全文获取类型
收费全文 | 30016篇 |
免费 | 510篇 |
国内免费 | 360篇 |
专业分类
测绘学 | 808篇 |
大气科学 | 2174篇 |
地球物理 | 6036篇 |
地质学 | 10321篇 |
海洋学 | 2702篇 |
天文学 | 6994篇 |
综合类 | 70篇 |
自然地理 | 1781篇 |
出版年
2021年 | 258篇 |
2020年 | 298篇 |
2019年 | 362篇 |
2018年 | 693篇 |
2017年 | 665篇 |
2016年 | 849篇 |
2015年 | 534篇 |
2014年 | 816篇 |
2013年 | 1523篇 |
2012年 | 940篇 |
2011年 | 1270篇 |
2010年 | 1113篇 |
2009年 | 1487篇 |
2008年 | 1315篇 |
2007年 | 1331篇 |
2006年 | 1314篇 |
2005年 | 999篇 |
2004年 | 991篇 |
2003年 | 897篇 |
2002年 | 857篇 |
2001年 | 690篇 |
2000年 | 728篇 |
1999年 | 635篇 |
1998年 | 633篇 |
1997年 | 586篇 |
1996年 | 453篇 |
1995年 | 475篇 |
1994年 | 483篇 |
1993年 | 372篇 |
1992年 | 371篇 |
1991年 | 308篇 |
1990年 | 376篇 |
1989年 | 328篇 |
1988年 | 312篇 |
1987年 | 341篇 |
1986年 | 289篇 |
1985年 | 383篇 |
1984年 | 423篇 |
1983年 | 399篇 |
1982年 | 378篇 |
1981年 | 304篇 |
1980年 | 334篇 |
1979年 | 269篇 |
1978年 | 260篇 |
1977年 | 254篇 |
1976年 | 227篇 |
1975年 | 238篇 |
1974年 | 214篇 |
1973年 | 217篇 |
1972年 | 135篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
R. K. Ray T. H. Syed Dipankar Saha B. C. Sarkar A. K. Patre 《Hydrogeology Journal》2017,25(8):2513-2525
Extracted groundwater, 90% of which is used for irrigated agriculture, is central to the socio-economic development of India. A lack of regulation or implementation of regulations, alongside unrecorded extraction, often leads to over exploitation of large-scale common-pool resources like groundwater. Inevitably, management of groundwater extraction (draft) for irrigation is critical for sustainability of aquifers and the society at large. However, existing assessments of groundwater draft, which are mostly available at large spatial scales, are inadequate for managing groundwater resources that are primarily exploited by stakeholders at much finer scales. This study presents an estimate, projection and analysis of fine-scale groundwater draft in the Seonath-Kharun interfluve of central India. Using field surveys of instantaneous discharge from irrigation wells and boreholes, annual groundwater draft for irrigation in this area is estimated to be 212 × 106 m3, most of which (89%) is withdrawn during non-monsoon season. However, the density of wells/boreholes, and consequent extraction of groundwater, is controlled by the existing hydrogeological conditions. Based on trends in the number of abstraction structures (1982–2011), groundwater draft for the year 2020 is projected to be approximately 307 × 106 m3; hence, groundwater draft for irrigation in the study area is predicted to increase by ~44% within a span of 8 years. Central to the work presented here is the approach for estimation and prediction of groundwater draft at finer scales, which can be extended to critical groundwater zones of the country. 相似文献
982.
Concentrations of Ni, Ga, Ge and Ir in 106 iron meteorites are reported. Three new groups are defined: IC, IIE and IIIF containing 10, 12 and 5 members, respectively, raising the number of independent groups to 12. Group IC is a cohenite-rich group distantly related to IA. Group IIE consists of those irons previously designated Weekeroo Station type and five others having similar compositions though diverse structures. The IIE irons are compositionally similar to the mesosiderites and pallasites, and the three groups probably formed at similar heliocentric distances. The mixing of the globular IIE silicates with the metal probably occurred during shock events. Group IIIF is a well-defined group of low-Ni and low-Ge irons. The compositions of these groups are summarized as follows:
Group | Ni (%) | Ga (ppm) | Ge (ppm) | Ir (ppm) |
IC | 6.1–6.8 | 42–54 | 85–250 | 0.07–10 |
IIE | 7.5–9.7 | 21–28 | 62–75 | 0.5–8 |
IIIF | 6.8–7.8 | 6.3–7.2 | 0.7–1.1 | 1.3–7.9 |