
that empirical analytical models can be effectively used to estimate ground motion due to rupture of active faults. 相似文献
59.
将稳定同位素效应引入CLM(Community Land Model),并对巴西马瑙斯站在平衡年的稳定水同位素的逐日变化进行模拟和分析.结果表明: 降水、水汽和地表径流中δ18O存在明显的季节变化,并与相应的水量存在显著的负相关关系,但凝结物中δ18O与地面凝结量存在显著的正相关关系,蒸发水汽中δ18O与蒸发量之间无显著的相关关系.受土壤贮水削峰功能的影响,表层土壤和根区水中δ18O的季节变化全无.植被层蒸发水汽中稳定同位素的丰度与大气的干湿程度存在密切联系: 当降水量少时,大气干燥,植被层的蒸发较少,植被蒸发中δ18O较高;当降水量较大时,空气湿润,植被层的蒸发量较大,蒸发中δ18O则较低.植被蒸腾中δ18O的变化与源区水体中δ18O的变化保持一致,尤其是与根区水中的δ18O.由于地下径流直接源自根区水的补充,因此,地下径流中δ18O等于根区水中的δ18O.模拟结果还显示,降水MWL (大气水线)的梯度项和常数项均比全球平均MWL略偏小.尽管主要来自降水的贡献,但地表径流和植被层水体的MWLs与降水MWL存在较大的差异,这一方面与两类水体在蒸发过程中的稳定同位素的富集作用有关,另一方面与CLM模拟的水量有关.大气水汽线与降水的MWL的梯度值相近,说明大气水汽与降水近似处于稳定同位素平衡状态.另外,模拟的地面的凝结线与植被层的凝结线均与全球大气水线相近,且具有非常高相关程度,说明CLM的模拟是合理的. 相似文献
60.