全文获取类型
收费全文 | 1035篇 |
免费 | 267篇 |
国内免费 | 29篇 |
专业分类
测绘学 | 12篇 |
大气科学 | 15篇 |
地球物理 | 872篇 |
地质学 | 218篇 |
海洋学 | 81篇 |
天文学 | 6篇 |
综合类 | 11篇 |
自然地理 | 116篇 |
出版年
2024年 | 13篇 |
2023年 | 5篇 |
2022年 | 7篇 |
2021年 | 63篇 |
2020年 | 83篇 |
2019年 | 35篇 |
2018年 | 53篇 |
2017年 | 51篇 |
2016年 | 48篇 |
2015年 | 49篇 |
2014年 | 62篇 |
2013年 | 117篇 |
2012年 | 38篇 |
2011年 | 41篇 |
2010年 | 42篇 |
2009年 | 32篇 |
2008年 | 74篇 |
2007年 | 54篇 |
2006年 | 63篇 |
2005年 | 32篇 |
2004年 | 37篇 |
2003年 | 39篇 |
2002年 | 40篇 |
2001年 | 22篇 |
2000年 | 38篇 |
1999年 | 25篇 |
1998年 | 25篇 |
1997年 | 26篇 |
1996年 | 28篇 |
1995年 | 8篇 |
1994年 | 11篇 |
1993年 | 12篇 |
1992年 | 12篇 |
1991年 | 4篇 |
1990年 | 9篇 |
1989年 | 6篇 |
1988年 | 3篇 |
1987年 | 8篇 |
1986年 | 1篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1973年 | 1篇 |
1971年 | 2篇 |
排序方式: 共有1331条查询结果,搜索用时 531 毫秒
961.
The longitude of the western limit of the equatorial Pacific upwelling is a key parameter for studies of carbon budget and pelagic fisheries variability. Although it is well defined at the surface on the equator by a salinity front and a sharp variation of the partial pressure of CO2, data from two equatorial cruises make it clear that this hydrological limit does not necessarily coincide with the boundary of the nitrate and chlorophyll enriched area. In January-February 1991 during a non-El Niño period, when trade winds and the South Equatorial current (SEC) were favorable to upwelling, the two limits were at the same longitude. Conversely, in September-October 1994 during El Niño conditions, when the equatorial upwelling had stopped, the nitrate and chlorophyll enriched zone was found a few degrees of longitude east of the hydrological boundary (5.5° at the surface and 2.5° for the 50 m upper layer), whereas no such offset was observed for zooplankton biomass. A simple model, based on the HNLC (High Nutrient - Low Chlorophyll) ecosystem functioning, was initialized with nitrate uptake measurements and estimates of upwelling break duration. The model results support the hypothesis that zonal separation of the limits arises from biological processes (i.e. nitrate uptake and phytoplankton grazing) achieved during that upwelling break. 相似文献
962.
A geometrical model, including different geometrical shapes influencing thermal conductivity of snow is proposed. The geometrical
model has been assumed to comprise of unit cells having solid (ice) inclusion as an aggregation of spherical, cylindrical
or cubical shapes with vertical connection, arranged in a cubic packing. From the geometrical model and one-dimensional heat
transfer theory, the effective thermal conductivity has been computed. For this purpose, coupled one-dimensional heat transfer
equations have been solved for steady-state condition to account for conduction in ice, conduction in air and latent heat
transfer due to water vapour sublimation through air. The model demonstrates the dependency of thermal conductivity on density,
grain-spacing, grain contact ratio and temperature. Spherical inclusions give highest conductivity while cubical inclusion
estimates lowest value for the same density. Thermal conductivity has been found increasing sharply near to the packing density
for all three shapes. Empirical model results and results obtained from existing microstructure based models have also been
compared with the present model. 相似文献
963.
Surface energy balance of seasonal snow cover for snow-melt estimation in N-W Himalaya 总被引:2,自引:0,他引:2
Prem Datt P. K. Srivastava P. S. Negi P. K. Satyawali 《Journal of Earth System Science》2008,117(5):567-573
This study describes time series analysis of snow-melt, radiation data and energy balance for a seasonal snow cover at Dhundi
field station of SASE, which lies in Pir Panjal range of the N-W Himalaya, for a winter season from 13 January to 12 April
2005. The analysis shows that mean snow surface temperature remains very close to the melting temperature of snow. It was
found close to −1°C for the complete observational period which makes the snow pack at Dhundi moist from its beginning. The
average air temperature over this period was found to be 3.5°C with hourly average variation from −5.5°C to 13°C. The snow
surface at this station received a mean short wave radiation of 430W m−2, out of which 298W m−2 was reflected back by the snow surface with mean albedo value of 0.70. The high average temperature and more absorption of
solar radiation resulted in higher thermal state of the snowpack which was further responsible for faster and higher densification
of the snowpack. Net radiation energy was the major component of surface energy budget with a mean value of 83W m−2. Bulk transfer model was used to calculate turbulent fluxes. The net energy was utilized for satisfying cold content and
snow-melt by using measured snow surface temperature and density of snow pack. The mean square error between calculated and
measured daily snow-melt was found to be approximately 6.6mm of water equivalent. 相似文献
964.
陆面水文过程是全球/区域气候模式十分重要而又十分薄弱的环节。本文通过科学文献计量法,分析了陆面水文—气候耦合研究的发展状况及研究热点和趋势,并进一步对研究中存在的问题和挑战进行综述和探讨。现有多数气候模式中的陆面模式主要基于一维垂向结构设计,缺乏对流域尺度水文过程的精细描述,尤其缺乏下垫面人类活动影响的描述。因此,为了科学认识水文过程与气候变化的相互反馈作用机制,大量研究主要通过耦合流域水文模型与气候模式,研究不同时空尺度下水文过程变化的气候反馈效应。陆面过程模式中水文过程的改进和大尺度水文模型发展为陆面水文—气候耦合模拟奠定基础,在此基础上,陆面水文—气候耦合研究正从传统的单向耦合研究逐步发展为考虑气候—水文反馈的双向耦合研究。然而,双向耦合研究远未成熟,其问题集中表现为陆面水文—气候模型耦合过程如何匹配并提高系统稳定性、研制有效的尺度转换方案、完善参数化方案并评估参数不确定性、研制有效参数移植方法并提高模型适用性以及高分辨率甚至超分辨率模拟等方面,逐步解决上述问题并提高模拟精度是未来水文—气候耦合模拟研究的重要发展方向。 相似文献
965.
A sound understanding of groundwater recharged from various sources occurring at different time scales is crucial for water management in arid and semi-arid river basins. Groundwater recharge sources and their geochemical evolution are investigated for the Heihe River Basin(HRB) in northwest China on the basis of a comprehensive compilation of geochemical and isotopic data. Geochemical massbalance modeling indicates that mountain-block recharge accounts for a small fraction(generally less than 5%) of the shallow and deep groundwater sustaining the oasis, whereas infiltration of rivers and irrigation water contribute most of the groundwater recharge. Dedolomitization is the primary process responsible for the changes in groundwater chemical and carbon isotope compositions from the piedmont to the groundwater discharge zone, where the dedolomitization is very likely enhanced by modern agricultural activities affecting the shallow groundwater quality. Analysis of radioactive isotopes suggests that these primary recharge sources occur at two different time scales. Radiocarbon-derived groundwater age profiles indicate a recharge rate of approximately 12 mm/year, which probably occurred during 2000–7000 years B.P., corresponding to the mid-Holocene humid period. The recharge of young groundwater on the tritium-dated time scale is much higher, about 360 mm/year in the oasis region. Infiltration from irrigation canals and irrigation return flow are the primary contributors to the increased young groundwater recharge. This study suggests that groundwater chemistry in the HRB has been influenced by the complex interaction between natural and human-induced geochemical processes and that anthropogenic effects have played a more significant role in terms of both groundwater quantity and quality. 相似文献
966.
气候变化对水文水资源影响评价的不确定性分析 总被引:8,自引:1,他引:8
在评价气候变化对水文水资源影响的各个环节中都存在不确定性。以蓝塘和花园两流域为例,应用MonteCarlO和非参数方法研究区域水文评价模型参数和径流的不确定性估计。结果表明:径流对降水的变化要比对气温升高敏感;径流系数较小的流域,其变化幅度反而大。若考虑信度水平为5%的不确定度和最不利的气候情景组合,蓝塘和花园两流域的洪峰流量将分别增大45.21%和49.27%,这将对防洪工程造成重大和不利的影响。 相似文献
967.
968.
本文讨论了在完整岩溶泉域内,从灰岩裸露的补给区,至灰岩隐伏于地下的径流区宜至深埋区,可溶岩矿物与水相互作用所发生的不同化学作用和分带性;以及不同化学作用下限确定原则和方法;岩溶水化学类型随深度变化规律;可溶岩矿物溶蚀(解)量的方法。 相似文献
969.
970.
无机水化学离子在实验流域降雨径流过程中的响应及其示踪意义 总被引:1,自引:0,他引:1
为研究无机水化学离子在降雨径流示踪中的可能性,在实验流域实测降雨、地面径流、壤中流、地下水径流过程和流域内17个测孔的地下水过程中,施测了Na+,K+,Ca2+,Mg2+,Cl-,SO42-,HCO3-+CO32-,NO3-,F-,NH4-,PO42-,SiO2和pH,EC,18O的同步过程,还施测了少数土壤水水样。结果是:①在Ca2+与Cl-/SO42-和EC与Na+/(Na++Ca2+)的关系中以及除NO3-,NH4-,PO42-外的所有离子,都可识别出上述各种径流组分;②径流离子过程均与降雨离子过程相仿,随径流组分从地面到地下而渐趋坦化;③除个例外,离子浓度过程均以降雨为最小并从地面径流到地下水径流渐次增大;④降雨和地下水离子过程表现为径流离子过程的两端元;⑤在降雨和各径流组分中,18O过程与大部分离子过程有一定的同步性。从以上结果认为,降雨并不是流域径流离子输出的主要来源,然而却是形成它的主要控制因素。此外,有关试验结果还对应用Cl-进行地下水补给的估算方法提出了问题。 相似文献