全文获取类型
收费全文 | 1374篇 |
免费 | 320篇 |
国内免费 | 112篇 |
专业分类
测绘学 | 53篇 |
大气科学 | 93篇 |
地球物理 | 145篇 |
地质学 | 233篇 |
海洋学 | 746篇 |
天文学 | 1篇 |
综合类 | 66篇 |
自然地理 | 469篇 |
出版年
2024年 | 12篇 |
2023年 | 47篇 |
2022年 | 56篇 |
2021年 | 76篇 |
2020年 | 68篇 |
2019年 | 65篇 |
2018年 | 51篇 |
2017年 | 86篇 |
2016年 | 75篇 |
2015年 | 41篇 |
2014年 | 105篇 |
2013年 | 103篇 |
2012年 | 81篇 |
2011年 | 121篇 |
2010年 | 85篇 |
2009年 | 66篇 |
2008年 | 72篇 |
2007年 | 76篇 |
2006年 | 69篇 |
2005年 | 55篇 |
2004年 | 57篇 |
2003年 | 59篇 |
2002年 | 40篇 |
2001年 | 37篇 |
2000年 | 34篇 |
1999年 | 26篇 |
1998年 | 25篇 |
1997年 | 23篇 |
1996年 | 16篇 |
1995年 | 21篇 |
1994年 | 7篇 |
1993年 | 6篇 |
1992年 | 8篇 |
1991年 | 5篇 |
1990年 | 4篇 |
1989年 | 2篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 2篇 |
排序方式: 共有1806条查询结果,搜索用时 15 毫秒
941.
942.
943.
金沙江下游近40年来土壤侵蚀变化--以云南彝良为例 总被引:7,自引:0,他引:7
云南金沙江流域是我国水土流失最为严重的区域之一,尤其以金沙江下游区最为突出。为揭示该区域土壤侵蚀动态变化的基本规律,为今后水土保持和土地资源可持续利用提供依据,选择该区域较为典型的彝良县作为研究实例,采用航片判读、实地调查、GIS等方法编制了1960、1980和2000年土壤侵蚀图,据此分析该县1960~2000年的土壤侵蚀变化特点,并应用马尔柯夫模型预测在当前土地利用方式下土壤侵蚀动态变化趋势,为该县制定水土保持措施和土地利用规划提供了重要依据。 相似文献
944.
Mairi Jay 《New Zealand geographer》2005,61(1):14-28
Abstract: This paper addresses the issue of conservation of native biodiversity on privately owned farmland in New Zealand. Based on surveys of Waikato dairy farmers as exemplars of intensive agricultural practice, it examines factors that influence the survival of native forest on land with potential for commercial production. Results suggest that a significant proportion of Waikato dairy farmers regard native forest favourably, although the proportion of farmers who actively conserve their forest is small. Factors that assist the persistence of native forest on dairy farms include personal characteristics of the farmer, past accidents of history which have left forest remnants in place, and physical characteristics of the farm such as topography. While the conservation of native biodiversity within this intensively farmed landscape is strongly influenced by political economy pressures that encourage production, non‐utilitarian motives such as aesthetic enjoyment and family heritage can serve to counter the production ethic. 相似文献
945.
Financial efficiency of major soil and water conservation measures in West Usambara highlands, Tanzania 总被引:2,自引:0,他引:2
Soil and Water Conservation (SWC) measures are needed to control soil erosion and sustain agricultural production on steep slopes of West Usambara mountains. However, the adoption by farmers of the recommended soil and water conservation measures is low and soil erosion continues to be a problem. It could well be that the reason for the low adoption is that the costs to invest in soil and water conservation are higher than the eventual benefits. This research assessed the costs and benefits of bench terraces, grass strips and fanya juu, which are major SWC measures. Financial Cost Benefit Analysis (FCBA) was undertaken for farmers with low, moderate and high opportunity costs of labour at different slopes and soil types. The results show that labour is the major cost item in implementing SWC measures and is higher with bench terraces than with fanya juu and grass strips. The results also show that the costs of establishing the three SWC measures exceed the returns in the initial 2 years. However, in the long term, the three SWC measures are profitable to farmers with low to medium opportunity costs of labour on gentle to moderate slopes. It was also found that SWC measures are not financially attractive to most farmers with off-farm activities and other sources of income. It is concluded that high investment costs and initial negative returns are the major hindrances to the adoption of SWC measures by smallholder farmers in West Usambara mountains. Options to overcome the initial investment costs include the gradual investment in SWC measures, introduction of high value crops and small credit facilities. The promotion of dairy cattle under zero grazing system can also increase the adoption of SWC measures because of the high benefits from grasses used to stabilise SWC measures. 相似文献
946.
Michael O'Neal Campbell 《Singapore journal of tropical geography》2005,26(2):151-169
Despite recent interest in sacred (fetish) groves as remnant forests, few studies have investigated their sustainability and conservation role in West Africa. This article employs a Geographical Information Systems (GIS) analysis of time series images (1960‐98), comprehensive social surveys and ecological field methods to evaluate four sacred groves and eight unprotected tree stands in the coastal savanna of Ghana and compare these with vegetation in the distant forested hinterland. There were strong similarities and substantial differences in tree species between different sacred groves, and between these and the unprotected stands and proximate deciduous forests. In addition, far fewer tree losses were documented in the sacred groves than in the local unprotected stands. Although these sacred groves were on average only partially representative of deciduous forest vegetation, their stronger sustainability compared with unprotected tree stands may be important to consider in detail for conservation. 相似文献
947.
Dnes Schmera 《Limnologica》2004,34(3):274-278
Wildlife conservation requires methods to express the conservation status of assemblages in order to identify those of importance. Expressions of diversity and a number of different conservation status indices are widely used techniques for assessing assemblages. Some of these indices are based on species lists, while others use abundances. In this study, the conservation statuses of 21 light-traped adult caddisfly assemblages were compared using species lists and abundance data. The results showed that the assessment based on species list provided a higher conservation status than that based on species abundances. Consequently, indices incorporating species abundances were not comparable with indices using only species lists. The difference between the two measures might be derived from the unequal weighting of species categories. In conclusion, incorporating species abundance reduces sampling method bias and increases the precision of the conservation status evaluation. 相似文献
948.
Hydrological responses to conservation practices in a catchment of the Loess Plateau,China 总被引:2,自引:0,他引:2
Since the late 1950s a series of soil conservation practices have been implemented in the Loess Plateau. It is important to assess the impact of these practices on hydrology at the catchment scale. The Jialuhe River catchment, a tributary of the Yellow River, with a drainage area of 1117 km2 in the Loess Plateau, was chosen to investigate the hydrological responses to conservation practices. Parametric and non‐parametric Mann–Kendall tests were utilized to detect trends in hydrological variables or their residuals. Relationships between precipitation and hydrological variables were developed to remove the impact of precipitation variability. Significant linear decreasing trends in annual surface runoff and baseflow were identified during the treated period from 1967 to 1989, and the rate of reduction was 1·30 and 0·48 mm/year, respectively. As result, mean annual surface runoff and baseflow decreased by 32% over the period of 1967 to 1989. Seasonal runoff also decreased during the treated period with the greatest reduction occurring in summer and the smallest reduction in winter. The response of high and low daily flow to conservation practices was greater than average flows. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
949.
This paper introduces a new consistent dissipation operator. It is based on the explicit square conservation scheme and the theory of consistent dissipation. The operator makes full use of the advantages of the Leap-frog scheme, i.e., its second order time precision and its explicit solution manner. Meanwhile, it overcomes the fatal disad-vantage, the absolute instability in computations, of the scheme. When it is applied to the explicit square conservation scheme, the time precision of the scheme reaches to third order. Especially, the computational stability of this scheme is as good as the third order explicit Runge-Kutta scheme. The CPU time required in computations by the scheme is less than that required by the explicit square conservation scheme with the consistent dissipation operator constructed from the Runge-Kutta method. Therefore, the new operator is an economical one. The application of the operator to the improvement of the dynamical model of the L2IAP AGCM shows its time-saving property and its good effects 相似文献
950.