全文获取类型
收费全文 | 1044篇 |
免费 | 159篇 |
国内免费 | 30篇 |
专业分类
财政金融 | 6篇 |
工业经济 | 33篇 |
计划管理 | 151篇 |
经济学 | 23篇 |
综合类 | 30篇 |
运输经济 | 3篇 |
旅游经济 | 5篇 |
贸易经济 | 145篇 |
农业经济 | 162篇 |
经济概况 | 16篇 |
水利工程 | 659篇 |
出版年
2024年 | 7篇 |
2023年 | 21篇 |
2022年 | 32篇 |
2021年 | 52篇 |
2020年 | 57篇 |
2019年 | 50篇 |
2018年 | 65篇 |
2017年 | 47篇 |
2016年 | 56篇 |
2015年 | 62篇 |
2014年 | 87篇 |
2013年 | 72篇 |
2012年 | 96篇 |
2011年 | 101篇 |
2010年 | 60篇 |
2009年 | 43篇 |
2008年 | 48篇 |
2007年 | 46篇 |
2006年 | 43篇 |
2005年 | 45篇 |
2004年 | 37篇 |
2003年 | 27篇 |
2002年 | 22篇 |
2001年 | 13篇 |
2000年 | 10篇 |
1999年 | 7篇 |
1998年 | 7篇 |
1997年 | 3篇 |
1996年 | 6篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1989年 | 1篇 |
1985年 | 1篇 |
1983年 | 1篇 |
排序方式: 共有1233条查询结果,搜索用时 31 毫秒
101.
102.
Steven W. Effler MaryGail Perkins Feng Peng Christopher Strait Alan D. Weidemann Martin T. Auer 《Journal of Great Lakes research》2010
Features of light absorption are critical to optical aspects of water quality and in regulating the signal available for remote sensing. Spectral characteristics and spatial patterns of light-absorbing components, and their relationships with optically active constituents, are documented for the Sturgeon River, Keweenaw Bay, and Lake Superior based on analyses of samples collected on two cruises (2006 and 2007, 20 sites). The absorption coefficient, a (m− 1), is partitioned according to the additive components (ax) of colored dissolved organic matter (aCDOM), non-algal particles (aNAP), phytoplankton (a?), and water itself (aw; known). The role of minerogenic particles and their iron content in regulating aNAP is evaluated based on paired measurements by an individual particle analysis technique (Peng et al., 2009), through empirical analyses and Mie theory calculations of absorption by these particles (am). Spectral characteristics of aNAP and a? were consistent with those reported for other case 2 (i.e., phytoplankton not dominant) systems. However, the slope values that describe aCDOM spectra for the bay and the lake were unusually low, suggesting an atypical composition for the lake's CDOM. The dominant absorbing component in the blue wavelengths was CDOM, representing ≥ 75% of a at a wavelength of 440 nm at all sites in the 2006 survey. A general gradient in both aCDOM and aNAP extended from the Sturgeon River, through the bay, into eastern Lake Superior in that survey. Relationships between ax and optically active constituents were within the broad ranges reported for other case 2 systems. Minerogenic particles, related to their iron content, are demonstrated to be an important component of aNAP. 相似文献
103.
104.
105.
1999年至2010年呼伦贝尔市重要湿地遥感动态变化研究 总被引:1,自引:0,他引:1
以1999年、2002年和2010年3期TM/ETM影像为数据源,综合应用RS与GIS技术,以呼伦贝尔市西南六县为重点研究区,对其湿地进行专题信息提取和分类,了解呼伦贝尔湿地近10多年来的动态变化。结果表明:近11年来,研究区湿地面积萎缩严重,总面积减少3 522.07 km2,变化率为36.5%。其中新巴尔虎右旗和陈巴尔虎旗湿地面积减少最多,变化主要包括湿地向非湿地的转变和草甸沼泽湿地向草本沼泽湿地的转变。初步反映了呼伦贝尔盟湿地的退化现状,为进一步加强湿地保护与生态恢复提供了依据。 相似文献
106.
农业灌溉水量估算和区域农业用水的校核是用水统计、监管的难点。现行基于典型调查和定额推算的方法依赖统计人员经验,易受人为因素干扰,准确性难以控制;采用基于水量平衡的灌溉用水推算方法时,区域蓄水变量和耗水量等要素仅依靠传统地面监测条件难以获取。遥感技术能够解决水量平衡方程中区域蓄水变量和蒸散发的估算问题,可以提高水量平衡方法在计算灌溉用水中应用的可行性,但大多数遥感产品存在区域适用性和精度的问题,需要将多种产品结合,通过相互校验和组合控制误差;此外,遥感产品与水量平衡要素之间还存在一定程度的概念差异,需要进行匹配与转换。本文构建了一种基于多源遥感数据和水量平衡原理的灌溉用水分析方法,利用区域水量平衡方程检验多源遥感数据的一致性和可靠性,建立遥感数据与水量平衡要素的对应关系,选取合理的区域蒸散发数据,再利用田间水量平衡将总蒸散发分解为降水蒸散发和灌溉蒸散发,推算得到灌溉耗水量和灌溉用水量的合理范围。应用本文提出的方法进行山东省济南市2012—2015年灌溉用水分析,并与济南市水资源公报统计的农业用水进行对比,结果表明:在降水量接近多年平均的2012、2013、2015年,二者结果较为接近;在干旱的2014年,本文的分析结果与公报结果相差较大。结合同期农田作物长势、区域地下水位变化分析,认为该年份存在较大的灌溉用水,本文结果比较合理。本文方法可为校核区域灌溉用水统计数据的合理性提供技术支撑。 相似文献
107.
Zahir Nikraftar Esmaeel Parizi Seiyed Mossa Hosseini Behzad Ataie-Ashtiani 《Journal of Great Lakes research》2021,47(4):955-969
Lake Urmia is the second-largest hypersaline lake in the world. There has been a drastic water level drop of 7.2 m from 1995 to 2016. Beginning in October 2013, the Lake Urmia Restoration Plan (LURP) launched a 10-year program. An increase in water level and a relative improvement in Lake Urmia condition has been observed since 2017. It is an undecided and controversial issue whether the recent positive trend of Lake Urmia has been due to the LURP activities or it is a natural contribution of climate factors variations. To shed some light on this issue, we examine three other lakes, adjacent to the Lake Urmia basin, with similar rainfall variability to investigate their status during the same period. Van (Turkey), Mosul, and Tharthar (both in Iraq), are evaluated as well as Lake Urmia. Three decades of remotely sensed data including precipitation (), water level (), and lake extent () were considered for the mentioned lakes. A significant correlation was observed between standardized , and over the long-term period, especially for the recent three years (2 = 0.63–0.87). We show that the cumulative precipitation in the antecedent months played a major role in the improvement of these lakes' situation but with different time lags (up to 6 months for Van and Mosul lakes and up to 36 months for Lake Urmia and Tharthar lake). These findings could inform the planners of LURP to adopt strategies for achieving a sustainable state of Lake Urmia based on a more realistic outlook. 相似文献
108.
E. S. Papastergiadou A. Retalis A. Apostolakis Th. Georgiadis 《Water Resources Management》2008,22(5):579-594
Loss and degradation of terrestrial and aquatic habitats and degraded water quality are major environmental concerns worldwide.
Especially wetlands are sensitive ecosystems that are subject to stress from human activities. Remote sensing techniques have
been primarily used to generate information on land cover/use changes. Geographical Information Systems (GIS) and remote sensing
can be used to provide a rapid or a large-scale understanding of lake change and in developing lake management strategies.
The principal objectives of this study are to monitor and assess the spatial and temporal changes in land cover/use by using
GIS, and to determine the main environmental factors affecting these changes. This paper presents a case study for the application
of integrated remote sensing and GIS data for the classification and monitoring of the spatial and temporal changes in land
use types. The study was conducted in a small natural wetland of Lake Cheimaditida, located in the East Mediterranean region
of Northern Greece. Data analysis was conducted using GIS software. During the past several decades Lake Cheimaditida wetland
has been influenced by many anthropogenic activities. The variables chosen for the assessment included condition of wetland
and lake areas, present extent of wetlands relative to historic area, cover of natural habitat, wetland disturbances, etc.
These variables address catchments properties that are important for maintaining and improving wetland habitats and water
quality and assessment of trends useful for environmental monitoring. Land cover/land use patterns were assessed and compared
using aerial photographs taken in 1945, 1969, 1982, and 1996. Over this period, reed beds enormously increased by 1,655.19%,
while open-water areas and peat lands decreased by 74.05 and 99.5%, respectively. The significance of the changes in land
cover distribution within the Lake Cheimaditida wetland are discussed in relation to the historical pattern of reed beds colonization,
the importance of Phragmites australis in the process and the implications for strategic management of freshwater wetland resources. 相似文献
109.
基于水泥回转窑旋风预热器远程监控系统的要求,应用多线程技术实现了旋风预热器整个运行过程中的数据采集处理、图像动态显示、数据库访问以及故障报警等多任务的同步运行,从而提高了整个系统的运行效率和速度。 相似文献
110.
当前,我国经济正处于快速增长期,用水量大幅度增长,水资源供需矛盾突出。文章以广西来宾市远程自动抄表系统在取水计量实时监控中的应用为例,从战略上分析了做好节水工作的重要性。 相似文献