全文获取类型
收费全文 | 21135篇 |
免费 | 4262篇 |
国内免费 | 4980篇 |
专业分类
测绘学 | 1517篇 |
大气科学 | 3462篇 |
地球物理 | 5423篇 |
地质学 | 10704篇 |
海洋学 | 3926篇 |
天文学 | 393篇 |
综合类 | 1476篇 |
自然地理 | 3476篇 |
出版年
2024年 | 70篇 |
2023年 | 269篇 |
2022年 | 630篇 |
2021年 | 929篇 |
2020年 | 901篇 |
2019年 | 979篇 |
2018年 | 905篇 |
2017年 | 985篇 |
2016年 | 935篇 |
2015年 | 1067篇 |
2014年 | 1339篇 |
2013年 | 1669篇 |
2012年 | 1330篇 |
2011年 | 1415篇 |
2010年 | 1287篇 |
2009年 | 1411篇 |
2008年 | 1427篇 |
2007年 | 1491篇 |
2006年 | 1490篇 |
2005年 | 1259篇 |
2004年 | 1201篇 |
2003年 | 1015篇 |
2002年 | 954篇 |
2001年 | 805篇 |
2000年 | 722篇 |
1999年 | 617篇 |
1998年 | 549篇 |
1997年 | 499篇 |
1996年 | 387篇 |
1995年 | 359篇 |
1994年 | 323篇 |
1993年 | 277篇 |
1992年 | 213篇 |
1991年 | 156篇 |
1990年 | 117篇 |
1989年 | 111篇 |
1988年 | 69篇 |
1987年 | 53篇 |
1986年 | 33篇 |
1985年 | 39篇 |
1984年 | 19篇 |
1983年 | 14篇 |
1982年 | 7篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1978年 | 11篇 |
1976年 | 2篇 |
1971年 | 3篇 |
1954年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 687 毫秒
961.
The influence of land use and land cover on ecological environment is a focus of global change research. The paper chooses an industrial city-Shuicheng in Guizhou Province-as a study area because the karst water quality around the city is deteriorating with land use and land cover change. The natural susceptibility of karst water system is an important factor leading to karst water pollution. But land use and land cover change is also a main factor according to the chemical analysis of karst water quality and land use change. So it is a good way to protect karst water through rational planning and managing of land use and land cover. 相似文献
962.
An economic analysis was conducted for temporal yield variations of four local grape cultivars grown in rain-fed microcatchments in the Bajgah area of Fars province, in Islamic Republic of Iran. For this study, four local cultivars of grapes were considered. The cultivars examined were Black Rishbaba, Rotabi, Asgari and Black grapes. The soil type was a gravely loam with an average slope of 5–6%, in microcatchment systems, and in plots without microcatchments. Analysis of yield data with a multiple regression model indicated that a 9 m2 (3×3 m) microcatchment area for each individual plant (vine) was the most appropriate area for vineyards in this region. The results also demonstrated that by using this area for each grapevine, yields (kg/ha) were 40% greater than yields obtained for vines in the standard vineyard (without microcatchments) in this area. Economic analysis was performed and the cost and income were compared for a depreciation period (life-span period) of 50 years and interest rate of 14%. The analysis showed that a microcatchment area of 9 m2 is economically feasible. The Black Rishbaba rain-fed vines were the most suitable cultivar for this region. The benefit probabilities of rain-fed Black Rishbaba, Rotabi, Asgari and Black grapes with a microcatchment area of 9 m2 and without such an area were 63.3% (i.e. two of 3 years), 52.4% (i.e. one of 2 years), 32.9% (i.e. one of 3 years), 13.3% (i.e. one of 8 years), and 59.5% (i.e. two of 3 years), 39.4% (i.e. two of 5 years), 31.8% (i.e. one of 3 years), and 10.7% (i.e. one of 9 years), respectively. Therefore, the Black Rishbaba and Rotabi cultivars are economically more stable in microcatchment system, while only the Black Rishbaba cultivar would be such in the standard vineyard system. 相似文献
963.
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
964.
965.
966.
M. D. Sharma 《Geophysical Journal International》2004,159(2):703-710
967.
968.
利用1960~1999年全国600个站月平均降水资料,对黄河三门峡水库—小浪底水库间的夏季降水总量年际变化进行了统计分析,并应用功率谱诊断方法提取了夏季各月降水的振荡周期。在此基础上,选取了黄河三门峡水库—小浪底水库间降水量多与少的年份,利用NCEP再分析资料研究了其上空的水汽变化以及水汽输送量变化。结果表明,黄河三门峡水库—小浪底水库间降水量多的年份,其上空为较为明显的水汽辐合;降水量比较少的年份,其上空为明显的水汽辐散。选取多雨年(1982年)及少雨年(1997年),结合NCEP再分析资料以及TBB资料,进一步验证了上述结论。 相似文献
969.
970.