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   641篇
  免费   63篇
  国内免费   38篇
测绘学   7篇
大气科学   99篇
地球物理   162篇
地质学   137篇
海洋学   110篇
天文学   20篇
综合类   11篇
自然地理   196篇
  2024年   1篇
  2023年   3篇
  2022年   22篇
  2021年   11篇
  2020年   19篇
  2019年   15篇
  2018年   15篇
  2017年   34篇
  2016年   28篇
  2015年   30篇
  2014年   41篇
  2013年   70篇
  2012年   33篇
  2011年   32篇
  2010年   24篇
  2009年   28篇
  2008年   47篇
  2007年   46篇
  2006年   44篇
  2005年   32篇
  2004年   23篇
  2003年   19篇
  2002年   25篇
  2001年   17篇
  2000年   17篇
  1999年   7篇
  1998年   7篇
  1997年   6篇
  1996年   10篇
  1995年   7篇
  1994年   5篇
  1993年   2篇
  1992年   6篇
  1991年   3篇
  1990年   1篇
  1988年   1篇
  1987年   2篇
  1986年   1篇
  1985年   1篇
  1984年   1篇
  1983年   4篇
  1982年   1篇
  1979年   1篇
排序方式: 共有742条查询结果,搜索用时 468 毫秒
81.
ABSTRACT

The Australian government’s proposal to expand the Snowy Hydro Scheme to include a second pumped hydro energy storage (PHES) system, and support for feasibility studies for PHES in Tasmania, offer an opportunity to incorporate more intermittent renewable energy generation into the National Energy Market. However, the infrastructure construction required for PHES expansion may have negative effects for biodiversity in subalpine and alpine areas. To identify the potential effects of PHES on biodiversity in Kosciuszko National Park (KNP), this systematic literature review assesses: (i) the key environmental disturbances likely to arise from PHES construction; and (ii) the specific impacts of these construction processes on biodiversity in subalpine and alpine environments. We find that the effects of PHES construction-related disturbances are likely to be negative for subalpine and alpine biodiversity, with impacts including the proliferation of exotic flora, habitat loss resulting from vegetation clearing, altered landscape hydrology and reduced water quality. Management recommendations to limit these impacts are provided and further research is recommended to assess: (i) the effects of PHES on fish populations in alpine reservoirs; (ii) the utility of spoil as artificial habitat for endemic fauna; and (iii) the chemical and physical impacts of spoil dumping in alpine reservoirs.  相似文献   
82.
Mountain and lowland watersheds are two distinct geographical units with considerably different hydrological processes. Understanding their hydrological processes in the context of future climate change and land use scenarios is important for water resource management. This study investigated hydrological processes and their driving factors and eco-hydrological impacts for these two geographical units in the Xitiaoxi watershed, East China, and quantified their differences through hydrological modelling. Hydrological processes in 24 mountain watersheds and 143 lowland watersheds were simulated based on a raster-based Xin'anjiang model and a Nitrogen Dynamic Polder (NDP) model, respectively. These two models were calibrated and validated with an acceptable performance (Nash-Sutcliffe efficiency coefficients of 0.81 and 0.50, respectively) for simulating discharge for mountain watersheds and water level for lowland watersheds. Then, an Indicators of Hydrological Alteration (IHA) model was used to help quantify the alterations to the hydrological process and their resulting eco-hydrological impacts. Based on the validated models, scenario analysis was conducted to evaluate the impacts of climate and land use changes on the hydrological processes. The simulation results revealed that (a) climate change would cause a larger increase in annual runoff than that under land use scenario in the mountain watersheds, with variations of 19.9 and 10.5% for the 2050s, respectively. (b) Land use change was more responsible for the streamflow increment than climate change in the lowland watersheds, causing an annual runoff to increase by 27.4 and 16.2% for the 2050s, respectively. (c) Land use can enhance the response of streamflow to the climatic variation. (d) The above-mentioned hydrological variations were notable in flood and dry season in the mountain watersheds, and they were significant in rice season in the lowland watersheds. (e) Their resulting degradation of ecological diversity was more susceptible to future climate change in the two watersheds. This study demonstrated that mountain and lowland watersheds showed distinct differences in hydrological processes and their responses to climate and land use changes.  相似文献   
83.
84.
程绍文  李艳  张晓梅  李照红 《地理科学》2017,37(10):1569-1576
以武当山为研究案例地,通过游客问卷与访谈调查,采用SPSS18.0统计软件定量分析武当山旅游体验对游客认知、思想、行为意愿等的影响。研究结果表明,① 武当山游客的旅游动机有宗教朝圣、文化体验、娱乐放松及感情交流4类;② 宗教文化旅游影响游客对目的地的形象感知及对道教文化的认知、态度及践行意愿;宗教文化旅游对游客的影响强弱因动机而不同,自强至弱依次为宗教朝圣、文化体验、感情交流及娱乐放松。  相似文献   
85.
安徽省人口年龄结构特征及其社会经济影响研究   总被引:1,自引:0,他引:1  
汪静 《地理科学》2017,37(11):1728-1735
根据人口普查和抽样调查的数据资料,采用对比分析、聚类分析等研究方法分析了1953~2015年安徽省人口年龄结构时间特征、空间特征。从时间上看,安徽省人口年龄结构不断演变,少儿人口数量从不断增加转变为不断减少,劳动适龄人口比重比较稳定,人口老龄化程度不断加深,总抚养系数和少年儿童抚养系数大幅度下降,老年抚养系数上升;从空间上看,人口年龄结构存在较大地区差异。安徽省人口年龄结构对社会经济发展的影响主要表现为:① 人口老年化导致劳动生产率和经济增长速度的减缓,家庭规模小型化,并对社会养老保障和医疗卫生的挑战;② 少儿人口的不断减少影响教育资源的配置;③ 人口年龄结构的区域差异进一步加剧了安徽区域发展的不平衡性。为了尽可能地减缓人口年龄结构演变对安徽省社会经济发展的不利影响,建议采取如下措施:一是充分利用“人口红利”机遇期,加快经济发展,不断促进社会进步;二是要加强引导,因地施策;三是完善社会保障制度,努力扩大社会保障制度的覆盖面,力争全覆盖,尽量减小城乡差距;四是大力发展养老产业,探索建立满足居家养老、社区养老、机构养老等多种方式需求的政府主导、政策扶持、社会参与、市场运作的社会化养老服务体系。  相似文献   
86.
Soil erosion has become a serious environmental problem worldwide, and slope land is the main source of soil erosion. As a primary cover of slope land, crops have an important influence on the occurrence and development of runoff and soil erosion on slope land. This paper reviews the current understanding of runoff and soil erosion on slope cropland. Crops mainly impact splash detachment, slope runoff, and sediment yield. In this review paper, the effects of crop growth and rainfall on the splash detachment rate and the spatial distribution of splash detachment are summarized. Crop growth has a significant impact on runoff and sediment yield. Rainfall intensity and slope gradient can influence the level of erosive energy that causes soil erosion. Furthermore, other factors such as antecedent soil water content, soil properties, soil surface physical crust, and soil surface roughness can affect soil anti-erodibility. The varying effects of different crops and with different influence mechanisms on runoff and soil erosion, as well as changes in their ability to influence erosion under different external conditions should all remain focal points of future research. The effect of crop vegetation on runoff and soil erosion on slope land is a very important factor in understanding large-scale soil erosion systems, and in-depth study of this topic is highly significant for both theory and practice.  相似文献   
87.
北京不同区域气溶胶辐射效应   总被引:1,自引:0,他引:1       下载免费PDF全文
采用大气辐射传输模式SES2以及2013年1月—2015年10月欧洲中期天气预报中心细网格再分析资料计算了北京地区4个观测站地面接收的短波辐射通量,分析了晴天和云天北京城郊气溶胶对总辐射的定量影响时空变化特征。结果表明:北京城区和近郊区气溶胶对总辐射的影响约为远郊区的2倍,北京南部和西部气溶胶对辐射的影响较大,晴天和云天北京城区和近郊区气溶胶对总辐射的削减值分别为146.23~180.99 W·m-2和202.11~217.02 W·m-2,晴天总辐射削减空间差异较大;秋冬季气溶胶对总辐射的影响明显大于春夏季,北京市观象台秋冬季气溶胶对总辐射的削减作用最大可达60%,较春夏季高10%~20%;北京城郊总辐射和直接辐射削减率与气溶胶光学厚度变化均呈线性关系,近地面PM2.5浓度对辐射的影响不容忽视。  相似文献   
88.
Due to the uplift of Qinghai-Tibet Plateau(QTP), the cryosphere gradually developed on the higher mountain summits after the Neocene, becoming widespread during the Late Quaternary. During this time, permafrost on the QTP experienced repeated expansion and degradation. Based on the remains and cross-correlation with other proxy records such as those from glacial landforms, ice-core and paleogeography, the evolution and changes of permafrost and environmental changes on the QTP during the past 150,000 years were deduced and are presented in this paper. At least four obvious cycles of the extensive and intensive development, expansion and decay of permafrost occurred during the periods of 150–130, 80–50, 30–14 and after 10.8 ka B.P.. During the Holocene, fluctuating climatic environments affected the permafrost on the QTP, and the peripheral mountains experienced six periods of discernible permafrost changes:(1) Stable development of permafrost in the early Holocene(10.8 to 8.5–7.0 ka B.P.);(2) Intensive permafrost degradation during the Holocene Megathermal Period(HMP, from 8.5–7.0 to 4.0–3.0 ka B.P.);(3) Permafrost expansion during the early Neoglacial period(ca. 4,000–3,000 to 1,000 a B.P.);(4) Relative degradation during the Medieval Warm Period(MWP, from 1,000 to 500 a B.P.);(5) Expansion of permafrost during the Little Ice Age(LIA, from 500 to 100 a B.P.);(6) Observed and predicted degradation of permafrost during the 20 th and 21 st century. Each period differed greatly in paleoclimate, paleoenvironment, and permafrost distribution, thickness, areal extent, and ground temperatures, as well as in the development of periglacial phenomena. Statistically, closer dating of the onset permafrost formation, more identification of permafrost remains with richer proxy information about paleoenvironment, and more dating information enable higher resolution for paleo-permafrost reconstruction. Based on the scenarios of persistent climate warming of 2.2~2.6 °C in the next 50 years, and in combination of the monitored trends of climate and permafrost changes, and model predictions suggest an accelerated regional degradation of plateau permafrost. Therefore, during the first half of the 21 st century, profound changes in the stability of alpine ecosystems and hydro(geo)logical environments in the source regions of the Yangtze and Yellow rivers may occur. The foundation stability of key engineering infrastructures and sustainable economic development in cold regions on the QTP may be affected.  相似文献   
89.
90.
Erosion and sediment yield are a significant problem in the Guadalquivir River basin. Such phenomena are largely driven by a land use devoted to intensive cultivation of olive trees, with a large socioeconomic influence in Andalusia. This sediment overload in rivers causes serious impacts on all fluvial ecosystem components.In this study we assess the chronic effect of sediment yield on fish communities at 104 river sites located in two different sub-catchments – the Bembézar and Guadajoz rivers – both with different lithological composition and erosion rates. Sediment yield was estimated using a semi-quantitative Factorial Score Model (FSM), developed specifically for Spanish rivers. The fish populations of both basins were evaluated in composition and abundances by the study of Fernández-Delgado et al., 2014. The influence of sediment yield on the fish community was analyzed using General Additive Models.The sediment yield was higher in the Guadajoz basin (921 T/Km2 per year) than in Bembézar (701 T/Km2 per year). In the former, fish communities were poorer in both fish density and diversity, with Luciobarbus sclateri as the only substantially present species and a significant relationship between sediment yield and load, and fish density. In contrast, in the Bembézar basin, sediment yield was correlated with total fish density, including Luciobarbus sclateri, Pseudochondrostoma willkommii, Cobitis paludica, Iberochondrostoma lemmingii, Anaecypris hispanica, and Cyprinus carpio. Intermediate values of sediment yield led to maximum densities, while those higher decreased the density of these species.  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