全文获取类型
收费全文 | 3702篇 |
免费 | 793篇 |
国内免费 | 1267篇 |
专业分类
测绘学 | 173篇 |
大气科学 | 1477篇 |
地球物理 | 925篇 |
地质学 | 1610篇 |
海洋学 | 682篇 |
天文学 | 51篇 |
综合类 | 265篇 |
自然地理 | 579篇 |
出版年
2024年 | 28篇 |
2023年 | 65篇 |
2022年 | 124篇 |
2021年 | 150篇 |
2020年 | 159篇 |
2019年 | 185篇 |
2018年 | 139篇 |
2017年 | 177篇 |
2016年 | 169篇 |
2015年 | 173篇 |
2014年 | 243篇 |
2013年 | 263篇 |
2012年 | 257篇 |
2011年 | 263篇 |
2010年 | 223篇 |
2009年 | 278篇 |
2008年 | 272篇 |
2007年 | 282篇 |
2006年 | 256篇 |
2005年 | 201篇 |
2004年 | 209篇 |
2003年 | 169篇 |
2002年 | 177篇 |
2001年 | 156篇 |
2000年 | 133篇 |
1999年 | 116篇 |
1998年 | 131篇 |
1997年 | 118篇 |
1996年 | 103篇 |
1995年 | 73篇 |
1994年 | 87篇 |
1993年 | 80篇 |
1992年 | 60篇 |
1991年 | 47篇 |
1990年 | 41篇 |
1989年 | 28篇 |
1988年 | 35篇 |
1987年 | 17篇 |
1986年 | 18篇 |
1985年 | 13篇 |
1984年 | 13篇 |
1983年 | 1篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1978年 | 14篇 |
1977年 | 1篇 |
1973年 | 2篇 |
1954年 | 2篇 |
排序方式: 共有5762条查询结果,搜索用时 15 毫秒
1.
Jeffrey Wade Laura Lautz Christa Kelleher Philippe Vidon Julianne Davis Julio Beltran Casey Pearce 《水文研究》2020,34(26):5340-5353
Beaver dam analogues (BDAs) are a cost-effective stream restoration approach that leverages the recognized environmental benefits of natural beaver dams on channel stability and local hydrology. Although natural beaver dams are known to exert considerable influence on the hydrologic conditions of a stream system by mediating geomorphic processes, nutrient cycling, and groundwater–surface water interactions, the impacts of beaver-derived restoration methods on groundwater–surface water exchange are poorly characterized. To address this deficit, we monitored hyporheic exchange fluxes and streambed porewater biogeochemistry across a sequence of BDAs installed along a central Wyoming stream during the summer of 2019. Streambed fluxes were quantified by heat tracing methods and vertical hydraulic gradients. Biogeochemical activity was evaluated using major ion porewater chemistry and principal component analysis. Vertical fluxes of approximately 1.0 m/day were observed around the BDAs, as was the development of spatially heterogeneous zones of nitrate production, groundwater upwelling, and anaerobic reduction. Strong contrasts in hyporheic zone processes were observed across BDAs of differing sizes. This suggests that structures may function with size-dependent behaviour, only altering groundwater–surface water interactions after a threshold hydraulic step height is exceeded. Patterns of hyporheic exchange and biogeochemical cycling around the studied BDAs resemble those around natural beaver dams, suggesting that BDAs may provide comparable benefits to channel complexity and near-stream function over a 1-year period. 相似文献
2.
MohammedHazaea HUKe MohammedMattash CHENBing M.O.AI-JabaliAref MohammedAbdussalam 《东北亚地学研究》2004,7(2):181-184
The chemical composition of several thermal springs around Changbaishan area has been investigated. Cenozoic basaltic rocks are widely distributed in Northeast China and geothermal characteristics have been described. About one hundred hot springs exist around Changhaishan Volcano at the border between China and D.P.R.Korea with high temperature about 82~C. The pH values of the spring water range from 6.9 to 7.1 and the total flow rate is about 4.8 L/sec. The chemical composition of the thermal springs is sodium carbonate; the high-mineral contents of thermal water are believed to have medicinal properties. Bathhouses are already built along the hot springs to take the advantage of the supposed healing properties. The high quality of those hot springs is believed to be utilitized for mineral water. The chemical equilibrium temperatures were estimated at about 160℃ based on the Na-K-Ca geochemical thermometer. 相似文献
3.
讨论了应用曲面拟合技术和利用离散的重力观测点的重力变化对特定研究区的重力场变化进行数值模拟以获取该区重力变化空间梯度场图像的具体方法;并应用该方法获得了2004年3月福建安溪3.6级地震前的重力变化空间梯度场图像,发现空间梯度场图像能清晰地反映出地震前重力场群体性变化的差异性;利用此方法对2005年6月~2006年3月间台湾海峡西岸重力场变化进行数值模拟,获得其空间梯度场图像。 相似文献
4.
This paper proposes a multi‐level parallelized substructuring–frontal combined algorithm for the analysis of the problem of thermo/hydraulic/mechanical behaviour of unsaturated soil. Temperature, displacement, pore water pressure and pore air pressure are treated as the primary variables in a non‐linear analysis. Details are given firstly of the substructuring–frontal combined approach. The incorporation of the algorithm in a multi‐level parallel strategy is then discussed. The parallel processing can thus be carried out at different substructural levels. The method thus developed impacts, in a positive way, on both computer storage requirement and execution time. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
5.
INTRODUCTIONTriploidanimalsareusefulbecausetheyaresterile ,characterizedbypositivegrowthinthere productiveseason ,usefulfortesting ,forintroductionofnon nativespecies ,andforprotectionofde velopedstrains.Theinductionoftriploidyhadbeenreportedinmanyaquaculturespecies .Triploidshavebeensuccessfullyinducedinshrimpusingtemperatureshockorchemicalshock (CB ,6 DMAP)in 4species :Sicyoniaingentis (Xiangetal.,1 991 ) ,Fenneropenaeuschinensis (Xiangetal.,1 992 ;Daietal.,1 993;Baoetal.,1 993;Li… 相似文献
6.
Surface waters of Alsea Bay, an unpolluted estuary on the Oregon coast, were analysed for nitrous oxide, nitrate and nitrite on a weekly or biweekly basis during the summer of 1979. The estuary was found to be a variable source of N2O to the atmosphere. Large and rapid increases in the concentrations of N2O, NO3?, and NO2? occurred at the beginning of the sampling period and are attributed to the influx of nutrient-rich upwelling water into the estuary with the tide. The subsequent decline in concentrations of nitrate, nitrite and nitrous oxide over the remainder of the summer is attributed to a decrease in upwelling intensity, a decline in nitrification rates and to assimilatory nitrate reduction. Measurements of nitrous oxide at six stations along the Alsea River were also made in September and October before and after the onset of the rainy season. Samples taken after flood conditions were established were systematically 50% higher than pre-flood samples. The data suggest that soil runoff results in elevated concentrations of N2O in rivers. 相似文献
7.
随着河南区域化探扫面工作的结束,异常评价是摆在我们面前的当务之急。近年来,随着地质工作的不断深入,我们在异常评价程序及方法上作了一些探索,下面以桐柏县固庙—新集1:20万金银异常评价方法及效果为例,重点加以讨论。 相似文献
8.
D. Prandle 《Estuarine, Coastal and Shelf Science》1985,20(5):615-635
An examination is made of the circulation in narrow estuaries subject to a predominant tidal forcing. Velocity structures are derived separately for residual flow components associated with (a) river flow, (b) wind stress, (c) a well-mixed longitudinal density gradient and (d) a fully stratified saline wedge. Dimensionless parameters are introduced to indicate the magnitude of each component and these parameters are evaluated for 9 major estuaries, thereby revealing their sensitivity to each component.For a channel of constant breadth and depth, formulae are deduced for the length of saline intrusion, L. Comparisons with observed data show that such formulae may be used with confidence to predict changes in L arising from variations in river flow, tidal range or channel depths.The level of stratification is shown to be related to a product of two parameters, one associated with velocity structure and a second involving the square of the ‘flow ratio’ (i.e. residual velocity/amplitude of the tidal velocity). This relationship provides a simple classification system for estuarine stratification which can be used to indicate the sensitivity of any particular estuary to changing conditions. 相似文献
9.
10.
Ian C. Grieve 《水文研究》1989,3(4):339-346
Parameters for ion exchange selectivity and aluminium hydroxide dissolution in the soil chemical submodels used in applications of the Birkenes model and of MAGIC are compared and several discrepancies identified for organic soils. A laboratory column simulation of the soil chemical submodels is proposed and applied to soils from the Loch Dee area in Galloway. Experimental results were well predicted by a simplified version of MAGIC, with ion exchange selectivity parameters similar to those used in a previous simulation of one subcatchment of Loch Dee. The aluminium hydroxide dissolution parameter used previously was found to be too low for the organic soil materials, where a value of 106 predicted the experimental results more closely. The model developed also included a simple silicate weathering reaction to release base cations into the system. It is concluded that such simple laboratory simulations are useful for independent calibration of the soil chemical submodel of catchment models. 相似文献