全文获取类型
收费全文 | 2415篇 |
免费 | 515篇 |
国内免费 | 773篇 |
专业分类
测绘学 | 62篇 |
大气科学 | 1614篇 |
地球物理 | 302篇 |
地质学 | 597篇 |
海洋学 | 129篇 |
天文学 | 18篇 |
综合类 | 131篇 |
自然地理 | 850篇 |
出版年
2024年 | 14篇 |
2023年 | 38篇 |
2022年 | 66篇 |
2021年 | 79篇 |
2020年 | 98篇 |
2019年 | 124篇 |
2018年 | 76篇 |
2017年 | 77篇 |
2016年 | 85篇 |
2015年 | 119篇 |
2014年 | 165篇 |
2013年 | 218篇 |
2012年 | 167篇 |
2011年 | 178篇 |
2010年 | 137篇 |
2009年 | 128篇 |
2008年 | 152篇 |
2007年 | 207篇 |
2006年 | 160篇 |
2005年 | 156篇 |
2004年 | 165篇 |
2003年 | 161篇 |
2002年 | 119篇 |
2001年 | 117篇 |
2000年 | 123篇 |
1999年 | 93篇 |
1998年 | 94篇 |
1997年 | 76篇 |
1996年 | 74篇 |
1995年 | 53篇 |
1994年 | 44篇 |
1993年 | 44篇 |
1992年 | 30篇 |
1991年 | 26篇 |
1990年 | 7篇 |
1989年 | 12篇 |
1988年 | 8篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有3703条查询结果,搜索用时 515 毫秒
991.
992.
993.
A Conceptual Framework for Enhancing the Utility of Rainfall Hazard Forecasts for Agriculture in Marginal Environments 总被引:1,自引:0,他引:1
Muhammad?T.?UsmanEmail author Emma?Archer Peter?Johnston Mark?Tadross 《Natural Hazards》2005,34(1):111-129
Semi-arid and dry sub-humid areas (especially in the tropics) are characterized by high inter-annual and intra-seasonal rainfall variability. Agriculture, which employs the bulk of the rapidly increasing populations, is largely rain-fed, low-input based and highly resource dependent. Recent spates of drought have, therefore, exacerbated the now-too-familiar specter of famine and starvation in these areas with glaring examples being the recurring episodes in sub-Saharan Africa since the great Sahel drought of 1969–1973. A great need for accurate and timely hazard forecast products in aid of agriculture thus exists.Several schemes are currently employed by various agencies around the globe in this direction. There does remain, however, a gap between product provision and user expectations. This paper examines this gap suggesting a five-point framework within which it can be addressed as an action agenda for the climate science community. The paper posits that changes are possible to existing methodologies (related to three of these points), which, within the context of current science, can greatly enhance the utility of forecast products for agriculture in marginal areas. The remaining two points have, however, been identified as requiring additional applied research and necessary pointers for addressing these issues are provided. First is the need for appropriate impact-related indicators for intra-seasonal and interannual rainfall variability that are easy to compute, amenable to forecasting and follow closely the experiences of farmers in marginal areas. The second is a consideration of appropriate forecast information formatting and communication medium that guarantee effective feedback between forecast producers and users. Specific examples of the status quo and of work currently underway are cited from southern Africa – a region currently attracting international attention as a result of recent droughts and the threat of famine. 相似文献
994.
CLIMATIC VARIATIONS IN CHINA OVER THE LAST 2000 YEARS 总被引:4,自引:0,他引:4
1 INTRODUCTIONIn the last years, evidence for climate changes showed that the timing of the two great climatic events of “Medieval Warm Period” (MWP) and “Little Ice Age” (LIA) differed geographically (HUGHES et al., 1994, JONES and BRADLEY, 1992). In other words, the cold or warm periods in one region were often not coincident with those in other regions. The Northern Hemisphere warmth degree in the Middle Ages was lower than or at most comparable to that in the mid-20… 相似文献
995.
福建省干旱和水资源开发研究 总被引:2,自引:0,他引:2
分析了福建省干旱分布和气候、水资源供需现状和水资源开发前景。结果表明:国民经济的快速发展和城镇化水平的加快,使大致稳定的降水量与明显增长的供水需求间的矛盾不断尖锐,水资源短缺制约了经济的发展和影响了公众的日常生活。分析了福建省主要干旱季节对流云系含水量和降水效率,结果表明:福建省旱期主要降水云系对流云含水量丰富,降水效率偏低,可以通过人工增雨提高降水效率,达到调节水资源短缺的目的。 相似文献
996.
997.
We investigate how smallholder farmers at two sites in Kenya and Tanzania cope with climate stress and how constraints and opportunities shape variations in coping strategies between households and over time during a drought. On the basis of this analysis, we draw out implications for adaptation and adaptive policy. We find that households where an individual was able to specialize in one favoured activity, such as employment or charcoal burning, in the context of overall diversification by the household, were often less vulnerable than households where each individual is engaged in many activities at low intensity. Many households had limited access to the favoured coping options due to a lack of skill, labour and/or capital. This lack of access was compounded by social relations that led to exclusion of certain groups, especially women, from carrying out favoured activities with sufficient intensity. These households instead carried out a multitude of less favoured and frequently complementary activities, such as collecting indigenous fruit. While characterized by suitability to seasonal environmental variations and low demands on time and cash investments, these strategies often yielded marginal returns. Both the marginalization of local niche products and the commercialization of forest resources exemplify processes leading to differential vulnerability. We suggest that vulnerability can usefully be viewed in terms of the interaction of such processes, following the concept of locality. We argue that coping is a distinct component of vulnerability and that understanding the dynamism of coping and vulnerability is critical to developing adaptation measures that support people as active agents. 相似文献
998.
999.
早期智人古老型——智人大荔亚种的年代 总被引:2,自引:0,他引:2
文中综合研究了有关大荔人头骨化石的产出地层、伴随出现的古生物化石、古气候演变的条件等 ,结合用热释光技术测定地层年代这一基本思路 ,将大荔人的生存年代确定为不老于 30万年。 相似文献
1000.
PERSISTENCES IN THE δ~(13)C AND δ~(18)O FROM TS95, A LAKE CORE AT THE SOUTH FRANK OF WEST KUNLUN MOUNTAIN 相似文献