全文获取类型
收费全文 | 966篇 |
免费 | 92篇 |
国内免费 | 72篇 |
专业分类
测绘学 | 52篇 |
大气科学 | 112篇 |
地球物理 | 85篇 |
地质学 | 191篇 |
海洋学 | 5篇 |
天文学 | 1篇 |
综合类 | 94篇 |
自然地理 | 590篇 |
出版年
2024年 | 2篇 |
2023年 | 12篇 |
2022年 | 40篇 |
2021年 | 37篇 |
2020年 | 41篇 |
2019年 | 41篇 |
2018年 | 41篇 |
2017年 | 33篇 |
2016年 | 33篇 |
2015年 | 43篇 |
2014年 | 50篇 |
2013年 | 71篇 |
2012年 | 58篇 |
2011年 | 62篇 |
2010年 | 56篇 |
2009年 | 37篇 |
2008年 | 51篇 |
2007年 | 39篇 |
2006年 | 50篇 |
2005年 | 55篇 |
2004年 | 38篇 |
2003年 | 30篇 |
2002年 | 45篇 |
2001年 | 30篇 |
2000年 | 28篇 |
1999年 | 19篇 |
1998年 | 12篇 |
1997年 | 20篇 |
1996年 | 10篇 |
1995年 | 8篇 |
1994年 | 7篇 |
1993年 | 7篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 6篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
排序方式: 共有1130条查询结果,搜索用时 187 毫秒
31.
本文通过对豫东、豫北等河南农业主产区80年代以来工业化进程的调研分析,在总结经验教训的基础上,针对这些地区工业发展的特殊条件,提出了平原农区工业化的基本思路和途径,主导产业的选择和注意处理的几个问题。 相似文献
32.
33.
吉林省西部沙化土地持续农业体系建立的实践 总被引:2,自引:1,他引:2
吉林省沙化土地的87.9%分布在吉林省西部白城、松原半干旱地区,统属科尔沁和松嫩沙地,面积9216.5km2,占吉林省西部总面积的19.6%。8年(1986年~1993年)实践证明:针对沙地环境特点建立的“乔、灌、草”、“果、药、杂”、“田、林、草”为主体的三种持续农业体系效果明显。 相似文献
34.
中国干旱半干旱区农业生态地质环境系统工程研究 总被引:1,自引:0,他引:1
干旱半干旱区是当前我国农业经济发展最有潜力的地区。可是,该区生态环境脆弱,并受到一定程度的破坏。利用系统工程学的原理,从农业生态地质环境容量的角度出发,对该区的农业生态地质环境的演化过程和演化模式进行了研究,进而提出了农业生态地质环境系统工程研究模式。在调控社会人口和经济发展的基础上,合理开发利用水、土、生物资源,保护治理生态环境,提高自然资源环境的承载力,抑制对资源环境的破坏力,使该区农业生态环境得以恢复和重建,建立起稳定持续发展的农业基地。 相似文献
35.
柴达木盆地近40a气候变化及其对农业影响的研究 总被引:7,自引:2,他引:7
利用柴达木盆地6个代表站逐日平均气温、逐月降水资料以及逐日最低气温资料。对盆地40a来的气候变化特征进行分析。同时分析了气候变化对农业生产的影响。为充分利用变化了的农业气候资源。减少不利因素带来的影响提出了对策和建议。 相似文献
36.
37.
Water and water related matters are important issues in water scarce countries like the Republic of South Africa. This is
especially true for the agricultural sector that relies heavily on the availability of water for irrigation to ensure a secure
food supply. In South Africa, present day irrigation projects rely on government subsidies and are usually large undertakings
beyond the reach of individuals or small communities. The sustainability of such large irrigation projects is still a matter
of debate. A 100 years ago a different set of rules applied. Irrigation could only be achieved by hand-dug furrows that supplied
water in relatively small quantities to small farming communities. This paper investigated such a small community, the Leeukloof
farming community in the Western Cape Province of the Republic of South Africa. For the past 93 years this community has maintained
a furrow that supplies water not only to farms bordering the Perdeberg River, which supplies the furrow with water, but also
to farms not bordering the river. The authors try to reconstruct the changes in agricultural practices brought about by the
furrow, methods of water extraction during each phase, as well as the development and role of the furrow system. The perception
by the community of the necessity of this method of water extraction from the river system is also investigated.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
38.
During the last decade of the 20th century, extensive conversion in agricultural land use took place in Northeast China. The goal of this study is to ascertain its spatial distribution and regional differentiation, determine its causes, and analyze its environmental impact. Especially we attempt to elucidate how institutional constraints have facilitated the change at a time of agrarian restructuring when newly emerging free market was hybridized with the former planned economy. Information on six categories of land use was mapped from interpretation of Landsat TM images recorded in 1990, 1995 and 2000. Most of land use changes took place during the first half of the decade, coinciding with abrupt and chaotic changes in government directives. Farmland was changed mainly to woodland, water body and built-up areas while woodland and grassland were converted chiefly to farmland. Spatially, the change from farmland to woodland was restricted to the west of the study area. The change from grassland to farmland took place in the grazing and farming interlocked west. These chaotic and occasionally conflicting changes were largely caused by lack of stability and consistency in agricultural land use policies promulgated. They have exerted adverse impacts on the local environment, including land degradation, increased flooding, and modified climate regime. 相似文献
39.
What is the source of baseflow in agriculturally fragmented catchments? Complex groundwater/surface‐water interactions in three tributary catchments of the Wabash River,Indiana, USA
下载免费PDF全文
![点击此处可从《水文研究》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Marty D. Frisbee Zachary P. Meyers Noah S. Stewart‐Maddox Marc W. Caffee Philine Bogeholz Madison N. Hughes 《水文研究》2017,31(22):4019-4038
Some conceptual models suggest that baseflow in agriculturally fragmented watersheds may contain little, if any, groundwater. This has critical implications for stream quality and ecosystem functioning. Here, we (a) identify the sources and flowpaths contributing to baseflow using 222Rn and 87Sr/86Sr and (b) quantify mean apparent ages of groundwater and baseflow using multiple isotopic tracers (CFC, SF6, 36Cl, and 3H) in 4 small (0.08 to 0.64 km2) tributary catchments to the Wabash River in Indiana, USA. 222Rn activities and 87Sr/86Sr ratios indicate that baseflow in 3 catchments is sourced primarily from groundwater; baseflow in the fourth is dominated by a source similar to agricultural run‐off. CFC‐12 data indicate that springs in 1 catchment are discharging significant proportions of water that recharged between 1974 (42 ± 2 years) and 1961 (55 ± 2 years). Those same springs have 36Cl/Cl ratios between 1,381.08 ± 29.37 (×10?15) and 1,530.64 ± 27.65 (×10?15) indicating that a substantial proportion of the discharge likely recharged between 1975 (41 years) and 1950 (66 years). Groundwater samples collected from streambed mini‐piezometers in a separate catchment have CFC‐12 concentrations indicating that a large proportion of the recharge occurred between 1948 (68 ± 2 years) and 1950 (66 ± 2 years). Repeat sampling conducted in September 2015 after above‐average summer rainfall did not show significant decreases in mean apparent age. The relatively old ages observed in 3 of the catchments can be explained by geological complexities that are likely present in all 4 catchments, but overwhelmed by flow from the shallow phreatic aquifer in the fourth catchment. 相似文献
40.