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   16041篇
  免费   2805篇
  国内免费   3320篇
测绘学   686篇
大气科学   1594篇
地球物理   4113篇
地质学   8804篇
海洋学   2676篇
天文学   771篇
综合类   1037篇
自然地理   2485篇
  2024年   137篇
  2023年   298篇
  2022年   482篇
  2021年   711篇
  2020年   680篇
  2019年   750篇
  2018年   630篇
  2017年   680篇
  2016年   646篇
  2015年   739篇
  2014年   938篇
  2013年   1178篇
  2012年   907篇
  2011年   995篇
  2010年   934篇
  2009年   982篇
  2008年   1019篇
  2007年   1070篇
  2006年   1100篇
  2005年   933篇
  2004年   889篇
  2003年   781篇
  2002年   703篇
  2001年   600篇
  2000年   555篇
  1999年   472篇
  1998年   433篇
  1997年   358篇
  1996年   277篇
  1995年   275篇
  1994年   241篇
  1993年   183篇
  1992年   136篇
  1991年   104篇
  1990年   72篇
  1989年   85篇
  1988年   46篇
  1987年   36篇
  1986年   24篇
  1985年   28篇
  1984年   13篇
  1983年   12篇
  1982年   4篇
  1981年   9篇
  1980年   6篇
  1979年   2篇
  1978年   9篇
  1973年   1篇
  1971年   3篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
781.
The Zhujiang (Pearl) River Delta is the rapid economic development region in China since the opening and reform door policy was carried out in 1978. Being the rapid development of industry and city, the impact on the aquatic environmental quality was significant. The pollution caused the water quality descended and the ecological system degraded, and also impeded the economic development. The characteristics and problems of the aquatic environment are: the capacity of aquatic environment is large but hasn’t been utilized rationally, the water quality is influenced by saline sea water and tide current, the main pollutants are organic matter and the pollution is going heavier, the concentration of pollutants change seasonally. The countermeasures of aquatic environmental protection are: carrying out the environmental functional regionalization and controlling the total amount of pollutant discharge, revising the industrial structure and making a rational industrial arrangement, raising the rate of waste water treatment and making a full assessment of the water conservancy project.  相似文献   
782.
本文在讨论西安市地面沉降特征的基础上,论述了西安市地下水开采量、承压水位变化与地面沉降三者之间在时间和空间上的对应关系,初步认定大量开采地下水是地面沉降主要原因。典型水准点沉降量与邻近自备井承压水位变化相关分析表明,两者间相关性极强。经综合分析市区地貌、第四系地质、工程地质和水文地质特征并进行工程地质分层后,可确定出市区地面沉降的第一和第二主要压缩层。据地面沉降与承压水位下降值回归方程所预测的地面沉降发展趋势表明,西安地面沉降今后还会继续发展,将对西安城市建设产生严重影响,故应采取相应治理措施。  相似文献   
783.
渤海与环渤海地区年降水量的统计分析   总被引:4,自引:1,他引:4  
李宁  刘珍  顾卫 《地理研究》2006,25(6):1022-1030
本文以渤海海域及环渤海地区分水岭内为研究区域,利用环渤海地区分水岭内92个气象站点自1971年至2000年30年的降水量资料,基于ARC/INFO地理信息系统支持下的泰森多边形法,对研究区域内的平均年降水量进行了统计分析。结论如下:环渤海地区分水岭以内区域的平均年降水量占整个环渤海地区2000年降水量的63.88%;渤海海域海水直接利用量占该海域平均年降水量的8.82%,其中辽东湾的海水利用量占该湾平均年降水量的3.09%,渤海湾为15.86%,莱州湾为17.38%;辽东湾2002年的海冰资源量占该湾平均年降水量的6.08%。在渤海海域水量平衡的关系研究中,本研究是初步的,基础的。划出分水岭内的研究区域进行平均年降水量的统计较对整个环渤海地区进行统计更具精确性。  相似文献   
784.
Knowledge of sub-pixel heterogeneity, particularly at the passive microwave scale, can improve the brightness temperature (and ultimately the soil moisture) estimation. However, the impact of surface heterogeneity (in terms of soil moisture, soil temperature and vegetation water content) on brightness temperature in an agricultural setting is relatively unknown. The Soil Moisture Active Passive Validation Experiment 2012 (SMAPVEX12) provided an opportunity to evaluate sub-pixel heterogeneity at the scale of a Soil Moisture Ocean Salinity (SMOS) or the Soil Moisture Active Passive (SMAP) radiometer footprint using field measured data. The first objective of this study was to determine if accounting for surface heterogeneity reduced the error between estimated brightness temperature (Tb) and Tb measured by SMOS. It was found that when accounting for variation in surface soil moisture, temperature and vegetation water content within the pixel footprint, the error between the modelled Tb and the measured Tb was less than if a homogeneous pixel were modelled. The correlation between the surface parameters and the error associated with not accounting for surface heterogeneity were investigated. It was found that there was low to moderate correlation between the error and the coefficient of variance associated with the measured soil moisture, soil temperature and vegetation volumetric water content during the field campaign. However, it was found that the correlations changed depending on the stage of vegetation growth and the amount of time following a precipitation event. At the start of the field campaign (following a precipitation event), there was strong correlation between the error and all three surface parameters (r  0.75). Following a precipitation event close to the middle of the field campaign (during which there was rapid growth in vegetation), there was strong correlation between the error and the variability in vegetation water content (r = 0.89), moderate correlation with soil moisture (r = 0.61) and low correlation with soil temperature (r = 0.26).  相似文献   
785.
近几年来,河池市地下水总硬度出现逐年升高趋势。本文采用地下水化学成分随机抽样回归分析法分析其形成机理。认为该市地下水总硬度的组成是以重碳酸盐和硫酸盐为主,而硬度的升高是以组成暂时的重碳酸盐的增大表现出来的。并对防止地下水硬度升高提出具体意见。  相似文献   
786.
Sea level rise could increase the salinity of an estuary by altering the balance between fresh water and salt water. The implications of sea level rise for increasing salinity have been examined in the Changjiang (Yangtze) River estuary. By correlative analysis of chlorinity, discharge and tidal level and calculation of two-dimensional chlorinity, distribution of the Changjiang River estuary, the changes of the intensity and lasting hours of salt water intrusion at Wusong Station and the changes of chlorinity distribution in the South Branch of the Changjiang River estuary have been estimated when future sea level rises 50-100 cm. The intensity of salt water intrusion in the future will be far more serious than current trend.  相似文献   
787.
In this paper, the contents and various forms of Cd, Cu, Zn and Pb in the sediments of the Xiangjiang River have been investigated. The presentation of this paper focuses on the various forms and characteristics of the heavy metals in the column sediments at Xiawan Port of Zhuzhou, which has been severely polluted by a metallurgical plant.The forms of the heavy metals discussed in this paper are: The concentration of the exchangeable forms of Cd, Cu, Zn and Pb, in the forms of carbonates and Fe/ Mn oxide in water, the combination of Cu with organic matter and form, and the proportion of the residue form at each section have been studied.  相似文献   
788.
North China, whose total area is 420,000 km2, covers 2 provinces and 2 cities (Shanxi and Hebei provinces, Beijing and Tianjin) and the territories of Henan and Shandong provinces to the north of the Huanghe (Yellow) River, and part of the Inner Mongolia Autonomous Region and Liaoning Province. Most of this region, which is situated in the semi-humid, semi-arid continental climate temperate zone, gets an annual precipitation of 500-600 mm, whereas, the perennial average amount of total water resource is 50.99 billion m3, including 33.82 billion m3 of surface water, 32.94 billion m3 of ground water and 15.77 billion m3 of their duplication. Due to the intensive exploitation of water resource caused by the deficiency of water resource in North China, key elements of water balance was changed, runoff volume reduced, evaporation increased, vertical movement of moisture strengthened, water circulation pattern transformed from open system into regionally closed system; meanwhile, due to the incompetent water  相似文献   
789.
对邢台地震台地下流体数字化水位观测资料的内在质量及年、月、日动态特征等进行分析.结果表明:所辖流体观测井水位资料连续、可靠,内在质量较高,4个测点均是较理想的井水位观测点,其中,衡水井、辛集井水位对远场大震的同震效应明显;宁晋井因受井孔条件所限,同震效应不明显.井水位年变均为趋势性下降变化,但下降幅度均大于5 m,不利...  相似文献   
790.
孙孝波  朱玉祥  孙珂  王小国 《水文》2012,32(3):66-68,28
水资源承载能力制约和决定了经济社会可持续发展,拓展提高水资源承载能力,能为经济社会可持续发展提供更大的水资源支撑空间。黄河两岸的浅层地下水含水层,具备相当大的蓄储调蓄功能,形成了巨大的地下水库。计算结果显示,在原有水资源总量不增加的情况下,运用地下水库调蓄黄河水和大气降水,使得中原沿黄经济带2020年75%、90%年型的缺水率从11.4%、21.7%下降为0.5%和2.1%。采用地下水库调蓄很大程度上减少了中原沿黄经济带水资源开发利用的缺水率,使该地区的水资源承载能力大大提高。  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