全文获取类型
收费全文 | 7144篇 |
免费 | 1739篇 |
国内免费 | 2510篇 |
专业分类
测绘学 | 86篇 |
大气科学 | 8558篇 |
地球物理 | 197篇 |
地质学 | 1289篇 |
海洋学 | 258篇 |
天文学 | 2篇 |
综合类 | 159篇 |
自然地理 | 844篇 |
出版年
2024年 | 58篇 |
2023年 | 241篇 |
2022年 | 323篇 |
2021年 | 333篇 |
2020年 | 308篇 |
2019年 | 368篇 |
2018年 | 268篇 |
2017年 | 304篇 |
2016年 | 255篇 |
2015年 | 343篇 |
2014年 | 462篇 |
2013年 | 449篇 |
2012年 | 391篇 |
2011年 | 448篇 |
2010年 | 410篇 |
2009年 | 421篇 |
2008年 | 416篇 |
2007年 | 488篇 |
2006年 | 404篇 |
2005年 | 431篇 |
2004年 | 381篇 |
2003年 | 444篇 |
2002年 | 414篇 |
2001年 | 385篇 |
2000年 | 358篇 |
1999年 | 250篇 |
1998年 | 292篇 |
1997年 | 322篇 |
1996年 | 282篇 |
1995年 | 268篇 |
1994年 | 225篇 |
1993年 | 162篇 |
1992年 | 139篇 |
1991年 | 139篇 |
1990年 | 97篇 |
1989年 | 80篇 |
1988年 | 9篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1974年 | 1篇 |
1957年 | 2篇 |
1955年 | 2篇 |
1954年 | 1篇 |
1943年 | 2篇 |
1942年 | 1篇 |
1941年 | 3篇 |
1936年 | 2篇 |
1935年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
991.
992.
993.
994.
995.
氮沉降和降水格局改变对草原生态系统的结构、功能及关键过程具有重要影响。依托内蒙古乌拉特荒漠草原研究站的全球变化实验平台,研究了氮添加和增减雨(+50%、-50%)及其交互作用对荒漠草原植物群落光合速率和植被特征的影响,分析了荒漠草原植物群落光合速率与植被特征的关系。结果表明:(1)短期氮添加对荒漠草原植物群落光合速率和植被特征没有显著影响(P > 0.05);(2)降水格局改变显著影响荒漠植物群落光合速率(P < 0.05),减雨50%显著降低了荒漠草原植物群落光合速率和优势种植物高度(P < 0.05),而降水增加50%没有改变群落光合速率和植被特征,降水改变下的土壤水分能很好地解释群落光合速率;(3)氮添加和增加降水的交互效应显著提高了群落的光合速率和优势种植物高度(P < 0.05),而减少降水与氮添加没有显著影响;(4)荒漠草原植物群落盖度、优势种盖度、优势种平均高度与群落的光合速率呈现出指数增加关系,解释率为40%~58%。干旱极大地抑制荒漠草原植物群落光合速率,而氮沉降则依赖于降水增加来提高群落的光合速率,荒漠草原植物群落光合速率与水肥处理下的植物生长特征具有密切的关系。 相似文献
996.
997.
达州市气候变化的特征分析 总被引:1,自引:0,他引:1
基于达县测站1961-2007年的地面常规观测资料,主要采用小波分析,结合趋势分析、功率谱分析等方法分析了达州市47年来近地表气温、降水随时间的变化特征.结果表明,达州市近50年来冬季(年平均)地表气温变暖幅度约为0.9℃(0.4℃),增温速率接近0.18℃/10a(0.08℃/10a),低于全球的升温幅度.利用小波分析方法研究冬季气温的周期性发现,达州市的冬季气温存在着明显的8年和50年以上的特征时间尺度和周期性振荡.通过对降水的趋势分析发现,夏季降水的增加幅度比年均显著.小波分析方法结果显示达州市的夏季降水同样存在50年以上的特征时间尺度,但在小尺度上20世纪70年代末前后存在不同的变化特征. 相似文献
998.
提出基于模糊Hopfield神经网络的大气降水粒子分类,它是通过计算样本间的模糊关系的相似程度,再加上Hopfield网络所特有的高稳定性,对大气降水粒子进行分类.通过对各降水粒子所对应雷达回波数据的分析,验证了方法的可行性. 相似文献
999.
Impacts of Climatic Factors on Runoff Coefficients in Source Regions of the Huanghe River 总被引:4,自引:0,他引:4
CHEN Liqun LIU Changming LI Yanping WANG Guoqiang 《中国地理科学(英文版)》2007,17(1):047-055
Runoff coefficients of the source regions of the Huanghe River in 1956–2000 were analyzed in this paper. In the 1990s runoff of Tangnaihai Hydrologic Station of the Huanghe River experienced a serious decrease, which had at- tracted considerable attention. Climate changes have important impact on the water resources availability. From the view of water cycling, runoff coefficients are important indexes of water resources in a particular catchment. Kalinin baseflow separation technique was improved based on the characteristics of precipitation and streamflow. After the separation of runoff coefficient (R/P), baseflow coefficient (Br/P) and direct runoff coefficient (Dr/P) were estimated. Statistic analyses were applied to assessing the impact of precipitation and temperature on runoff coefficients (including Dr/P, Br/P and R/P). The results show that in the source regions of the Huanghe River, mean annual baseflow coefficient was higher than mean annual direct runoff coefficient. Annual runoff coefficients were in direct proportion to annual pre- cipitation and in inverse proportion to annual mean temperature. The decrease of runoff coefficients in the 1990s was closely related to the decrease in precipitation and increase in temperature in the same period. Over different sub-basins of the source regions of the Huanghe River, runoff coefficients responded differently to precipitation and temperature. In the area above Jimai Hydrologic Station where annual mean temperature is –3.9oC, temperature is the main factor in- fluencing the runoff coefficients. Runoff coefficients were in inverse relation to temperature, and precipitation had nearly no impact on runoff coefficients. In subbasin between Jimai and Maqu Hydrologic Station Dr/P was mainly affected by precipitation while R/P and Br/P were both significantly influenced by precipitation and temperature. In the area be-tween Maqu and Tangnaihai hydrologic stations all the three runoff coefficients increased with the rising of annual precipitation, while direct runoff coefficient was inversely proportional to temperature. In the source regions of the Huanghe River with the increase of average annual temperature, the impacts of temperature on runoff coefficients be-come insignificant. 相似文献
1000.
Evaporation (E) rate and precipitation (P) rate are two significant meteorological elements required in the ocean baroclinic
modeling as external forcings. However, there are some uncertainties in the currently used E/P rates datasets, especially
in terms of the data quality. In this study, we collected E/P rates data from ERA-40, NCEP/NCAR Reanalysis, HOAPS for the
Bohai Sea and nine routine stations around Laizhou Bay, and made comparisons among them. It was found that the differences
in E/P rates between land and sea are remarkable, which was due to the difference in underlying surfaces. Therefore, the traditional
way of using E/P rates acquired on land directly at sea is not correct. Since no final conclusion has been reached concerning
the net water transport between the Bohai Sea and the Yellow Sea, it is unfeasible to judge the adequacy of the four kinds
of data by using the water budget equation. However, the E/P rates at ERA-40 sea points were considered to be the optimal
in terms of temporal/spatial coverage and resolution for the hindcast of salinity variation in the Bohai Sea. Besides, using
the 3-D hydrodynamic model HAMSOM (HAMburg Shelf Ocean Model), we performed numerical experiments with different E/P datasets
and found that the E/P rates at sea points from ERA-40 dataset are better than those from NCEP/NCAR Reanalysis dataset. If
NCEP/NCAR Reanalysis E/P rates are to be used, they need to be adjusted and tested prior to simulation so that more close-to-reality
salinity values can be reproduced. 相似文献