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   1969篇
  免费   81篇
  国内免费   10篇
测绘学   71篇
大气科学   165篇
地球物理   411篇
地质学   691篇
海洋学   152篇
天文学   362篇
综合类   5篇
自然地理   203篇
  2021年   26篇
  2020年   26篇
  2019年   28篇
  2018年   47篇
  2017年   49篇
  2016年   65篇
  2015年   51篇
  2014年   45篇
  2013年   123篇
  2012年   100篇
  2011年   99篇
  2010年   81篇
  2009年   116篇
  2008年   97篇
  2007年   100篇
  2006年   71篇
  2005年   80篇
  2004年   54篇
  2003年   59篇
  2002年   54篇
  2001年   39篇
  2000年   54篇
  1999年   32篇
  1998年   27篇
  1997年   23篇
  1996年   25篇
  1995年   22篇
  1994年   17篇
  1993年   27篇
  1992年   22篇
  1991年   15篇
  1990年   23篇
  1989年   17篇
  1988年   19篇
  1987年   19篇
  1986年   16篇
  1985年   16篇
  1984年   27篇
  1983年   28篇
  1982年   20篇
  1981年   18篇
  1980年   16篇
  1977年   11篇
  1976年   12篇
  1975年   15篇
  1974年   12篇
  1973年   10篇
  1972年   19篇
  1971年   13篇
  1970年   8篇
排序方式: 共有2060条查询结果,搜索用时 328 毫秒
951.
This paper introduces a new modeling framework to understand and improve regional natural disaster risk management in the USA, including the interactions among key stakeholders and between the two important risk management mechanisms of insurance and retrofit. The framework includes a stochastic programming optimization to represent insurer decisions, which interacts with a utility-based model of individual homeowners’ decisions to insure and/or retrofit. Reinsurer and government roles are represented as inputs, and the decision models are integrated with a detailed regional catastrophe loss estimation model. This modeling framework is applied to a full-scale, realistic case study for hurricane risk to residential buildings in Eastern North Carolina. Several alternative system configurations are considered that affect the incentives for adoption of alternative risk management methods. They include providing a government subsidy for insured homeowners to encourage retrofit, providing both a government subsidy and insurance rebate to reduce retrofit costs, and mandating insurance purchase with a cap on insurance premiums. For each configuration, outcomes are presented from the perspectives of all key stakeholders—primary insurer, homeowners (insured and uninsured, in high- and low-risk areas), reinsurers, and the government. Results suggest that it is possible to design policies in which all stakeholders can be better off simultaneously. Retrofit incentives for insured homeowners can be effective in linking and strengthening the benefits of retrofit and insurance. Mandatory insurance coupled with capped profit loading factors and possibly retrofit rebates from the insurer to the homeowner can also reduce overall system risk.  相似文献   
952.
The last major eruption of La Soufrière volcano in Guadeloupe (Lesser Antilles) in 1976–1977 caused the mass evacuation of part of the population, whereby a total of 76,000 people were displaced for a period of 3–6 months. This evacuation has left a bitter memory among the inhabitants who believed that the political authorities of the time had not anticipated the possibility of an eruption crisis and that decisions were taken in haste. La Soufrière remains active, and future eruptions could once again lead to partial or even total evacuation of the population if there were a major Plinian eruption. This article offers an investigation of future evacuation procedures, questioning different aspects of Guadeloupe’s current territorial and social challenges (the multi-risk context, the reporting to the scientists and to the authorities, the importance of local solidarity). In order to do so, we used the Focus Group Discussion method, making it possible to identify resources and gaps in crisis management on the basis of previous event history.  相似文献   
953.
Rapidly growing human populations have caused heavy modifications to the watersheds of many Mediterranean climate estuaries, subjecting them to excessive nutrient enrichment and harmful macroalgal blooms. Despite these impacts, comprehensive studies in these systems are rare and comparisons between systems are lacking. We surveyed five southern California estuaries that ranged in size from 93 to 1,000 ha and incorporated differing land usages and watershed sizes. We sampled environmental variables (sediment redox potential, organic content, total nitrogen and total phosphorus, water column nitrate, ammonium, and salinity) and macroalgal cover and biomass quarterly at three locations within each estuary over 15 months to compare spatial and wet vs. dry season patterns. Maximum mean water column nitrate concentration across all estuaries ranged from 47 to 1,700 μM, showing that all estuaries were highly enriched with nitrogen, at least at some times. Mean macroalgal biomass ranged from 0 to 1,500 g wet wt m?2. However, neither nutrient concentrations nor algal biomass showed consistent seasonal patterns as maximum values occurred in different seasons in different estuaries. Three-dimensional principal components analysis followed by regression analyses confirmed that macroalgal abundance was not directly related to water or sediment N concentrations. Rather each of these southern California estuaries showed individual patterns in all measured variables, which were most likely induced by a suite of physical modifications unique to each system and its watershed.  相似文献   
954.
Radon gas is a human health hazard; long-term exposure to high radon concentrations through inhalation is the second leading cause of lung cancer. Nova Scotia has been previously identified as a potential high risk region because of the geology. As such, the gas transport through Halifax’s fine grained leucomonzogranite (FGL) unit of the South Mountain Batholith needed to be quantified to further remediation efforts. Using controlled laboratory experiments, four different soil columns were created using the Halifax Regional Municipality’s (HRM) highest producing field tills and bedrock. Permeability, diffusivity, radon-222 gas concentrations, and gas transit time/speed were measured in both dry tills (field moisture) and wet tills (simulated rain event moisture). Columns with HRM till displayed the highest radon concentrations, and were less permeable with additional moisture. Radon diffusivity calculated from CO2 was 7.52 × 10?8 m2 (dry), and 3.37 × 10?8 m2 (wet); diffusivity calculated from 222Rn was 7.30 × 10?7 m2 (dry), and 6.47 × 10?7 m2 (wet). The average FGL transit time in a 60 cm column was 3.57 days (dry), and 3.82 days (wet). Locally this study presents two different methods for diffusivity calculations, for a unit lacking previous diffusivity information. The radon gas concentrations and transport speeds quantified the transport mechanisms within the till. Globally, the correlation between soil moisture, and radon/permeability values was established using these results. The link between diffusivity and permeability was also confirmed using field tills. Implications were made for building foundations, as well as the depth and type of material necessary to reduce radon gas from reaching the surface.  相似文献   
955.
Spatial accuracy of structural data is an important factor for construction of a properly scaled model of crustal structures. Digital Geological Mapping methods can add the required spatial accuracy to the data, and also can improve the versatility of the geological/structural map in many ways, which are difficult to achieve through conventional mapping techniques. The present paper describes the methodology of digital mapping and discusses its applicability in structural analyses in the Canisp Shear Zone (CSZ) — a reactivated continental basement structure in the Precambrian Lewisian Gneiss complex of NW Scottish Highland. The CSZ is reinterpreted as a major dextral transpressional shear zone developed during Laxfordian deformation. It overprinted an earlier Inverian shear zone fabric, and was later reactivated and/or overprinted by brittle shearing of different phases and different scales. Spatial analyses within the ArcGISTM environment has helped bring out the geological relationships between different types of structural data in the shear zone, highlighting the partitioning of deformation into zones of high and low strain. A kinematic interpretation based on the geospatial data analysis, combined with conventional stereographic projection technique, is presented.  相似文献   
956.
The everyday implications of a volatile geopolitical climate are increasingly recognised, but far less is known about how people’s emotional geographies are affected by geopolitical change. This paper offers a critical examination of how some young people in different parts of the world navigate fears and hopes that might be considered ‘global’ in nature, and those that might be considered ‘everyday’. We report from participatory research conducted with young people from a range of ethnic and cultural backgrounds living in New Zealand and the United Kingdom. We examine how personal fears and hopes intersect with wider anxieties about youth, urban crime and terrorism. The research suggests that global-everyday emotions are not separated out in young people’s analyses. They are critically reflexive about wider discourses of fear, while undertaking the day to day business of navigating what are sometimes challenging emotional topographies.  相似文献   
957.
Detailed outcrop studies at the flanks of Al Kufrah Basin, Libya, reveal the nature of glacially-related sedimentation and post-depositional deformation styles produced in association with the Late Ordovician glaciation, during which ice sheets expanded northward over North Africa to deposit the Mamuniyat Formation. At the SE basin flank (Jabal Azbah), the Mamuniyat Formation is sand-dominated, and incises interfingering braidplain and shallow marine deposits of the Hawaz Formation. The glacially-related sediments include intercalations of mud-chip bearing tabular sandstones and intraformational conglomerates, which are interpreted as turbidite and debrite facies respectively. These record aggradation of an extensive sediment wedge in front of a stable former ice margin. An increase in mudstone content northward is accompanied by the occurrence of more evolved turbidites. A widespread surface, bearing streamlined NW–SE striking ridges and grooves, punctuates this succession. The structures on the surface are interpreted to have formed during a regional north-westward ice advance. Above, siltstones bearing Arthrophycus burrows, and Orthocone-bearing sandstones beneath tidal bars testify to glaciomarine conditions for deposition of the underflow deposits beneath. By contrast, the northern basin margin (Jabal az-Zalmah) is appreciably different in recording shallower water/paralic sedimentation styles and major glaciotectonic deformation features, although facies analysis also reveals northward deepening. Here, a siltstone wedging from 8 to 50 m toward the north was deposited (lower delta plain), succeeded by climbing ripple cross-laminated sandstones up to 60 m in thickness (distal through proximal delta mouth bar deposits) with occasional diamictite interbeds. These rocks are deformed by thrusts and > 50 m amplitude fault-propagation folds, the deformation locally sealed by a diamictite then overlain by conglomeratic lag during ultimate deglaciation. Integrating observations from both basin margins, a model of fluvial-dominated delta systems feeding a pulsed debrite and turbidite fan system in a shallow proglacial shelf is proposed.  相似文献   
958.
Atomistic model was proposed to describe the thermodynamics of mixing in the diopside-K-jadeite solid solution (CaMgSi2O6-KAlSi2O6). The simulations were based on minimization of the lattice energies of 800 structures within a 2 × 2 × 4 supercell of C2/c diopside with the compositions between CaMgSi2O6 and KAlSi2O6 and with variable degrees of order/disorder in the arrangement of Ca/K cations in M2 site and Mg/Al in Ml site. The energy minimization was performed with the help of a force-field model. The results of the calculations were used to define a generalized Ising model, which included 37 pair interaction parameters. Isotherms of the enthalpy of mixing within the range of 273–2023 K were calculated with a Monte Carlo algorithm, while the Gibbs free energies of mixing were obtained by thermodynamic integration of the enthalpies of mixing. The calculated T-X diagram for the system CaMgSi2O6-KAlSi2O6 at temperatures below 1000 K shows several miscibility gaps, which are separated by intervals of stability of intermediate ordered compounds. At temperatures above 1000 K a homogeneous solid solution is formed. The standard thermodynamic properties of K-jadeite (KAlSi2O6) evaluated from quantum mechanical calculations were used to determine location of several mineral reactions with the participation of the diopside-K-jadeite solid solution. The results of the simulations suggest that the low content of KalSi2O6 in natural clinopyroxenes is not related to crystal chemical factors preventing isomorphism, but is determined by relatively high standard enthalpy of this end member.  相似文献   
959.
This paper explores the following general question: Why are Fair Trade coffee farmers in Nicaragua burdened by debt? Nearly five years from “the end” of the coffee crisis, peasant farming households committed to Fair Trade standards continue to struggle with a legacy of indebtedness caused by years of low farm-gate prices and declining productivity between 2000 and 2004. Through ethnographic observation and interviews with peasant farmers in Nicaragua, I explore how farmers experience a simple reproduction squeeze that hinders them from “bootstrapping” their own economic development. I argue that the effort to raise the Fair Trade minimum price and premium for coffee through Fair Trade Labelling Organizations International (FLO) in 2008 demonstrated a good first step toward improving farm-gate prices, however, these gains must be understood in the context of long-term indebtedness as well as rising production costs and household consumption costs.  相似文献   
960.
Despite the increasing urbanization of the Aboriginal population in Canada over the past 50 years, most municipalities have not developed services and programs designed to meet their unique social and cultural needs. Faced with numerous health and social problems, the Aboriginal population is mainly forced to rely on the non-Aboriginal social services sector. However, little is known about the extent to which such sectors seek to accommodate Aboriginal populations in their programming. We examine the extent to which the recovery system makes space for Aboriginal healing through the provision of culturally-appropriate services and programming. Through the use of 24 in-depth interviews with staff members at seven treatment facilities in Winnipeg (Canada), we find an entrenched reluctance, indifference and lack of desire to create Aboriginal spaces of healing in treatment, save for one facility where Aboriginal healing spaces serve as a focal point of treatment. We discuss the implications in terms of the effectiveness of the non-Aboriginal recovery system (along with other social services) in meeting the needs of Canada’s urban Aboriginal population.  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