全文获取类型
收费全文 | 19733篇 |
免费 | 2340篇 |
国内免费 | 3494篇 |
专业分类
测绘学 | 1476篇 |
大气科学 | 1304篇 |
地球物理 | 3332篇 |
地质学 | 7597篇 |
海洋学 | 2171篇 |
天文学 | 5253篇 |
综合类 | 1227篇 |
自然地理 | 3207篇 |
出版年
2024年 | 52篇 |
2023年 | 140篇 |
2022年 | 493篇 |
2021年 | 583篇 |
2020年 | 614篇 |
2019年 | 740篇 |
2018年 | 615篇 |
2017年 | 657篇 |
2016年 | 722篇 |
2015年 | 740篇 |
2014年 | 1048篇 |
2013年 | 1187篇 |
2012年 | 1188篇 |
2011年 | 1343篇 |
2010年 | 1281篇 |
2009年 | 1614篇 |
2008年 | 1475篇 |
2007年 | 1524篇 |
2006年 | 1436篇 |
2005年 | 1253篇 |
2004年 | 1048篇 |
2003年 | 930篇 |
2002年 | 759篇 |
2001年 | 672篇 |
2000年 | 607篇 |
1999年 | 538篇 |
1998年 | 458篇 |
1997年 | 289篇 |
1996年 | 251篇 |
1995年 | 204篇 |
1994年 | 225篇 |
1993年 | 207篇 |
1992年 | 120篇 |
1991年 | 110篇 |
1990年 | 79篇 |
1989年 | 77篇 |
1988年 | 59篇 |
1987年 | 32篇 |
1986年 | 38篇 |
1985年 | 35篇 |
1984年 | 28篇 |
1983年 | 17篇 |
1982年 | 21篇 |
1981年 | 9篇 |
1980年 | 14篇 |
1979年 | 4篇 |
1978年 | 7篇 |
1977年 | 16篇 |
1954年 | 3篇 |
1875年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
171.
J. L. Simón M. A. Soriano L. E. Arlegui J. Gracia C. L. Liesa A. Pocoví 《Environmental Geology》2008,53(5):1057-1065
Space and time variations of alluvial karst subsidence in the central Ebro Basin are analysed in trenches (paleodolines),
aerial photographs (historical dolines) and field surveys (present-day active dolines). The measured subsidence rates, as
well as a comparison between densities of paleodolines computed in randomly selected scan-lines and density of active dolines
computed in maps, suggest that present-day subsidence in favourable areas is more intense than that of Pleistocene times.
According to diachronic maps drawn from (a) different aerial photographs taken after 1946 and (b) field surveys on deformations
in urbanized areas, subsidence behaved as a nearly steady process while the whole area was used for agriculture. In contrast,
rapid changes (through periods of several tens of years) have occurred in urbanized areas, in which dolines expand their boundaries
through small marginal collapses, shifting sharply their sinking centres to neighbouring sites. These rapid changes do not
have an equivalent in observed paleodolines. 相似文献
172.
Participation in payments for ecosystem services: Case studies from the Lacandon rainforest, Mexico 总被引:3,自引:0,他引:3
Understanding people’s willingness to participate in projects and programmes of payments for ecosystem services (PES) has not been a key analytical concern of the scholarly literature around this new field of environmental policy and practice. This paper analyses participation in four communities benefiting from payments for biodiversity and carbon fixation in Mexico, and contrasts the results for each case with neighbouring communities that do not receive payments. We take a holistic approach that accounts for procedural rules, actors’ interactions, institutions and values, and individuals’ characteristics. We show that the nature of PES rules and the effectiveness of communication with government officers and NGOs influence resource managers’ ability and willingness to participate. We highlight community size, resource managers’ ability to diversify livelihood activities and local perspectives on the conservation of common forests, particularly sacred values and intergenerational concerns on forest conservation, as critical participation drivers. This analysis provides insights on why and how these new institutions may be attractive for some resource managers and permits to draw some recommendations for the future design of PES projects and programmes. 相似文献
173.
The analysis of the history of the management and distribution of electricity, water and sanitation networks in Santiago de Chile throws light on a problem which is central to thinking on the relationship between urban services management and territorial splintering: what is the impact of urban policy, in particular housing policy and urban planning, on access to services and on the potential levels of social segregation and institutional splintering of metropolises?The Chilean example is eminently anchored in its political and institutional history. We shall see, for example, that the authoritarian urban policy of the military government, associated with early liberal reform, enabled the improvement of access to networks in every municipality of the Greater Santiago area. However, this success, leading to the integration by urban services of a large territory, does not signify that networks remained neutral as regards the process of spatial segregation. Inversely, we can defend the theory that the presence of an integrated service was a necessary condition and a reinforcing factor of the process of urban spread and residential segregation. This paradoxical argument is a result of the ambivalent liberal network and urban policies of the military government. Urban expansion was largely sustained by the development of integrated infrastructures and universal services. The processes of liberalising land and urban services were mutually sustaining and provided the necessary conditions for the birth of an extremely intense movement of urban sprawl and segregation between 1985 and 2000. 相似文献
174.
Thembela Kepe 《Geoforum》2008,39(2):958-968
Different types of vegetation found in local environments are of value to rural livelihoods of many African households. However, the dominant way of expressing this value, which is mainly through economic valuation studies, is potentially limiting this knowledge’s usefulness in policy and research, due to the inability to present full picture. Using literature review and insights from field work, the paper argues that realized and notional values of vegetation to rural livelihoods are socially constructed and contested, and - in addition to understanding local livelihood context, which include social difference, and ecological dynamics - a focus on social institutions as terrains of negotiation is crucial. This means that resource value in rural livelihoods can be realized through contested and negotiated access arrangements that are mediated by complex institutions at local and external levels. 相似文献
175.
Quantitative sinkhole hazard assessments in karst areas allow calculation of the potential sinkhole risk and the performance
of cost-benefit analyses. These estimations are of practical interest for planning, engineering, and insurance purposes. The
sinkhole hazard assessments should include two components: the probability of occurrence of sinkholes (sinkholes/km2 year) and the severity of the sinkholes, which mainly refers to the subsidence mechanisms (progressive passive bending or
catastrophic collapse) and the size of the sinkholes at the time of formation; a critical engineering design parameter. This
requires the compilation of an exhaustive database on recent sinkholes, including information on the: (1) location, (2) chronology
(precise date or age range), (3) size, and (4) subsidence mechanisms and rate. This work presents a hazard assessment from
an alluvial evaporite karst area (0.81 km2) located in the periphery of the city of Zaragoza (Ebro River valley, NE Spain). Five sinkholes and four locations with features
attributable to karstic subsidence where identified in an initial investigation phase providing a preliminary probability
of occurrence of 0.14 sinkholes/km2 year (11.34% in annual probability). A trenching program conducted in a subsequent investigation phase allowed us to rule
out the four probable sinkholes, reducing the probability of occurrence to 0.079 sinkholes/km2 year (6.4% in annual probability). The information on the severity indicates that collapse sinkholes 10–15 m in diameter
may occur in the area. A detailed study of the deposits and deformational structures exposed by trenching in one of the sinkholes
allowed us to infer a modern collapse sinkhole approximately 12 m in diameter and with a vertical throw of 8 m. This collapse
structure is superimposed on a subsidence sinkhole around 80 m across that records at least 1.7 m of synsedimentary subsidence.
Trenching, in combination with dating techniques, is proposed as a useful methodology to elucidate the origin of depressions
with uncertain diagnosis and to gather practical information with predictive utility about particular sinkholes in alluvial
karst settings: precise location, subsidence mechanisms and magnitude, and timing and rate of the subsidence episodes. 相似文献
176.
177.
This paper explores the links between a strategic policy, urban consolidation, and house prices by examining the changes in
the mix of housing and in house price for the period 1991–2004. We contend that urban consolidation could be seen as a source
of additional supply, (which might be expected to be felt in lower prices and so contribute to a local policy objective) but
also as a stimulus to demand (by developers who could bid up the price of lots where it was understood more housing could
be built). Analyses were carried out at the metropolitan and sub-regional scales using correlation tests. The research finds
very weak statistical connections, and concludes that this policy has not been associated with price changes. 相似文献
178.
The Cambrian-Vendian groundwater body plays an important role in drinking water supply of capital Tallinn and other settlements
in northern Estonia. Water quality in this deep-seated confined groundwater body is influenced by the intrusion of present-day
seawater and pumping-induced upward migration of deeper saline groundwater. The aim of this study is to evaluate the threshold
values applying the methodology worked out within the EU 6th FP project “Background criteria for the identification of groundwater
thresholds (BRIDGE)” and to compare the results with the values proposed by local authorities. A database containing 250 monitoring
points was completed and used for the calculation of natural background levels (as the 90 and 97.7 percentile) in groundwater.
The calculation of threshold values is based on natural background levels and reference values. In case of Cambrian-Vendian
groundwater body the receptor is drinking water, thus limit values set by Estonian drinking water standard were used as reference
values. The threshold values proposed by local authorities are more or less the same as calculated by BRIDGE methodology.
Exceptionally different values are derived in case of chloride and ammonium. 相似文献
179.
180.
P-III分布参数的概率权重矩法S函数计算 总被引:4,自引:1,他引:3
概率权重矩法是一种估计统计分布参数的方法.本文根据不完全Γ函数在无限区间积分,推导了P-Ⅲ分布参数的S函数的计算公式.通过现有计算公式比较,其计算结果具有较高的计算精度,避免了的大量的数值积分计算.文中公式只要借助于计算编程进行求解,给定超几何函数项一定的截断误差,其运算具有较高的运行速度.文中计算公式是一种P-Ⅲ分布参数S函数的计算途径. 相似文献