全文获取类型
收费全文 | 2958篇 |
免费 | 635篇 |
国内免费 | 331篇 |
专业分类
测绘学 | 178篇 |
大气科学 | 149篇 |
地球物理 | 1044篇 |
地质学 | 1374篇 |
海洋学 | 277篇 |
天文学 | 7篇 |
综合类 | 180篇 |
自然地理 | 715篇 |
出版年
2024年 | 10篇 |
2023年 | 25篇 |
2022年 | 72篇 |
2021年 | 132篇 |
2020年 | 146篇 |
2019年 | 146篇 |
2018年 | 121篇 |
2017年 | 147篇 |
2016年 | 141篇 |
2015年 | 128篇 |
2014年 | 172篇 |
2013年 | 249篇 |
2012年 | 163篇 |
2011年 | 159篇 |
2010年 | 132篇 |
2009年 | 163篇 |
2008年 | 180篇 |
2007年 | 208篇 |
2006年 | 194篇 |
2005年 | 150篇 |
2004年 | 166篇 |
2003年 | 144篇 |
2002年 | 126篇 |
2001年 | 103篇 |
2000年 | 81篇 |
1999年 | 73篇 |
1998年 | 79篇 |
1997年 | 70篇 |
1996年 | 43篇 |
1995年 | 47篇 |
1994年 | 39篇 |
1993年 | 19篇 |
1992年 | 19篇 |
1991年 | 13篇 |
1990年 | 13篇 |
1989年 | 12篇 |
1988年 | 12篇 |
1987年 | 6篇 |
1986年 | 7篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1979年 | 1篇 |
排序方式: 共有3924条查询结果,搜索用时 890 毫秒
991.
992.
1INTRODUCTIONErgodichypothesisisoneofthebasicconceptsofmoderngeomorphology.Itsuggeststhatundercertaincircumstancessamplinginspacecanbeequivalenttosamplingthroughtime;andthatspace-timetransformationsarepermissibleasaworkingtool.Thisisbasedontheassumptionthat,whenindividualmembersofapopulationoflandformsarechangingregularlythroughtime,thespatialfrequencyofoccurrenceofgiventypesoflandformsisinverselyproportionaltotheirrateofchange(Chorley,etal.,1985).TheconceptofthecycleofDavis'erosioninwhi… 相似文献
993.
1GENERALTraditionally,theLowerYellowRiverstartsatHuayuankouandrunsonaplainforabout800km,beforeemptyingintotheBohaiSea.Theriverisfedwithextremelyheavysedimentload(1.6billiontonsannuallyuptotheeighties)andrelativelysmallrunoff(46billionm3uptotheeighties)fromitsdrainagebasinupstream.Thisloadofsedimentispartlydepositedinthechannelresultinginaggradationorincreaseinthebedslope.TheremainderisdischargedtotheBobalSea.AsthetidesoftheBohaiSeaareweak,thedepositionintheareaaroundtherivermouthisusual… 相似文献
994.
CHEN Yang SUN Jingshi
Associate Professor Tianjin Research Institute of Water Transport Engineering Tianjin P. R. China.
Professor Tianjin Research Institute of Water Transport Engineering Tianjin P. R. China. 《中国海洋工程》1999,(2)
-By use of the parabolic equation of numerical simulation of wave which is suitable forlarge-angle propagation and Crank-Nicolson differential method,the wave field at the Kemema Rivermouth has been studied for analysis of sediment movement in the area.In order to reflect wave energy lossaccurately,the Bretchneider-Reid formula is quoted and the friction coefficient in the formula is discussedin this paper.The calculation results indicate that the wave becomes a little damped at the mouth ofKemena River influenced by the topography and bottom friction,and the wave at the east beach is higherthan that at the west beach,because the east beach extends out. 相似文献
995.
996.
Ankara Creek is often subjected to overflowing of sewage caused by rainfall or direct discharge of raw sewage. Alluvial aquifers
adjacent to Ankara Creek and its tributaries have considerable groundwater potential. The present status of groundwater quality
is far from drinking water standards. Groundwater contamination in Ankara is suspected to be caused by Ankara Creek which
is heavily polluted by raw sewage discharge, surface runoff and other common sources. In order to investigate the influence
of heavily polluted Ankara Creek on the groundwater contamination in the adjacent alluvial aquifers, five sampling stations
on Ankara Creek and 25 water wells were monitored during 1996. At five different sampling periods, water samples were collected
from both surface water and groundwater. Chemical analyses of basic ions, pollution parameters and heavy metals in natural
waters were carried out. The organic pollution prevails in Ankara Creek whereas total dissolved solids (TDS) and heavy metal
concentrations are considerably low. Starting from the idea that Ankara Creek somewhat influences the groundwater quality
and the contaminants in groundwater should attenuate with respect to distance, a series of water wells in a certain area,
each having different distance from the creek, were examined using four pollution parameters. It is concluded that Ankara
Creek barely influences the aquifer systems in connection. This is attributed to two reasons: rapid attenuation of contaminants
due to dilution in groundwater and a blanket of very fine sized materials covering the bottom of Ankara Creek.
Received: 28. April 1997 · Accepted: 23. February 1998 相似文献
997.
G. Stamatis 《Environmental Geology》1999,38(2):126-140
Peneos is the mainstream for the drainage of Thessaly's basin. It contributes significantly in recharging the aquifers, as
well as to the direct irrigation water, since there is intense agricultural activity in the basin of Thessaly. The investigation
of the hydrochemical conditions of the surface water system of Peneos showed that the chemical composition is related to the
lithology of the aquifers of the basin, the anthropogenic influence and the biological activity. The chemical composition
of this system changes in time and space. The salinity decreases during the winter period, in contrast to the summer period
when it increases drastically. The lower values of TDS appear in the upstream part of the river (95–124 mg/l) whereas downstream,
the values of TDS increase significantly (400–632 mg/l). The hydrochemical type Ca-Mg-HCO3 dominates the largest part of the river while it changes to NA-Cl in the estuary. The concentrations of the dissolved species
in the main Peneos river are higher than in the spring waters. Peneos is the most polluted river in Greece. The concentration
of the pollutants is higher at the downstream part of the river, but is lower than the accepted upper limits. The use of the
surface water of the system of Peneos river for irrigation purposes based on the SAR factor is acceptable because the absorption
ratio for Na and salinity are low and medium respectively.
Received: 27 October 1997 · Accepted: 21 September 1998 相似文献
998.
赤潮流界是表征河口径潮相互作用的关键界面,受各种动力作用在空间上不断上下迁移。本文基于2012—2019年间长江口岸直水道五峰山至南支河段大范围实测资料,分析了46种径潮流组合下,长江主汊河段主深槽潮流界位置的变化。研究结果表明:(1)在径流条件一定时,随着潮差的减小潮流界位置逐渐向下游移动,初始时期潮流界位置下移速度较慢,后期加快。(2)在潮差一定时,当上边界径流量以同样速度增加时,枯水期潮流界向下游移动的速度快,平水期与丰水期潮流界向下游移动的速度会逐渐减慢。(3)潮流界距大通的距离与潮径比呈显著相关的对数关系,可用于今后潮流界位置的定量计算。潮流界位置随着潮径比的减小而下移,当潮径比约为0.56时,潮流界位置在江阴附近。新水沙条件下,长江潮流界位置的研究对"长江大保护"战略下河口海岸地区的开发规划具有重要现实意义。 相似文献
999.
1000.
This study challenges the use of three nature‐inspired algorithms as learning frameworks of the adaptive‐neuro‐fuzzy inference system (ANFIS) machine learning model for short‐term modeling of dissolved oxygen (DO) concentrations. Particle swarm optimization (PSO), butterfly optimization algorithm (BOA), and biogeography‐based optimization (BBO) are employed for developing predictive ANFIS models using seasonal 15 min data collected from the Rock Creek River in Washington, DC. Four independent variables are used as model inputs including water temperature (T), river discharge (Q), specific conductance (SC), and pH. The Mallow's Cp and R2 parameters are used for choosing the best input parameters for the models. The models are assessed by several statistics such as the coefficient of determination (R2), root‐mean‐square error (RMSE), Nash–Sutcliffe efficiency, mean absolute error, and the percent bias. The results indicate that the performance of all‐nature‐inspired algorithms is close to each other. However, based on the calculated RMSE, they enhance the accuracy of standard ANFIS in the spring, summer, fall, and winter around 13.79%, 15.94%, 6.25%, and 12.74%, respectively. Overall, the ANFIS‐PSO and ANFIS‐BOA provide slightly better results than the other ANFIS models. 相似文献