首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   4287篇
  免费   647篇
  国内免费   971篇
测绘学   548篇
大气科学   675篇
地球物理   955篇
地质学   1471篇
海洋学   1170篇
天文学   31篇
综合类   328篇
自然地理   727篇
  2024年   24篇
  2023年   66篇
  2022年   138篇
  2021年   172篇
  2020年   214篇
  2019年   268篇
  2018年   171篇
  2017年   192篇
  2016年   217篇
  2015年   255篇
  2014年   325篇
  2013年   342篇
  2012年   286篇
  2011年   315篇
  2010年   231篇
  2009年   254篇
  2008年   238篇
  2007年   258篇
  2006年   278篇
  2005年   209篇
  2004年   203篇
  2003年   164篇
  2002年   160篇
  2001年   106篇
  2000年   107篇
  1999年   100篇
  1998年   93篇
  1997年   91篇
  1996年   78篇
  1995年   55篇
  1994年   60篇
  1993年   45篇
  1992年   43篇
  1991年   27篇
  1990年   24篇
  1989年   8篇
  1988年   13篇
  1987年   10篇
  1986年   10篇
  1985年   18篇
  1984年   11篇
  1983年   9篇
  1982年   9篇
  1981年   5篇
  1978年   2篇
  1977年   1篇
排序方式: 共有5905条查询结果,搜索用时 15 毫秒
991.
Based on the power function of velocity and the friction velocity,a velocity profile is obtained.By solving the Schmidt’s diffusion equation,an equilibrium suspended sediment concentration profile is further deduced.The profiles of velocity and suspended sediment concentration agree well with the field data,and the profile of suspended sediment concentration avoids the unreasonableness of the classical Rouse profile such as a zero value at the water surface.According to these profiles,an expression which is easy to use for calculating the suspended sediment transport rate is derived.  相似文献   
992.
利用重磁资料研究莺-琼盆地构造分界及其两侧断裂特征   总被引:1,自引:0,他引:1  
莺-琼盆地(即莺歌海盆地和琼东南盆地)位于印支地块与华南大陆交汇处,该区域地质构造复杂,为南海西北走滑型和伸展型陆缘的交汇区,也是印澳-欧亚板块碰撞“挤出-逃逸构造区”和“古南海俯冲-拖曳构造区”.莺-琼盆地的北部以红河断裂带中1号断裂为界,前人对其认识比较统一,但莺-琼盆地南部由于地质和地球物理资料较少、研究程度低,以致其构造格局及分界位置不明确.通过对重、磁资料的处理,认为在莺-琼盆地分界的北段存在明显的重力异常和磁力异常梯级带,该梯级带与1号断裂位置相对应;在其南段存在明显的重力异常和磁力异常高值带,推断该高值带为一“中建凸起”,结合重、磁对应分析技术确定了该“中建凸起”的分布范围.采用重、磁异常归一化总水平导数垂向导数技术识别出控制莺-琼盆地分界两侧的断裂构造,在分界以西断裂走向主要为北北西向,分界以东主要为北东向.通过地质和地球物理综合研究认为莺-琼盆地的北段以1号断裂为界,南段以“中建凸起”为界.  相似文献   
993.
发生在2009年7月22日亚太地区的日全食,为人们研究电离层扰动提供了一次难得的机会.基于沙洋地区采集的高频(1 Hz)GPS数据,通过对日食当天与前后几天电离层VTEC(Vertical Total Electron Content)的变化分析,结果显示日食期间电离层VTEC含量均呈现出一种先下降再上升的“水槽”变化趋势,且VTEC下降达到2~5 TECU,下降幅度达33.1%,VTEC的最小值与食甚时刻延迟为25.2~147.6 s.在日全食后期有中等强度的磁暴发生,与日食相互重合叠加共同对电离层VTEC变化造成影响,使得VTEC变化明显异于前后几天,最后定量的分析了磁暴对VTEC的影响.  相似文献   
994.
A slow moving compound rock slide located in the northern Apennines of Italy was mapped and monitored through the integration of Airborne Laser Scanning (ALS), multi‐temporal long‐range Terrestrial Laser Scanning (TLS), and Automated Total Station (ATS) measurements. Landslide features were mapped using a High Resolution Digital Terrain Model (HR‐DTM) obtained by merging ALS and TLS data in an Iterative Closest Point (ICP) procedure. Slope movements in the order of centimeters to a few decimeters were quantified with Differential TLS (D‐TLS) based on a Surface Matching approach and supported by ATS data to define stable reference surfaces. The integrated approach allowed mapping of the composite geomorphic features of the rock slide under examination, revealing its complex dynamic nature and further proving that laser scanning is a versatile and widely applicable tool for slope process analysis. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   
995.
Abstract

Agricultural watersheds in the Czech Republic are one of the primary sources of non-point-source phosphorus (P) loads in receiving waters. Since such non-point sources are generally located in headwater catchments, streamflow and P concentration data are sparse. We show how very short daily streamflow and P concentration records can be combined with nearby longer existing daily streamflow records to result in reliable estimates of daily and annual P concentrations and loads. Maintenance of variance streamflow record extension methods (MOVE) can be employed to extend short streamflow records. Constituent load regressions are used to predict daily P constituent loads from streamflow and other time varying characteristics. Annual P loads are then estimated for individual watersheds. Resulting annual P load estimates ranged from 0.21 to 95.4 kg year-1 with a mean value of 11.77 kg year-1. Similarly annual P yield estimates ranged from 0.01 to 0.3 kg ha-1 year-1 with an average yield of 0.07 kg ha-1 year-1. We document how short records of daily streamflow and P concentrations can be combined with a national network of daily streamflow records in the Czech Republic to arrive at meaningful and reliable estimates of annual P loads for small agricultural watersheds.

Citation Beránková, T., Vogel, R. M., Fiala, D. & Rosendorf, P. (2010) Estimation of phosphorus loads with sparse data for agricultural watersheds in the Czech Republic. Hydrol. Sci. J. 55(8), 1417–1426.  相似文献   
996.
The effect of nutrient and surfactant addition on the biodegradation of phenanthrene was studied in a batch scale soil–slurry system using isolated Mycoplana sp. MVMB2strain. The study was conducted using an artificially phenanthrene spiked and as well as contaminated soil from petrochemical industrial site. Maximum phenanthrene degradation and subsequent high microbial growth were observed at optimum pH (pH 6) and C/N/P ratio (100:20:3). To investigate maximum substrate degradation potential of Mycoplana sp. MVMB2, very high concentrations of phenanthrene (50–200 mg/kg soil) were used. The organism was capable of degrading >60% for a concentration below 20 mg/kg soil and >40% for concentrations up to 200 mg/kg within 8 days. Further the influence of five different surfactants namely Span 80, Tween 20, Triton X‐100, cetyl trimethyl ammonium bromide, and sodium dodecyl sulfate were tested at their critical micelle concentration (CMC) levels for phenanthrene degradation in the soil. The addition of surfactant enhanced the biodegradation and a maximum of 84.49% was obtained for Triton X‐100. Complete phenanthrene degradation by Mycoplana sp. MVMB2 was observed at 3 CMC concentration of Triton X‐100. The optimized parameters obtained were used for the degradation of phenanthrene present in the contaminated soil and 98.6% biodegradation was obtained. Thus, the results obtained in the study suggested that biodegradation of phenanthrene by Mycoplana sp. MVMB2 appeared to be feasible to remediate phenanthrene rich contaminated sites.  相似文献   
997.
比赛用游泳馆泳池长度必须满足相应国际标准,对其精度有严格的质量控制。根据日照市游泳馆工程设计的特点及精度要求,提出了运用TCA2003型全站仪对游泳池长度进行全过程长度控制的方法,并进行了精度分析。  相似文献   
998.
为了适应测绘科学技术的信息化、自动化应用发展的需求,掌上电脑(PersonalDigitalAssistant,PDA)在测绘野外数据采集得到广泛应用,本文运用面向对象的思想设计并开发了野外数据采集成果预处理系统。该系统主要由数据通信、数据记录与检查、测量数据概算、原始成果记录表格自动生成、数据格式转换五个模块组成。将该系统与全站仪结合可实现野外测量数据的高效自动化数据采集及预处理概算,为内外业数据处理一体化奠定基础。  相似文献   
999.
《Urban geography》2013,34(7):587-609
Poverty segregation increased in Southern California during the 1990s. A comparison of four indices reflecting various dimensions of segregation (e.g., evenness, concentration, isolation, and clustering) suggests that poor people are increasingly concentrated and isolated. However, cities within the region experienced different levels and types of segregation, with significant changes over time. Regression analysis using 1990 and 2000 census data highlights the role of demographic, economic, and fiscal variables in explaining the changing landscape of poverty. Whereas historically poor and older cities have seen a rise in poverty that translated into an increase in concentration and isolation, newer suburban communities have evolved differently. Cities where poverty has been high and increasing often maintained a high level of clustering and unevenness, but experienced higher levels of concentration and clustering in 2000. In contrast, wealthier cities typically faced a rise in unevenness and clustering as small pockets of poverty population emerged in the midst of high-income neighborhoods.  相似文献   
1000.
《Urban geography》2013,34(8):898-926
Manufacturing establishments are integral to the spatial structure of fast-growing Sunbelt metropolitan areas, but most concepts and theories of intrametropolitan location were largely developed for an earlier technological era and different spatial contexts. This article investigates the location of nine disaggregated manufacturing sectors in Phoenix, Arizona, showing varying degrees of central core concentration and metropolitan-wide clustering. Distinct sectoral co-location patterns are also evident. We interpret our results as evidence that the intrametropolitan location of postindustrial manufacturing is best understood as a series of spatial distributions with varying concentration, centralization, clustering, and other order-based characteristics. There is little evidence that randomly scattered discrete industrial zones have developed nor that spatial patterns are uniform. Enduring lock-in effects tied to transportation infrastructure are pivotal to understanding the locational distribution of manufacturing industries in metropolitan Phoenix. Results do not support a hypothesis that a positive relationship exists between establishment size and distance from sectoral mean centers.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号