全文获取类型
收费全文 | 8990篇 |
免费 | 2676篇 |
国内免费 | 1286篇 |
专业分类
测绘学 | 1700篇 |
大气科学 | 1242篇 |
地球物理 | 1756篇 |
地质学 | 4715篇 |
海洋学 | 1129篇 |
天文学 | 13篇 |
综合类 | 838篇 |
自然地理 | 1559篇 |
出版年
2025年 | 8篇 |
2024年 | 272篇 |
2023年 | 266篇 |
2022年 | 399篇 |
2021年 | 517篇 |
2020年 | 501篇 |
2019年 | 514篇 |
2018年 | 377篇 |
2017年 | 486篇 |
2016年 | 510篇 |
2015年 | 489篇 |
2014年 | 655篇 |
2013年 | 643篇 |
2012年 | 665篇 |
2011年 | 671篇 |
2010年 | 537篇 |
2009年 | 580篇 |
2008年 | 622篇 |
2007年 | 639篇 |
2006年 | 551篇 |
2005年 | 427篇 |
2004年 | 375篇 |
2003年 | 319篇 |
2002年 | 298篇 |
2001年 | 293篇 |
2000年 | 221篇 |
1999年 | 185篇 |
1998年 | 188篇 |
1997年 | 155篇 |
1996年 | 132篇 |
1995年 | 101篇 |
1994年 | 91篇 |
1993年 | 76篇 |
1992年 | 66篇 |
1991年 | 24篇 |
1990年 | 24篇 |
1989年 | 20篇 |
1988年 | 15篇 |
1987年 | 12篇 |
1986年 | 7篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1977年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
Analysis of reservoir water quality using fuzzy synthetic evaluation 总被引:12,自引:0,他引:12
R.-S. Lu S.-L. Lo J.-Y. Hu 《Stochastic Environmental Research and Risk Assessment (SERRA)》1999,13(5):327-336
A general methodology for fuzzy synthetic evaluation is developed and illustrated with a case study of trophic status assessment
for Fei-Tsui Reservoir in Taiwan. The historical data base was collected from the management agency of Fei-Tsui Reservoir
from 1987 to 1996. In fuzzy synthetic evaluation, the classification is determined by a matrix operation of the weighted vector
with the fuzzy evaluation matrix. After all individual membership functions of evaluated factors have been determined, the
fuzzy evaluation matrix can be established. The weighted vector is determined by the analytic hierarchy process method (AHP).
The results of this investigation show that the long-term change of water quality and the overturn phenomena cannot be observed
with the Carlson index from 1987 to 1992 but is expressed by fuzzy synthetic evaluation. Fuzzy synthetic evaluation is better
suited than the Carlson index to rating the trophic status of self-sustaining lakes. Interpretation of the results can provide
valuable information to decision makers and aid reservoir management. 相似文献
13.
Evaluation of density in layer compaction using SASW method 总被引:1,自引:0,他引:1
SASW test, which is non-intrusive and rapid in the field application, was used to evaluate the layer density in the roller compaction without performing the complicated inversion process. The concept of normalized shear wave velocity was introduced to minimize the effect of confinement in the density evaluation. SASW test was performed to determine the shear wave velocity of the layer, and the free–free resonant column (FF–RC) test was adopted to determine the correlation between the normalized shear wave velocity and density of the site, which is almost unique independent of confinement. Testing and data reduction procedures of both tests were briefly discussed and an evaluation procedure of the field density was proposed by effectively combining in-situ shear wave velocity determined by the SASW test with the correlation between the normalized shear wave velocity and the density determined by the FF–RC test. Finally, the feasibility of the proposed method was verified by performing a field case study at Hoengsung road construction site. Field densities determined by the proposed method matched well with those determined by sand cone tests, showing the potential of applying the proposed method in the field density evaluation. 相似文献
14.
塔里木盆地是我国最大的半封闭型内陆盆地, 干旱、大风、沙尘暴、洪水以及地震、雪灾等自然灾害频繁.为了提高塔里木盆地城市应对自然灾害的能力, 以阿克苏地区为例, 在广泛借鉴国内外灾害脆弱性评价的指标体系与评价模型的基础上, 以多灾种复合情况为背景, 构建塔里木盆地自然灾害脆弱性评价指标体系, 运用模糊综合评价法进行脆弱性评价, 并对各指标进行对比分析.结果表明: 阿克苏市、新和县、阿瓦提县的高脆弱度值最高, 分别为0.48、0.36及0.40, 属于高脆弱度; 温宿县、沙雅县的中脆弱度值较高, 分别0.24和0.26, 属于中脆弱度; 库车县、拜城县、乌什县、柯坪县的低脆弱度值较高, 分别为0.48、0.45、0.36及0.35, 属于低脆弱度.根据各县市自然灾害脆弱性评价结果, 结合区域特点, 提出了防灾抗灾的对策建议, 对塔里木盆地城市降低脆弱性、减轻自然灾害影响有一定借鉴意义. 相似文献
15.
阐述了岩质边坡岩体力学参数研究的意义,介绍了国内外常用的几种岩体力学参数的确定方法,但其准确性值得商榷和论证。针对Hoek-Brown强度准则D值的确定存在的困难,基于模糊综合评判,对产生扰动的影响因数综合考虑,确定了依托工程的D值,工程应用表明更加接近实际情况,并且探讨了岩质边坡的岩体以滑动面为界限划分扰动区与非扰动区的合理性,克服了其他公式的不足。以利比里亚邦矿铁矿岩体力学参数研究为例,分别用计算出的修正参数和其他几种方法进行计算分析对比,结果表明所计算出的修正D值更能反映实际的扰动程度,并论证了划分扰动区与非扰动区的合理性和意义,具有重要参考价值。 相似文献
16.
Heejun Chang 《The Professional geographer》2004,56(2):240-257
This study investigates potential changes in nitrogen and phosphorus loads under a warmer and wetter climate, urban growth, and combined changes in the Conestoga River Basin and its five subbasins in southeastern Pennsylvania. A GIS‐based hydrochemical model was employed for assessing the sensitivity of the basins to the projected changes in 2030. Under the HadCM2 climate change scenario, mean annual nitrogen and phosphorus loads are expected to increase, with great increases in spring but slight decreases in fall primarily because of changes in monthly precipitation. When climate change and urbanization occur concurrently, mean annual nitrogen loads further increase by 50% in the most urbanizing subbasin. Point source nitrogen control could mitigate negative effects of climate and land use changes, reducing mean annual nitrogen loads to the contemporary baseline level. 相似文献
17.
岩体工程质量分级应用研究 总被引:1,自引:0,他引:1
岩体质量评价采用的测试技术包括地质勘察、弹性波检测和室内岩石力学试验。结合水布垭水利枢纽大坝建基岩体工程质量评价实例,对岩体工程质量分级的应用进行了研究。 相似文献
18.
Quantitative assessment of vulnerability is a core aspect of wetland vulnerability research. Taking Baiyangdian (BYD) wetlands in the North China Plain as a study area and using the ‘cause-result’ model, 23 representative indicators from natural, social, sci-tech and economic elements were selected to construct an indicator system. A weight matrix was obtained by using the entropy weight method to calculate the weight value for each indicator. Based on the membership function in the fuzzy evaluation model, the membership degrees were determined to form a fuzzy relation matrix. Finally, the ecological vulnerability was quantitatively assessed based on the comprehensive evaluation index calculated by using a composite operator to combine the entropy weight matrix with the fuzzy relation matrix. The results showed that the ecological vulnerability levels of the BYD wetlands were comprehensively evaluated as Grade II, Grade Ⅲ, Grade IV, and Grade Ⅲ in 2010, 2011-2013, 2014, and 2015-2017, respectively. The ecological vulnerability of the BYD wetlands increased from low fragility in 2010 to general fragility in 2011-2013, and to high fragility in 2014, reflecting the fact that the wetland ecological condition was degenerating from 2010 to 2014. The ecological vulnerability status then turned back into general fragility during 2015-2017, indicating that the ecological situation of the BYD wetlands was starting to improve. However, the ecological status of the BYD wetlands on the whole is relatively less optimistic. The major factors affecting the ecological vulnerability of the BYD wetlands were found to be industrial smoke and dust emission, wetland water area, ammonia nitrogen, total phosphorus, rate of industrial solid wastes disposed, GDP per capita, etc. This illustrates that it is a systematic project to regulate wetland vulnerability and to protect regional ecological security, which may offer researchers and policy-makers specific clues for concrete interventions. 相似文献
19.
Remotely sensed assessment of water quality levels in the Pearl River Estuary, China 总被引:3,自引:0,他引:3
In this paper, a method of assessing water quality from satellite data is introduced. The composite pollution index (CPI) was calculated from measured chemical oxygen demand (COD) and nutrient concentration. The relationships between CPI and 240 band combinations of SeaWiFS water-leaving radiance were analyzed and the optimal band combination for estimating CPI was chosen from the 240 band combinations. An algorithm for retrieval of CPI was developed using the optimal band combination, (L443 × L510)/(L412 + L490). The CPI was estimated from atmospherically corrected SeaWiFS data by employing the algorithm. Furthermore, the CPI value range for each water quality level was determined based on data obtained from 850 samples taken in the Pearl River Estuary. The remotely sensed CPIs were then transferred to water quality levels and appropriate maps were derived. The remotely sensed water quality level maps displayed a similar distribution of levels based on in situ investigation issued by the State Ocean Administration, China. This study demonstrates that remote sensing can play an important role in water quality assessment. 相似文献
20.
本文从土地资源禀赋、土地利用效益、土地保护水平三个方面,初步建立了生态城市建设土地利用评价指标体系,并以扬州市为研究案例,得出扬州市2000~2008年的土地利用与保护综合水平。研究结果表明:2000年,扬州市土地利用与保护综合评价分值为52.9,处于生态恢复阶段中的治理小于破坏阶段;自2000~2008年扬州市土地利用与保护综合水平随时间呈递增趋势,特别是2004年以来递增速度明显加快,到2008年扬州市土地利用与保护综合分值为74.7,处于生态恢复阶段的末期,即将跨入生态保育阶段。最后,根据评价结论,提出了今后一段时期扬州生态城市建设土地利用与保护的建议。 相似文献