全文获取类型
收费全文 | 43369篇 |
免费 | 19576篇 |
国内免费 | 44376篇 |
专业分类
测绘学 | 3518篇 |
大气科学 | 22536篇 |
地球物理 | 14661篇 |
地质学 | 39663篇 |
海洋学 | 17023篇 |
天文学 | 1169篇 |
综合类 | 5319篇 |
自然地理 | 3432篇 |
出版年
2024年 | 218篇 |
2023年 | 489篇 |
2022年 | 961篇 |
2021年 | 1362篇 |
2020年 | 2409篇 |
2019年 | 5768篇 |
2018年 | 6075篇 |
2017年 | 5818篇 |
2016年 | 5948篇 |
2015年 | 5171篇 |
2014年 | 4774篇 |
2013年 | 5334篇 |
2012年 | 5071篇 |
2011年 | 4816篇 |
2010年 | 4756篇 |
2009年 | 4093篇 |
2008年 | 3304篇 |
2007年 | 3315篇 |
2006年 | 2925篇 |
2005年 | 2736篇 |
2004年 | 3063篇 |
2003年 | 2861篇 |
2002年 | 2724篇 |
2001年 | 2480篇 |
2000年 | 1974篇 |
1999年 | 2084篇 |
1998年 | 2027篇 |
1997年 | 2081篇 |
1996年 | 1666篇 |
1995年 | 1592篇 |
1994年 | 1412篇 |
1993年 | 1343篇 |
1992年 | 1138篇 |
1991年 | 833篇 |
1990年 | 797篇 |
1989年 | 650篇 |
1988年 | 549篇 |
1987年 | 426篇 |
1986年 | 340篇 |
1985年 | 277篇 |
1984年 | 301篇 |
1983年 | 178篇 |
1982年 | 220篇 |
1981年 | 157篇 |
1980年 | 122篇 |
1979年 | 117篇 |
1978年 | 50篇 |
1977年 | 45篇 |
1971年 | 54篇 |
1970年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
251.
252.
Has climate change driven spatio-temporal changes of cropland in northern China since the 1970s? 总被引:4,自引:0,他引:4
Wenjiao Shi Fulu Tao Jiyuan Liu Xinliang Xu Wenhui Kuang Jinwei Dong Xiaoli Shi 《Climatic change》2014,124(1-2):163-177
Improving the understanding of cropland change and its driving factors is a current focus for policy decision-makers in China. The datasets of cropland and cropland changes from the 1970s to the 2000s were used to explore whether climate change has produced spatio-temporal changes to cropland in northern China since the 1970s. Two representative indicators of heat and water resources, which are important determinants of crop growth and productivity, were considered to track climate change, including active accumulated temperatures ≥10 °C (AAT10) and the standardized precipitation evapotranspiration index (SPEI). Our results showed that rapid cropland change has occurred in northern China since the 1970s, and the area of cropland reclamation (10.23 million ha) was much greater than that of abandoned cropland (2.94 million ha). In the 2000s, the area of cropland with AAT10 higher than 3,000 °C·d increased, while the area of cropland with an SPEI greater than 0.25 decreased compared to the 1970s, 1980s and 1990s. It appears that climate warming has provided thermal conditions that have aided rapid cropland reclamation in northern China since the 1970s, and drier climatic conditions did not become a limiting factor for cropland reclamation, especially from the 1990s to the 2000s. Approximately 70 % of cropland reclamation areas were located in warmer but drier regions from the 1990s to the 2000s, and approximately 40 % of cropland abandonment occurred in warmer and wetter conditions that were suitable for agriculture during the periods from the 1970s to the 1980s and the 1990s to the 2000s. Our results suggest that climate change can be considered a driving factor of cropland change in the past several decades in northern China, in addition to socioeconomic factors. 相似文献
253.
Drought is one of the most complex natural hazards affecting agriculture, water resources, natural ecosystems, and society. The negative societal consequences of drought include severe economic losses, famine, epidemics, and land degradation. However, few studies have analyzed the complexity of drought characteristics, both at multiple time scales and with variations in evapotranspiration. In this study, drought occurrences were quantified using a new drought index, the Standardized Precipitation Evapotranspiration Index (SPEI), based on observed data of monthly mean temperature and precipitation from 1961 to 2013 in Henan province, central China. Based on the SPEI values of each weather station in the study, the frequency and severity of meteorological droughts were computed, and the monthly, seasonal, and annual drought frequency and intensity over a 53-year period were analyzed. The spatial and temporal evolution, intensity, and the primary causes of drought occurrence in Henan were revealed. The results showed that the SPEI values effectively reflected the spatial and temporal pattern of drought occurrence. As the time scale decreased, the amplitude of the SPEI increased and droughts became more frequent. Since 1961, drought has occurred at the annual, seasonal, and monthly scales, and the occurrence of drought has increased. However, regional distribution has been uneven. The highest drought frequency, 35%, was observed in the Zhoukou region, while the lowest value, ~26%, was measured in central and western Henan. The most severe droughts occurred in the spring and summer, followed by autumn. Annually, wide-ranging droughts occurred in 1966–1968, 1998–2000, and 2011–2013. The drought intensity showed higher values in north and west Henan, and lower values in its east and south. The maximum drought intensity value was recorded in Anyang, and the minimum occurred in Zhumadian, at 22.18% and 16.60%, respectively. The factors with the greatest influence on drought occurrence are increasing temperatures, the Eurasian atmospheric circulation patterns, and the El Niño effect. 相似文献
254.
255.
This work reports on a method using fuzzy membership functions to construct an aggregated interaction matrix in which the summation of variables is scaled according to the way rainfall and soil variables affect water availability to plants and hence influence rangeland productivity. Aggregation of the variables gives a comprehensive value which can be used to predict production. The model increases the predicability of production to 81% compared to models using rainfall alone and a multiplicative parametric one which give predictibilities of 61 and 76% respectively. The results showed that (1) the importance of rainfall in determining production was most important at lower rainfalls i.e. <350 mm; (2) soil texture and particularly slope were important through out the rainfall range (149–700 mm) investigated; and (3) soil depth was only important at the higher >350 mm rainfalls. The aggregated interaction matrix gives a measure of land productive capability. 相似文献
256.
In this paper, stable isotope(δ~(18)O, δD) investigations were completed in ground ice from a deep borehole in the Beiluhe Basin on northern Qinghai-Tibet Plateau to unravel the isotopic variations of ground ice and their possible source water. The δ~(18)O and δD of ground ice show distinctive characteristics compared with precipitation and surface water. The near-surface ground ice is highly enriched in heavier isotopes(δ~(18)O and δD), which were gradually depleted from top to bottom along the profile. It is suggestive of different origin and ice formation process. According to isotopic variations, the ice profile was divided into three sections: the near-surface ground ice at 2.5 m is frozen by the active-layer water which suffered evaporation. It is possible that ground ice between 3 and 4.2 m is recharged by the infiltration of snowmelt. From 5 to 6 m, the ground ice show complex origin and formation processes. Isotopic variations from 6 to 11.1 m and 20.55 m indicate different replenishment water. The calculated slope of freezing line(S=6.4) is larger than the experimental value(5.76), and is suggestive of complex origin and formation process of ground ice. 相似文献
257.
目的 探讨平均血小板体积等参数在急性胰腺炎病程中变化的临床意义及其对治疗效果评估的价值。方法 回顾性分析53例轻症急性胰腺炎(MAP)患者、25例重症急性胰腺炎(SAP)患者和40例健康者(对照组)的临床资料,检测平均血小板体积(MPV)、血小板计数(PLT)、血小板分布宽度(PDW)、白细胞计数(WBC)、C反应蛋白(CRP)及血清钙离子浓度(Ca2+)等指标。比较并分析上述指标在急性胰腺炎病程中及治疗前后的变化。结果 与对照组比较,急性胰腺炎患者入院时MPV显著升高,差异具有统计学意义(P<0.05);与MAP组比较,SAP组入院时MPV明显升高,差异具有统计学意义(P<0.05);SAP组中,MPV在入院后第3天较入院时进一步升高,至入院第5天、第7天逐渐下降;SAP组中,MPV在病程中各时间点与MAP组比较均显著升高,差异有统计学意义(P<0.05);与治疗前比较,急性胰腺炎患者治疗后MPV显著降低,差异有统计学意义(P<0.05)。结论 急性胰腺炎患者平均血小板体积与病情严重程度及治疗效果相关。 相似文献
258.
Environmental pollution, food safety and health are closely linked. A key challenge in addressing the problem of food safety and protecting public health is building an integrated knowledge base to inform policy and strengthen governance. This requires breaking down the trans-departmental information barrier across the environment, food and health domains to ensure the effective flow of data and the efficient utilization of resources, and facilitate the collaborative governance of food safety. Achieving this will be crucial for the development of health and medical care in China in the era of big data. Currently, the information resources commanded by various departments are incomplete and fragmented. Data resources are also organized in vertical silos and there is a lack of data sharing within and across policy streams. To provide the basis for more effective integrated collection and analysis of data in future, this study summarizes the information resources of various departments whose work relates to interactions between environment, food and health, and presents measures to strengthen top-down design, and establish unified data standards and a big data sharing platform. It also points to the need for increased training of data analysts with interdisciplinary expertise. 相似文献
259.
三维矿床模型及可视化研究是我国"数字化矿山"进程中的热点,本文应用数字化矿山软件Surpac,针对某钼矿66个钻孔数据进行三维矿床模型的构建,最终在三维可视化模型的基础上进行了钼矿资源储量估算。与传统计算方法对比结果表明,Surpac软件三维模型所计算的结果准确可靠,且该模型可依据品位的变化进行矿体的动态圈定。 相似文献
260.
Grasslands and agro-ecosystems occupy one-third of the global terrestrial area. However, great uncertainty still exists about their contributions to the global carbon cycle. This study used various com... 相似文献