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   19938篇
  免费   3785篇
  国内免费   4872篇
测绘学   1370篇
大气科学   3949篇
地球物理   5167篇
地质学   10208篇
海洋学   2418篇
天文学   867篇
综合类   2048篇
自然地理   2568篇
  2024年   105篇
  2023年   369篇
  2022年   1027篇
  2021年   1197篇
  2020年   1000篇
  2019年   1119篇
  2018年   1277篇
  2017年   1111篇
  2016年   1183篇
  2015年   1024篇
  2014年   1254篇
  2013年   1238篇
  2012年   1226篇
  2011年   1243篇
  2010年   1228篇
  2009年   1157篇
  2008年   1052篇
  2007年   932篇
  2006年   757篇
  2005年   728篇
  2004年   556篇
  2003年   493篇
  2002年   481篇
  2001年   532篇
  2000年   589篇
  1999年   845篇
  1998年   678篇
  1997年   652篇
  1996年   624篇
  1995年   506篇
  1994年   455篇
  1993年   422篇
  1992年   355篇
  1991年   236篇
  1990年   178篇
  1989年   175篇
  1988年   136篇
  1987年   96篇
  1986年   71篇
  1985年   56篇
  1984年   47篇
  1983年   34篇
  1982年   36篇
  1981年   18篇
  1980年   16篇
  1979年   16篇
  1978年   13篇
  1977年   7篇
  1975年   6篇
  1958年   17篇
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
951.
以分布式认知理论为基础,采用质性研究法、数量分析法以及GIS空间分析技术,探讨日喀则市藏族、汉族居民对城市空间环境认知的异同性。研究表明,藏族对城市空间环境的认知得分高于汉族对城市空间环境的认知得分。藏族、汉族对城市空间环境的认知水平可分为高、中、低3类。认知空间呈现圈层结构与混合结构模式,可分为藏汉一致性、藏族独立以及汉族独立3种认知空间类型。影响居民对城市空间环境的认知因素有共性因素和差异性因素。景观走廊质量、宜居性、基础设施两项是藏族居民关注的主要影响因素,汉族居民更关注公共服务设施的服务能力以及对当地文化的融入感。  相似文献   
952.
马延吉  艾小平 《地理科学》2019,39(3):487-495
结合联合国2030年可持续发展目标(SDGs)构建城镇化可持续发展评价指标体系,从省内、外两个角度研究吉林省城镇化可持续发展状况。结果表明:吉林省城镇化可持续发展水平波动上升,发展趋势良好,大致可分为3个阶段;研究期内各子系统可持续发展水平均呈上升趋势,其中社会子系统上升幅度最大,经济子系统上升幅度最小。城镇化可持续发展水平空间格局呈“核心-边缘”结构,即长春市水平高、其它地区水平低;经济、社会、资源和环境子系统可持续发展水平总体上表现为“中东部高,西部低”。研究期内吉林省城镇化可持续发展水平低于黑龙江省,2015年后高于辽宁省;相较辽黑两省,吉林省社会子系统可持续发展水平较低。  相似文献   
953.
长白山景区旅游安全风险动态评价研究   总被引:1,自引:0,他引:1  
孙滢悦  杨青山  陈鹏 《地理科学》2019,39(5):770-778
以长白山景区旅游安全为研究对象,以鱼骨图、动态贝叶斯、GIS技术等为基本研究方法,从研究区自然环境、社会环境及责任人为3个方面出发,筛选景区致险因子,构建景区旅游安全风险危险性评价指标体系,利用动态贝叶斯方法综合构建景区旅游安全风险动态评价模型;并以实测数据及景区统计数据为依据,划分景区旅游安全风险评价的4个动态时段,综合实现景区旅游安全风险动态风险评价。研究结果表明:中等以上风险区域呈条带状分布;高风险区域与主要景点重合;长白山景区安全风险发生高概率的时段发生在第三个时段(12:00~14:00);较高概率发生分别在第二个时段(10:00~12:00)与第四个时段(14:00~16:00);中等概率发生较高的时段在第四个时段(14:00~16:00);较低概率发生在第一个时段(8:00~10:00)。  相似文献   
954.
利用福州中心城区的社会调查、POI、道路网、用地性质等数据,在社区尺度探究城市建成环境对居民休闲型中高强度体力活动的影响。研究发现:低密度土地利用与体力活动时长呈正向关联,功能单一和邻近主干道则对体力活动起到抑制作用。体力活动目的地对不同人群具有差异化影响,体育用地和付费运动场所仅能促进中高阶层和中高收入者的体力活动,而周边公共运动设施的丰富程度更具普适性意义。通过纳入运动偏好和单独分析保障房、单位大院及房改房住户达到规避居住自选择的目的,证明上述建成环境特征对体力活动的影响是独立且稳健的。  相似文献   
955.
Microalgae, a sustainable source of multi beneficial components has been discovered and could be utilised in pharmaceutical, bioenergy and food applications. This study aims to investigate the sugaring-out effect on the recovery of protein from wet green microalga, Chlorella sorokiniana CY1 which was assisted with sonication. A comparison of monosaccharides and disaccharides as one of the phaseforming constituents shows that the monosaccharides, glucose was the most suitable sugar in forming the phases with acetonitrile to enhance the production of protein(52% of protein). The protein productivity of microalgae was found to be significantly influenced by the volume ratio of both phases, as the yield of protein increased to 77%. The interval time between the sonication as well as the sonication modes were influencing the protein productivity as well. The optimum protein productivity was obtained with 10 s of resting time in between sonication. Pulse mode of sonication was suitable to break down the cell wall of microalgae compared to continuous mode as a lower protein yield was obtained with the application of continuous mode. The optimum condition for protein extraction were found as followed: 200 g/L glucose as bottom phase with volume ratio of I:1.25, 10 s of resting time for ultrasonication, 5 s of ultrasonication in pulse mode and 0.25 g of biomass weight. The high yield of protein about 81% could be obtained from microalgae which demonstrates the potential of this source and expected to play an important role in the future.  相似文献   
956.
Yang  Bing  Hou  Yijun  Li  Min 《中国海洋湖沼学报》2019,37(3):938-956
Based on in-situ observation, satellite and reanalysis data, responses of the western North Pacific subtropical ocean(WNPSO) to the slow-moving category 5 super typhoon Nanmadol in 2011 are analyzed. The dynamical response is dominated by near-inertial currents and Ekman currents with maximum amplitude of 0.39 m/s and 0.15 m/s, respectively. The near-inertial currents concentrated around 100 m below the sea surface and had an e-folding timescale of 4 days. The near-inertial energy propagated both upward and downward, and the vertical phase speed and wavelength were estimated to be 5 m/h and 175 m, respectively. The frequency of the near-inertial currents was blue-shifted near the surface and redshifted in ocean interior which may relate to wave propagation and/or background vorticity. The resultant surface cooling reaches -4.35℃ and happens when translation speed of Nanmadol is smaller than 3.0 m/s.When Nanmadol reaches super typhoon intensity, the cooling is less than 3.0℃ suggesting that the typhoon translation speed plays important roles as well as typhoon intensity in surface cooling. Upwelling induced by the slow-moving typhoon wind leads to typhoon track confined cooling area and the right-hand bias of cooling is slight. The mixed layer cooling and thermocline warming are induced by wind-generated upwelling and vertical entrainment. Vertical entrainment also led to mixed layer salinity increase and thermocline salinity decrease, however, mixed layer salinity decrease occurs at certain stations as well. Our results suggest that typhoon translation speed is a vital factor responsible for the oceanic thermohaline and dynamical responses, and the small Mach number(slow typhoon translation speed) facilitate development of Ekman current and upwelling.  相似文献   
957.
Ma  Qin  Li  Mingzheng  Qin  Xiangchao  Liu  Huanzhang  Cao  Wenxuan 《中国海洋湖沼学报》2019,37(3):1090-1101
Floodplains are important and distinctive ecosystems around the world, and the recruitment of ichthyoplankton from river to floodplain lakes is crucial to maintain this unique ecosystem. However,this process has not been well documented. In this study, ichthyoplankton were sampled to investigate the details of ichthyoplankton recruitment from the Changjiang(Yangtze) River to the Dongting Lake via a floodway channel, the Songzi River, from May to July in 2013 and 2014. During the study period, 41 species of eggs and larvae were sampled. Among the samples, 16 were river-lake migratory species(RL), representing23.5% of the species in the Dongting Lake. In 2013, an estimated 130 million eggs and 3 180 million larvae drifted through the sampling section, and in 2014, an estimated 1 060 million eggs and 1 040 million larvae drifted through the sampling section. The amount of eggs and larvae of RL reached 3 210 million in 2013 and 1 850 million in 2014, respectively. These results demonstrated the importance of ichthyoplankton recruitment from the river to the lake, as species diversity will decrease sharply without this recruitment.Canonical correspondence analysis(CCA) showed that water temperature and water flow are the two most important factors influencing the spawning activities of fish. To maintain the high fish diversity in the Changjiang floodplain, we suggest to irrigate the channel to increase water discharge and increase the transport of ichthyoplankton from the Changjiang mainstream into the Dongting Lake.  相似文献   
958.
Ran  Weimin  Luan  Xiwu  Lu  Yintao  Liu  Hong  Yang  Jiajia  Zhao  Yang  He  Wenchang  Yan  Zhonghui 《中国海洋湖沼学报》2019,37(1):47-61

Analysis of 2D seismic data over 4 500 km in length from the Madura Strait Basin in the East Java Sea reveals seismic reflection characteristics of reefs and associated sedimentary bodies, including asymmetrical or symmetrical dome reflections, slope progradational reflections, chaotic reflections and discontinuous strong reflections inside the reef, which onlap the flank of the reef. It is concluded that the developmental paleo-environment of most reefs is mainly conducive to shallow marine carbonate platform facies and platform margin facies, based on well core data, variations in seismic facies and strata thickness. The formation and evolution of all reefs are primarily influenced by the tectonic framework of the Madura Strait Basin. Platform margin reefs are principally controlled by two types of structures: one is a series of E-W trending Paleogene normal faults, and the other is an E-W trending Neogene inversion structures. In addition, wave actions, tidal currents and other ocean currents play an accelerated role in sorting, rounding and redeposition for the accumulation and evolution of reefs. Tertiary reefs in the MSB can be divided into four types: 1) an open platform coral reef of Late Oligocene to Early Miocene, 2) a platform margin coral reef controlled by normal faults in Late Oligocene to Early Miocene, 3) a platform margin Globigerina moundreef controlled by a “hidden” inversion structure in Early Pliocene, and 4) a platform margin Globigerina mound-reef controlled by thrust faults in the early Pliocene. Patterns of the formation and evolution of reefs are also suggested.

  相似文献   
959.
Zhou  Liqing  Wang  Xuemei  Wu  Biao  Sun  Xiujun  Zhao  Qing  Zhang  Gaowei  Liu  Zhihong  Yang  Aiguo 《中国海洋湖沼学报》2019,37(1):169-175
Journal of Oceanology and Limnology - Blepharipoda liberate Shen is a commercially valuable seafood species that has important ecological significance in Shandong Province, China. Although B....  相似文献   
960.
Wang  Yanfeng  Chi  Liang  Liu  Qinghua  Xiao  Yongshuang  Ma  Daoyuan  Xiao  Zhizhong  Xu  Shihong  Li  Jun 《中国海洋湖沼学报》2019,37(1):350-360

Atlantic salmon reared in recirculating aquaculture system (RAS) may lead to inappropriately high stocking density, because fish live in a limited space. Finding the suitable stocking density of Atlantic salmon reared in RAS is very important for RAS industry. In this paper, the influence of stocking density on growth and some stress related physiological factors were investigated to evaluate the effects of stocking density. The fish were reared for 220 days at five densities (A: 24 kg/m3; B: 21 kg/m3; C: 15 kg/m3; D: 9 kg/ m3 and E: 6 kg/m3 ). The results show that 30 kg/m3 might be the maximum density which RAS can afford in China. The stocking densities under 30 kg/m3 have no effect on mortality of Atlantic salmon reared in RAS. However, the specific growth rate (SGR), final weight and weight gain in the high density group were significantly lower than the lower density groups and middle density groups. Moreover, feed conversion rate (FCR) had a negative correlation with density. Plasma hormone T3 and GH showed significant decrease with the increase of the stocking density of the experiment. Furthermore, thyroid hormone (T3), GH (growth hormone) activities were decreased with stocking density increase. However, plasma cortisol, GOT (glutamic oxalacetic transaminase) and GPT (glutamic pyruvic transaminase) activities were increase with stocking density increase. And the stocking density has no effects on plasma lysozyme of Atlantic salmon reared in RAS. These investigations would also help devise efficient ways to rear adult Atlantic salmon in China and may, in a way, help spread salmon mariculture in China.

  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