全文获取类型
收费全文 | 4874篇 |
免费 | 210篇 |
国内免费 | 52篇 |
专业分类
测绘学 | 122篇 |
大气科学 | 402篇 |
地球物理 | 1119篇 |
地质学 | 1812篇 |
海洋学 | 352篇 |
天文学 | 868篇 |
综合类 | 17篇 |
自然地理 | 444篇 |
出版年
2022年 | 28篇 |
2021年 | 63篇 |
2020年 | 67篇 |
2019年 | 90篇 |
2018年 | 142篇 |
2017年 | 149篇 |
2016年 | 152篇 |
2015年 | 133篇 |
2014年 | 184篇 |
2013年 | 259篇 |
2012年 | 203篇 |
2011年 | 266篇 |
2010年 | 207篇 |
2009年 | 261篇 |
2008年 | 212篇 |
2007年 | 189篇 |
2006年 | 176篇 |
2005年 | 174篇 |
2004年 | 184篇 |
2003年 | 161篇 |
2002年 | 161篇 |
2001年 | 81篇 |
2000年 | 89篇 |
1999年 | 73篇 |
1998年 | 90篇 |
1997年 | 68篇 |
1996年 | 65篇 |
1995年 | 61篇 |
1994年 | 55篇 |
1993年 | 63篇 |
1992年 | 46篇 |
1991年 | 39篇 |
1990年 | 40篇 |
1989年 | 41篇 |
1988年 | 47篇 |
1987年 | 50篇 |
1986年 | 47篇 |
1985年 | 57篇 |
1984年 | 67篇 |
1983年 | 51篇 |
1982年 | 57篇 |
1981年 | 47篇 |
1980年 | 48篇 |
1979年 | 46篇 |
1978年 | 44篇 |
1977年 | 37篇 |
1976年 | 32篇 |
1975年 | 32篇 |
1974年 | 33篇 |
1973年 | 32篇 |
排序方式: 共有5136条查询结果,搜索用时 15 毫秒
171.
Phytoplankton dynamics and cyanobacterial dominance in Murchison Bay of Lake Victoria (Uganda) in relation to environmental conditions 总被引:2,自引:0,他引:2
Sigrid Haande Thomas RohrlackRonald P. Semyalo Pål BrettumBente Edvardsen Anne Lyche-SolheimKai Sørensen Petter Larsson 《Limnologica》2011,41(1):20-29
Murchison Bay is a shallow embayment in the north-western part of Lake Victoria, strongly influenced by urban pollution from the Ugandan capital Kampala. Two stations, representing the semi-enclosed innermost part of the bay and the wider outer part of the bay, were sampled in the period from April 2003 to March 2004, in order to assess the phytoplankton community and the nutrient status in the bay. Murchison Bay was highly eutrophic with average concentrations (n=25) of total phosphorous >90 μg L−1 and total nitrogen >1100 μg L−1 in the inner part of the bay. The phytoplankton community was dominated by a variety of cyanobacterial species and diatoms. Cyanobacteria were dominant in the whole bay, whereas diatoms were more abundant in the outer part of the bay. Moreover, the proportion of N-fixing species like Anabaena sp. was higher in the outer part of the bay, whereas species like Microcystis sp. were more abundant in the inner part of the bay. The phytoplankton community, especially in the outer part of the bay, may be influenced by light limitation. Low NO3-N concentrations in the bay may also indicate a possible N-limitation, thus favouring growth of N-fixing cyanobacteria. The open bay is, however, a complex system, and additional environmental factors and loss processes most likely affect the phytoplankton community. 相似文献
172.
Poul L. Bjerg Kirsten Riigge Jesper Cortsen Per H. Nielsen Thomas H. Christensen 《Ground water》1999,37(1):113-121
173.
Saline water from a storm surge can flow down storm-damaged submerged water supply wells and contaminate boreholes and surrounding aquifers. Using data from conventional purging techniques, aquifer test response analysis, chemical analysis, and regression analysis of chloride/silica (Cl/Si) ratio, equations were derived to estimate the volume of saline water intrusion into a well and a porous media aquifer, the volume of water needed to purge a well shortly following an intrusion event, and the volume of water needed after delay of several or more months, when the saline plume has expanded. Purging time required is a function of volume of water and pumping rate. The study site well is located within a shoreline community of Lake Pontchartrain, St. Tammany Parish, in southeastern Louisiana, United States, which was impacted by two hurricane storm surges and had neither been rehabilitated nor chlorinated prior to our study. Chemical analysis of water samples in fall 2005 and purging of well and aquifer in June 6, 2006, indicated saline water had intruded the well in 2005 and the well and aquifer in 2006. The volume of water needed to purge the study well was approximately 200 casing volumes, which is significantly greater than conventionally used during collection of water samples for water quality analyses. 相似文献
174.
A review of paired catchment studies for determining changes in water yield resulting from alterations in vegetation 总被引:21,自引:0,他引:21
Alice E. Brown Lu Zhang Thomas A. McMahon Andrew W. Western Robert A. Vertessy 《Journal of Hydrology》2005,310(1-4):28-61
Paired catchment studies have been widely used as a means of determining the magnitude of water yield changes resulting from changes in vegetation. This review focuses on the use of paired catchment studies for determining the changes in water yield at various time scales resulting from permanent changes in vegetation. The review considers long term annual changes, adjustment time scales, the seasonal pattern of flows and changes in both annual and seasonal flow duration curves. The paired catchment studies reported in the literature have been divided into four broad categories: afforestation experiments, deforestation experiments, regrowth experiments and forest conversion experiments. Comparisons between paired catchment results and a mean annual water balance model are presented and show good agreement between the two methodologies. The results highlight the potential underestimation of water yield changes if regrowth experiments are used to predict the likely impact of permanent alterations to a catchment's vegetation. An analysis of annual water yield changes from afforestation, deforestation and regrowth experiments demonstrates that the time taken to reach a new equilibrium under permanent land use change varies considerably. Deforestation experiments reach a new equilibrium more quickly than afforestation experiments. The review of papers reporting seasonal changes in water yield highlights the proportionally larger impact on low flows. Flow duration curve comparison provides a potential means of gaining a greater understanding of the impact of vegetation on the distribution of daily flows. 相似文献
175.
Effective livelihood adaptation to climate change disturbance: Scale dimensions of practice in Mozambique 总被引:2,自引:0,他引:2
Natural resource-dependent societies in developing countries are facing increased pressures linked to global climate change. While social-ecological systems evolve to accommodate variability, there is growing evidence that changes in drought, storm and flood extremes are increasing exposure of currently vulnerable populations. In many countries in Africa, these pressures are compounded by disruption to institutions and variability in livelihoods and income. The interactions of both rapid and slow onset livelihood disturbance contribute to enduring poverty and slow processes of rural livelihood renewal across a complex landscape. We explore cross-scale dynamics in coping and adaptation response, drawing on qualitative data from a case study in Mozambique. The research characterises the engagements across multiple institutional scales and the types of agents involved, providing insight into emergent conditions for adaptation to climate change in rural economies. The analysis explores local responses to climate shocks, food security and poverty reduction, through informal institutions, forms of livelihood diversification and collective land-use systems that allow reciprocity, flexibility and the ability to buffer shocks. However, the analysis shows that agricultural initiatives have helped to facilitate effective livelihood renewal, through the reorganisation of social institutions and opportunities for communication, innovation and micro-credit. Although there are challenges to mainstreaming adaptation at different scales, this research shows why it is critical to assess how policies can protect conditions for emergence of livelihood transformation. 相似文献
176.
A mechanism for the generation of intrathermocline eddies (ITEs) at wind-forced fronts is examined using a high resolution numerical simulation. Favorable conditions for ITE formation result at fronts forced by “down-front” winds, i.e. winds blowing in the direction of the frontal jet. Down-front winds exert frictional forces that reduce the potential vorticity (PV) within the surface boundary in the frontal outcrop, providing a source for the low-PV water that is the materia prima of ITEs. Meandering of the front drives vertical motions that subduct the low-PV water into the pycnocline, pooling it into the coherent anticyclonic vortex of a submesoscale ITE. As the fluid is subducted along the outcropping frontal isopycnal, the low-PV water, which at the surface is associated with strongly baroclinic flow, re-expresses itself as water with nearly zero absolute vorticity. This generation of strong anticyclonic vorticity results from the tilting of the horizontal vorticity of the frontal jet, not from vortex squashing. During the formation of the ITE, high-PV water from the pycnocline is upwelled alongside the subducting low-PV surface water. The positive correlation between the ITE’s velocity and PV fields results in an upward, along-isopycnal eddy PV flux that scales with the surface frictional PV flux driven by the wind. The relationship between the eddy and wind-induced frictional PV flux is nonlocal in time, as the eddy PV flux persists long after the wind forcing is shut off. The ITE’s PV flux affects the large-scale flow by driving an eddy-induced transport or bolus velocity down the outcropping isopycnal layer with a magnitude that scales with the Ekman velocity. 相似文献
177.
Thomas R. Stewart 《Climatic change》1987,11(1-2):97-113
The Delphi technique for judgmental forecasting by expert groups is described and the controversy surrounding its use is summarized. The technique clearly does not eliminate all unwanted psychological effects on group judgment. Furthermore, the design of most Delphi studies makes it impossible to separate the signal from the noise in expert judgment. A methodological standard for evaluating judgmental forecasts is proposed. 相似文献
178.
Robert J. Beare Malcolm K. Macvean Albert A. M. Holtslag Joan Cuxart Igor Esau Jean-Christophe Golaz Maria A. Jimenez Marat Khairoutdinov Branko Kosovic David Lewellen Thomas S. Lund Julie K. Lundquist Anne Mccabe Arnold F. Moene Yign Noh Siegfried Raasch Peter Sullivan 《Boundary-Layer Meteorology》2006,118(2):247-272
Results are presented from the first intercomparison of large-eddy simulation (LES) models for the stable boundary layer (SBL), as part of the Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study initiative. A moderately stable case is used, based on Arctic observations. All models produce successful simulations, in as much as they generate resolved turbulence and reflect many of the results from local scaling theory and observations. Simulations performed at 1-m and 2-m resolution show only small changes in the mean profiles compared to coarser resolutions. Also, sensitivity to subgrid models for individual models highlights their importance in SBL simulation at moderate resolution (6.25 m). Stability functions are derived from the LES using typical mixing lengths used in numerical weather prediction (NWP) and climate models. The functions have smaller values than those used in NWP. There is also support for the use of K-profile similarity in parametrizations. Thus, the results provide improved understanding and motivate future developments of the parametrization of the SBL. 相似文献
179.
A horizontal shear flow having a Rossby number, Ro, greater than unity on a rotating plane can become unstable when its shear value is less than −f, the Coriolis frequency. In this paper, this instability is investigated for an O(10 km) submesoscale, sinusoidal shear flow in a thin homogeneous fluid layer as in an oceanic mixed layer or a shallow sea. The most unstable mode is shown by a linear analysis to occur in a narrow localized region centered around the maximum anticyclonic current shear. However, nonlinear numerical calculations show that the instability can grow to encompass both unstable and stable regions of the current. A consequence of this finite-amplitude evolution is the formation of surface convergence/shear fronts. The possibility that inertial instability mechanism is a source of some surface convergence/shear features seen in remote sensing images of the sea surface is discussed. A comparison is made with the shear-flow instability that can occur concurrently in a sinusoidal shear current, and inertial instability is shown to be the dominant instability mechanism in the immediate range above Ro=2. 相似文献
180.
The Kyoto Protocol allows Annex I countries to use afforestation (theconversion of non-forest landto forest) to meet emissions reduction targets. We present a new method forestimating the cost of CO2mitigation through afforestation based on econometric models of land use. Landuse models are developed from dataon observed land allocation decisions and quantify the relationship betweenthe share of land in forest and the netreturns to forestry, among other land use determinants. The econometricapproach measures the actual responsesby landowners to observed changes in net returns, in contrast to earlierstudies in which landowner responses aredictated by the researcher. Models are estimated for Maine, South Carolina,and Wisconsin. The estimated modelsare used to simulate subsidies for afforestation, which imply increases inforest area and net reductions inatmospheric CO2 concentrations. Average cost measures – totalsubsidies divided by total carbon sequestered –are derived for afforestation programs with and without timber harvesting. Theuse of econometric land use modelsin integrated assessments of climate change is explored. We model the effectson land use patterns and the costsof CO2 mitigation of changes in the net returns to agricultureinduced by climate change. 相似文献