全文获取类型
收费全文 | 3372篇 |
免费 | 178篇 |
国内免费 | 837篇 |
专业分类
测绘学 | 158篇 |
大气科学 | 354篇 |
地球物理 | 1417篇 |
地质学 | 1335篇 |
海洋学 | 379篇 |
天文学 | 139篇 |
综合类 | 341篇 |
自然地理 | 264篇 |
出版年
2024年 | 8篇 |
2023年 | 26篇 |
2022年 | 16篇 |
2021年 | 31篇 |
2020年 | 27篇 |
2019年 | 31篇 |
2018年 | 42篇 |
2016年 | 33篇 |
2015年 | 27篇 |
2014年 | 65篇 |
2013年 | 44篇 |
2012年 | 124篇 |
2011年 | 168篇 |
2010年 | 59篇 |
2009年 | 94篇 |
2008年 | 159篇 |
2007年 | 192篇 |
2006年 | 200篇 |
2005年 | 204篇 |
2004年 | 193篇 |
2003年 | 157篇 |
2002年 | 167篇 |
2001年 | 165篇 |
2000年 | 190篇 |
1999年 | 93篇 |
1998年 | 90篇 |
1997年 | 108篇 |
1996年 | 68篇 |
1995年 | 99篇 |
1994年 | 106篇 |
1993年 | 128篇 |
1992年 | 116篇 |
1991年 | 159篇 |
1990年 | 112篇 |
1989年 | 134篇 |
1988年 | 116篇 |
1987年 | 84篇 |
1986年 | 63篇 |
1985年 | 103篇 |
1984年 | 81篇 |
1983年 | 41篇 |
1982年 | 36篇 |
1981年 | 29篇 |
1980年 | 64篇 |
1979年 | 8篇 |
1978年 | 12篇 |
1977年 | 6篇 |
1974年 | 9篇 |
1947年 | 7篇 |
1939年 | 7篇 |
排序方式: 共有4387条查询结果,搜索用时 15 毫秒
71.
1INTRODUCTIONBydefinitionurbanizationistheprocessofruralareaschangingintourbanareasinnatureforasoci-etyitisanaturallyhistoricalprocessofthetransitionfromatraditionalruraloragriculturalsocietytoamodernizedurbansocietyGAO1991.Inbriefitrepresentsa… 相似文献
72.
1INTRODUCTIONWiththeaccelerationoftheeconomicglobaliza-tionthegradualshapingofthenewinternationaldivi-sionoflaborandthedevelopmentofthetransnationalcorporationstheworldurbansystemshavebegunanewroundofintegration.Ononehandthecitiesallovertheglobeare… 相似文献
73.
Slight perturbations of the oceanic DOC by addition of oxalic acid, tryptophan, and sodium humate did not affect the magnesium content of spontaneously precipitated Mg-calcites. However, the copresence of spontaneously precipitated aragonite and of Mg-calcite as a function of the initial degree of supersaturation was changed by the dissolved organic matter. Times of nucleation for carbonates was affected by organic matter at low organic concentrations because of coating and at high concentrations due to complexation. 相似文献
74.
Wild mushrooms are recognized as important non-wood forest products in mountainous ecosystems, but their real potential for generating rural economies has not been fully evaluated due to the difficulties in obtaining reliable productivity data, minimizing their true potential as contributor to rural economies. Mushroom yield models based on large data series from Pinus forest ecosystems in the region of Catalonia(Spain), combined with data from the Spanish National Forest Inventory allow us to estimate the potential mushroom productivity by forest ecosystems. The results of 24,500 tons/yr of mushrooms of which 16,300 tons are classified as edible and 7,900 tons are commonly marketed demonstrate the importance of mushroom productions in Catalonian pine forests, mostly located in mountainous areas where the development of agricultural activities is limited. Economic mushroom value is estimated at 48 million € for the edible mushroom and 32 million € for those corresponding to marketable yields, confirming the potential of this non-wood forest product. These production results and corresponding economic values provide a basis for the incorporation of wild mushrooms as significant non-wood forest products in the development of forest policies in mountainous areas. 相似文献
75.
KANG Shi chang QIN Da he YAO Tan dong PAUL A. Mayewski SALLIE Whitlow 《中国地理科学(英文版)》2000,10(3):218-225
Investigations of atmospheric composition in the Himalayas has been limited in both temporal and spatial scales, mainly due to difficult logistics. Ideal sites for monitoring atmospheric composition and its evolution should be free from local pollution and representative of the remote troposphere (HUEBERT et al., 1980). As the Himalayas are far removed from highly industrialized regions they provide suitable locations to monitor the chemistry of the remote troposphere and to study the evolu… 相似文献
76.
Payment schemes for environmental services face serious budget constraints in Mexico. Payments are equally distributed among forest owners, and the governmental agencies that make payments to beneficiaries do not evaluate the amount of environmental services produced by forested areas. This lack of evaluation could lead to economic and environmental deficiencies that restrict the possible benefits of such programmes. In general, these programmes do not achieve their conservation objectives, and local beneficiaries are not involved in the design or application of these programmes. Similarly, the Payments for Hydrological Environmental Services (PHES) programme of the State of Mexico, Mexico, makes equal payments to beneficiaries of forested areas considering tree density. The objective of the present work was to create a methodology for determining differentiated payments schemes for hydrological environmental services, prioritising areas that contribute the most to groundwater recharge and promote the participation of society. In the Nevado de Toluca Natural Protected Area, the environmental criteria of forest type, tree density and aquifer overexploitation as well as social participation were considered in a multi-criteria analysis implemented in a Geographic Information System. A methodology was developed to determine differentiated payments based on specific forest characteristics that influence the potential capacity of forests to recharge groundwater reserves in addition to the level of social participation. In particular, social participation is key as it could ensure the viability of PHES programmes as conservation alternatives in forested areas with high groundwater recharge potential. 相似文献
77.
K. Karami R. Mohebi 《中国天文和天体物理学报》2007,7(4):558-564
We introduce a new method to derive the orbital parameters of spectroscopic bi-nary stars by nonlinear least squares of (o-c). Using the measured radial velocity data of the four double lined spectroscopic binary systems, AI Phe, GM Dra, HD 93917 and V502 Oph, we derived both the orbital and combined spectroscopic elements of these systems. Our numerical results are in good agreement with the those obtained using the method of Lehmann-Filhe's. 相似文献
78.
79.
在8000至8670的波长范围内,利用一个由144颗恒星光谱组成的光谱厍,我们对57个巨椭圆和S0星系(M_B<-21)的光谱和它们的平均光谱进行了光谱合成.这个光谐库包括了光谱型为G,K和M的巨星和矮星,它具有的金属丰度[Fe/H]覆盖了-0.1到-0.5的范围,表面重力log g为1.0至5.0.光谱合成的结果表明:巨椭圆和S0星系的金属丰度约为太阳的一倍半;有效表面重力分布在3.2—4.1的范围内;矮星在8400附近光的贡献可与巨星相比较. 相似文献
80.
The atmospheric circulation of Titan is investigated with a general circulation model. The representation of the large-scale dynamics is based on a grid point model developed and used at Laboratoire de Météorologie Dynamique for climate studies. The code also includes an accurate representation of radiative heating and cooling by molecular gases and haze as well as a parametrization of the vertical turbulent mixing of momentum and potential temperature. Long-term simulations of the atmospheric circulation are presented. Starting from a state of rest, the model spontaneously produces a strong superrotation with prograde equatorial winds (i.e., in the same sense as the assumed rotation of the solid body) increasing from the surface to reach 100 m sec-1 near the 1-mbar pressure level. Those equatorial winds are in very good agreement with some indirect observations, especially those of the 1989 occultation of Star 28-Sgr by Titan. On the other hand, the model simulates latitudinal temperature contrasts in the stratosphere that are significantly weaker than those observed by Voyager 1 which, we suggest, may be partly due to the nonrepresentation of the spatial and temporal variations of the abundances of molecular species and haze. We present diagnostics of the simulated atmospheric circulation underlying the importance of the seasonal cycle and a tentative explanation for the creation and maintenance of the atmospheric superrotation based on a careful angular momentum budget. 相似文献