全文获取类型
收费全文 | 22417篇 |
免费 | 4267篇 |
国内免费 | 4664篇 |
专业分类
测绘学 | 1086篇 |
大气科学 | 1951篇 |
地球物理 | 4987篇 |
地质学 | 13586篇 |
海洋学 | 3873篇 |
天文学 | 61篇 |
综合类 | 1666篇 |
自然地理 | 4138篇 |
出版年
2024年 | 103篇 |
2023年 | 300篇 |
2022年 | 669篇 |
2021年 | 958篇 |
2020年 | 910篇 |
2019年 | 1091篇 |
2018年 | 879篇 |
2017年 | 965篇 |
2016年 | 949篇 |
2015年 | 1078篇 |
2014年 | 1336篇 |
2013年 | 1588篇 |
2012年 | 1279篇 |
2011年 | 1477篇 |
2010年 | 1317篇 |
2009年 | 1400篇 |
2008年 | 1415篇 |
2007年 | 1452篇 |
2006年 | 1567篇 |
2005年 | 1322篇 |
2004年 | 1292篇 |
2003年 | 1108篇 |
2002年 | 999篇 |
2001年 | 891篇 |
2000年 | 801篇 |
1999年 | 699篇 |
1998年 | 628篇 |
1997年 | 515篇 |
1996年 | 458篇 |
1995年 | 390篇 |
1994年 | 348篇 |
1993年 | 287篇 |
1992年 | 196篇 |
1991年 | 175篇 |
1990年 | 128篇 |
1989年 | 112篇 |
1988年 | 68篇 |
1987年 | 44篇 |
1986年 | 34篇 |
1985年 | 34篇 |
1984年 | 20篇 |
1983年 | 16篇 |
1982年 | 12篇 |
1981年 | 11篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1978年 | 10篇 |
1976年 | 2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 15 毫秒
991.
地理环境的结构,是指地理环境各组成要素之间和各组成部分之间相互关系性质的组合,它包括既有联系、又有区别的两个方面,即地理环境结构的整体性和差异性。笔者在亚、欧两洲自然地理数年的教学实践中,从自然环境结构的整体性和差异性特征入手,得出分析、综合、概括归纳和对比研究等方法是学习世界区域地理的主要方法。 相似文献
992.
993.
自适应空间信息移动服务 总被引:4,自引:0,他引:4
空间信息移动服务系统是空间信息系统继由传统PC计算环境向有线Web分布式计算环境扩展后,向移动计算环境的新发展。文中在分析移动计算环境概念和特点的基础上,讨论了空间信息移动服务的若干特点及关键技术,研究了自适应空间信息移动服务方案。 相似文献
994.
水下地形测量高程异常点剔除方法研究 总被引:2,自引:0,他引:2
本文针对水下地形测量过程中测深数据的粗差问题,结合在生产工作中的一些经验和有关的资料,提出了测深数据处理的方法,并编写了相应的程序用于生产。 相似文献
995.
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
996.
After dividing the source regions of the Yellow River into 38 sub-basins, the paper made use of the SWAT model to simulate streamflow with validation and calibration of the observed yearly and monthly runoff data from the Tangnag hydrological station, and simulation results are satisfactory. Five land-cover scenario models and 24 sets of temperature and precipitation combinations were established to simulate annual runoff and runoff depth under different scenarios. The simulation shows that with the increasing of vegetation coverage annual runoff increases and evapotranspiration decreases in the basin. When temperature decreases by 2oC and precipitation increases by 20%, catchment runoff will increase by 39.69%, which is the largest situation among all scenarios. 相似文献
997.
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
998.
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
999.
1000.