全文获取类型
收费全文 | 3381篇 |
免费 | 341篇 |
国内免费 | 249篇 |
专业分类
测绘学 | 1362篇 |
大气科学 | 181篇 |
地球物理 | 556篇 |
地质学 | 602篇 |
海洋学 | 204篇 |
天文学 | 6篇 |
综合类 | 320篇 |
自然地理 | 740篇 |
出版年
2024年 | 8篇 |
2023年 | 18篇 |
2022年 | 90篇 |
2021年 | 105篇 |
2020年 | 142篇 |
2019年 | 158篇 |
2018年 | 111篇 |
2017年 | 235篇 |
2016年 | 201篇 |
2015年 | 201篇 |
2014年 | 205篇 |
2013年 | 376篇 |
2012年 | 277篇 |
2011年 | 202篇 |
2010年 | 148篇 |
2009年 | 127篇 |
2008年 | 138篇 |
2007年 | 191篇 |
2006年 | 183篇 |
2005年 | 143篇 |
2004年 | 137篇 |
2003年 | 92篇 |
2002年 | 103篇 |
2001年 | 51篇 |
2000年 | 49篇 |
1999年 | 45篇 |
1998年 | 60篇 |
1997年 | 38篇 |
1996年 | 28篇 |
1995年 | 14篇 |
1994年 | 19篇 |
1993年 | 9篇 |
1992年 | 9篇 |
1991年 | 16篇 |
1990年 | 9篇 |
1989年 | 7篇 |
1988年 | 10篇 |
1987年 | 2篇 |
1986年 | 5篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 1篇 |
排序方式: 共有3971条查询结果,搜索用时 46 毫秒
1.
Average velocity in streams is a key variable for the analysis and modelling of hydrological and hydraulic processes underpinning water resources science and practice. The present study evaluates the impact of the sampling duration on the quality of average velocity measurements acquired with contemporary instruments such as Acoustic Doppler Velocimeters (ADV) an Acoustic Doppler Current Profilers (ADCP). The evaluation combines considerations on turbulent flows and principles and configurations of acoustic instruments with practical experience in conducting customized analysis for uncertainty analysis purposes. The study sheds new insights on the spatial and temporal variability of the uncertainty in the measurement of average velocities due to variable sampling durations acting in isolation from other sources of uncertainties. Sampling durations of 90 and 150 s are found sufficient for ADV and ADCP, respectively, to obtain reliable average velocities in a flow affected only by natural turbulence and instrument noise. Larger sampling durations are needed for measurements in most of the natural streams exposed to additional sources of data variability. 相似文献
2.
Subsurface flow measurements using passive flux meters in variably-saturated cold-regions landscapes
To date, passive flux meters have predominantly been applied in temperate environments for tracking the movement of contaminants in groundwater. This study applies these instruments to reduce uncertainty in (typically instantaneous) flux measurements made in a low-gradient, wetland dominated, discontinuous permafrost environment. This method supports improved estimation of unsaturated and over-winter subsurface flows which are very difficult to quantify using hydraulic gradient-based approaches. Improved subsurface flow estimates can play a key role in understanding the water budget of this landscape. 相似文献
3.
Studies of soil productivity must compensate for the effects of temporal trends in order to examine the pattern of crop yields along spatial gradients. An analysis of the published yield estimates for 30 soils in 233 counties, however, did not find consistent yield increases over the past three decades. On the contrary, the yield estimates for many soils were markedly uniform since 1972. The uniformity appears to have two causes: the acknowledged difficulty of making yield estimates in a time of increasing variability in soil and crop management, both within and between regions, and surveyors' awareness of data stored in a national soils data base. The effect is to cast doubt on soil productivity data reported in county soil surveys published between 1973 and 1988. 相似文献
4.
5.
气相色谱法测定地下水中六六六结果的不确定度评定 总被引:7,自引:4,他引:3
依照《测量不确定度评定与表示》,对气相色谱法测定地下水中六六六(HCH)四种单体结果进行了不确定度评定。分析了测量过程中引入的不确定度来源,包括提取液体积的量取、样品提取溶液的定容体积、分析仪器的进样量、标准系列溶液的测量以及仪器重复测定等分量引入不确定度及其各参数的采集和计算方法,最后合成标准不确定度,通过乘以95%概率下的扩展因子2,获得测量结果的扩展不确定度。 相似文献
6.
7.
Deborah F. Shmueli 《Geoforum》2008,39(6):2048-2061
When applied to environmental concerns, framing offers a rigorous conceptual and analytic approach with potentially practical significance for dealing with complex issues relevant to geography. The purpose of this article is to introduce framing concepts, typology and modes of analysis to address issues which geographers commonly treat: disputes over land ownership and uses, competition for water resources, cultural clashes over control of territory and resources and the impact of spatial patterns and structures when siting noxious sites. Framing analysis is applied to three cases of environmental conflict in Israel and, more generally, for understanding conflicts revolving around the management of human habitats in relation to the physical environment. 相似文献
8.
9.
10.