全文获取类型
收费全文 | 7905篇 |
免费 | 740篇 |
国内免费 | 696篇 |
专业分类
测绘学 | 1825篇 |
大气科学 | 535篇 |
地球物理 | 523篇 |
地质学 | 1599篇 |
海洋学 | 227篇 |
天文学 | 17篇 |
综合类 | 973篇 |
自然地理 | 3642篇 |
出版年
2024年 | 26篇 |
2023年 | 63篇 |
2022年 | 275篇 |
2021年 | 352篇 |
2020年 | 327篇 |
2019年 | 375篇 |
2018年 | 284篇 |
2017年 | 386篇 |
2016年 | 398篇 |
2015年 | 398篇 |
2014年 | 478篇 |
2013年 | 590篇 |
2012年 | 539篇 |
2011年 | 564篇 |
2010年 | 461篇 |
2009年 | 403篇 |
2008年 | 411篇 |
2007年 | 411篇 |
2006年 | 405篇 |
2005年 | 352篇 |
2004年 | 337篇 |
2003年 | 280篇 |
2002年 | 250篇 |
2001年 | 220篇 |
2000年 | 169篇 |
1999年 | 127篇 |
1998年 | 109篇 |
1997年 | 99篇 |
1996年 | 54篇 |
1995年 | 45篇 |
1994年 | 35篇 |
1993年 | 29篇 |
1992年 | 17篇 |
1991年 | 11篇 |
1990年 | 14篇 |
1989年 | 7篇 |
1988年 | 7篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1982年 | 2篇 |
排序方式: 共有9341条查询结果,搜索用时 687 毫秒
141.
This article presents an econometric analysis of land‐cover change in western Honduras. Ground‐truthed satellite image analysis indicates that between 1987 and 1996 net reforestation occurred in the 1,015‐km2 study region. While some reforestation can be attributed to a 1987 ban on logging, the area of reforestation greatly exceeds that of previously clear‐cut areas. Further, new area was also deforested between 1987 and 1996. Thus, the observed land‐cover changes represent a complex mosaic of changing land‐use patterns across time and space. The analysis contributes to the literature on land‐cover change modeling in that: (1) it compares two econometric approaches to capture complex and often bidirectional changes in land cover from 1987 to 1996 as a function of agricultural suitability and transportation costs, and (2) it addresses techniques to identify and correct for spatial autocorrelation in a categorical regression framework. 相似文献
142.
This article provides an analysis of a wetland site in southern Illinois from presettlement to the present. The study area is part of the Cache River‐Cypress Creek Wetland, which has international importance, as recognized by the Ramsar Convention on Wetlands. Land‐cover data for 1807, 1938, and 1993 were created and analyzed with a geographic information system (GIS). Land‐use change by topographic setting (uplands, transitional, and bottomlands) and soil productivity was quantified and studied. Interviews with local experts informed this analysis. Results illustrate the complexity of environmental change and its driving forces. First, notable forest and swamp acreage was converted to cropland between 1807 and 1938 and, to a lesser degree, from 1938 to 1993. Second, there were land‐use variations by topographic region. Between 1807 and 1938, the largest transformation occurred in the uplands, with substantial acreage converted from forest to cropland. Between 1938 and 1993, however, agriculture decreased in the upland areas as hilly areas reverted to forest cover. At the same time, agriculture expanded in the bottomlands as this land was drained for farming. Third, there are interesting patterns within these categories of land‐use change, as soil productivity is an indicator of what lands were taken out of cropland and converted back to grassland and forest. 相似文献
143.
144.
Mark Cleary 《Singapore journal of tropical geography》2005,26(3):359-374
The paper examines how land and forest management policies were elaborated in French Indochina circa 1900–40. It places their development in the context of a scientific and economic discourse about the value of land and forest resources, the most appropriate ways in which they might be exploited and the relationship between colonial science and indigenous knowledge. By focusing on debates and laws relating to the development of small‐scale and plantation farming systems (Land Code legislations) and forest management and exploitation (Forest Code legislations) the paper seeks to ground arguments about Western conceptions of the “tropics” within a discussion of national policy development and impacts. Focusing primarily on Cochinchina and Annam (southern and central Vietnam) and drawing on materials from French archives, the paper shows how changes in both attitudes and legislation have had lasting consequences on systems of property rights in forest management and on the place and status of indigenous peoples in Indochina. 相似文献
145.
Land use and land cover changes have a great impact on the regional hydrological process. Based on three periods of remote sensing data from the 1960s and the long-term observed data of groundwater from the 1980s, the impacts of land use changes on the groundwater system in the middle reach of Heihe River Basin in recent three decades are analyzed by the perspective of groundwater recharge and discharge system. The results indicate that with the different intensities of land use changes, the impacts on the groundwater recharge were 2.602×108 m3/a in the former 15 years (1969-1985) and 0.218×108 m3/a in the latter 15 years (1986-2000), and the impacts on the groundwater discharge were 2.035×108 m3/a and 4.91×108 m3/a respectively. When the groundwater exploitation was in a reasonable range less than 3.0×108 m3/a, the land use changes could control the changes of regional groundwater resources. Influenced by the land use changes and the large-scale exploitation in the recent decade, the groundwater resources present apparently regional differences in Zhangye region. Realizing the impact of land use changes on groundwater system and the characteristics of spatial-temporal variations of regional groundwater resources would be very important for reasonably utilizing and managing water and soil resources. 相似文献
146.
J. Anthony Abbott 《The Professional geographer》2005,57(2):198-212
In‐field conservation of locally domesticated crop varieties ameliorates agrobiodiversity losses, but the interaction among nationally regulated socioeconomic factors at the local scale tends to discourage this. Analyses of household surveys conducted in Ecuador demonstrate that state and nongovernmental institutions interact to discourage cultivation of locally domesticated varieties of beans (Phaseolus spp.). Land privatization, agricultural extension, and credit programs favor market production of introduced modern bean varieties, and locally domesticated varieties are noted for favorable nutrition, culinary, and agroecological qualities. Resolving disconnections between the market and social values for landrace beans may provide agrobiodiversity conservation opportunities. 相似文献
147.
Legal factors play an underrated but significant role as constraints on conservation management. In this paper, based on observations in both Northern Ireland and the Republic of Ireland, mode of land tenure is assessed to determine its influence on the conservation management of coastal dunes. The tenure types considered are: private ownership, corporate private ownership, tenancy, secured tenancy, leasing, common (joint) ownership, uncertain ownership, public ownership and its subset military ownership. It is suggested that multiple private ownership is least likely to favour effective conservation management. Public ownership by a statutory conservation authority and corporate private ownership by a conservation-orientated NGO are regarded as the optimum tenure types for conservation management of coastal dunes. 相似文献
148.
The effects of land use on runoff and soil nutrient losses in a gully catchment of the hilly areas: implications for erosion control 总被引:1,自引:0,他引:1
WANG Jun FU Bojie QIU Yang CHEN Liding CHEN Chen HU Baodong 《地理学报(英文版)》2005,15(4):396-404
1 Introduction Soiland waterlossisoneofthe worldwide environm entalissuesthreatening sustainable land use in semiarid areas.However,soiland water loss is highly variable in space and time,and its variability resultsfrom m any factorsoperating ata wide ran… 相似文献
149.
Cynthia S. Simmons 《GeoJournal》2005,64(4):307-317
Brazil possesses a long history of violent struggle for land, and its most recent phase is occurring predominantly in the
Amazon Basin. Consequently, this paper attempts to territorialize land conflict in the Brazilian Amazon, and in so doing,
to illuminate the place-specific intersection of historic social, political, and economic circumstance that created a violent
landscape in the so-called “South of Pará.” The paper’s premise is that such conflict can be best viewed as resulting from
a dialectic between general social processes operating across spatial scales, which create necessary conditions for conflict,
and place-specific historical circumstances that transform necessary into sufficient conditions. The paper presents a framework
integrating the theory of contentious politics and literatures addressing violence associated with the Amazonian frontier
and with resource scarcity (and abundance). The discussion and theoretical application deconstruct the environmental, cognitive,
and relational mechanisms that created violence in the South of Pará, and set the stage for the Eldorado dos Carajás massacre in April of 1996. 相似文献
150.