全文获取类型
收费全文 | 1529篇 |
免费 | 124篇 |
国内免费 | 99篇 |
专业分类
测绘学 | 89篇 |
大气科学 | 50篇 |
地球物理 | 272篇 |
地质学 | 247篇 |
海洋学 | 177篇 |
天文学 | 1篇 |
综合类 | 117篇 |
自然地理 | 799篇 |
出版年
2024年 | 9篇 |
2023年 | 29篇 |
2022年 | 54篇 |
2021年 | 70篇 |
2020年 | 45篇 |
2019年 | 59篇 |
2018年 | 49篇 |
2017年 | 69篇 |
2016年 | 78篇 |
2015年 | 63篇 |
2014年 | 61篇 |
2013年 | 62篇 |
2012年 | 103篇 |
2011年 | 123篇 |
2010年 | 80篇 |
2009年 | 98篇 |
2008年 | 131篇 |
2007年 | 93篇 |
2006年 | 113篇 |
2005年 | 69篇 |
2004年 | 89篇 |
2003年 | 63篇 |
2002年 | 33篇 |
2001年 | 27篇 |
2000年 | 18篇 |
1999年 | 16篇 |
1998年 | 13篇 |
1997年 | 9篇 |
1996年 | 10篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1985年 | 1篇 |
1983年 | 1篇 |
排序方式: 共有1752条查询结果,搜索用时 453 毫秒
481.
482.
近40 年来青藏高原典型高寒湿地系统的动态变化 总被引:38,自引:1,他引:37
选择青藏高原长江源区、黄河源区以及若尔盖地区等典型高寒湿地分布区域, 利用1969、1986、2000 和2004 年多期航片和卫星遥感数据, 从湿地主要组分分布、空间格局以及水生态功能等方面, 分析了近40 年来典型高寒湿地系统动态变化特征及其区域差异性。结果表明: 青藏高原典型高寒湿地退化具有普遍性, 湿地面积萎缩在10%以上。以长江源区的沼泽湿地退化最为严重, 退缩幅度达到29%, 同时大约有17.5%的长江源区内流小湖泊干涸消失, 黄河源区和若尔盖地区湿地系统空间分布格局的破碎化和岛屿化程度显著加剧。高寒 湿地系统退化使其水文功能发生变化, 表现在湿地退化较为强烈的长江源区与若尔盖地区枯 水期流量减少、稀遇较大流量径流发生频率增加而常遇流量发生频率减少、水涵养能力下降。湿地系统变化与区域气温显著升高有关, 在20 世纪80 年代以来区域增温幅度升高到过去40 年平均增温幅度的2.3 倍, 湿地系统退化程度也同步在20 世纪80 年代中期以后明显加剧。在降水量呈现增加以及冰川趋于消融的背景下, 高寒湿地退化是导致其流域径流持续递减的主要因素之一。 相似文献
483.
The hydrologic and water-quality characteristics of a small tropical riverine wetland at Ulakwo, near Owerri, Imo State,
Nigeria, were evaluated by analysis of stream hydrographs, the groundwater flow system, and geochemical analyses. This research
is an initial step toward providing information needed to develop a programme of sustainable development of the ecosystem.
The wetland is underlain by a layer of organic debris and hydromorphic soils, which in turn are underlain by an unconfined
alluvial sand aquifer about 80 m thick. Horizontal and upward vertical hydraulic head gradients of about 0.002 and 0.001,
respectively, and the results of a flow-net analysis suggest that considerable amounts of groundwater flow into the wetland.
Low concentrations of Fe, NO3, PO4, and SO4 in the wetland water column are probably due to short-term removal of these nutrients from the surface-water by adsorption
on the surficial wetland organic matter and bottom sediments. The groundwater flow system is important in the maintenance
of the wetland, which probably plays an important role in the flow stabilisation and improvement of the water quality of the
river.
Electronic Publication 相似文献
484.
485.
486.
洞庭湖湿地资源退化的生态经济损益评估 总被引:21,自引:2,他引:19
洞庭湖湿地是中国最大的淡水湿地生态系统之一,湿地资源十分丰富,但由于人类不合理的开发利用,导致湿地生态系统严重退化,湿地生态服务功能价值受损,制约了湖区经济的可持续发展。在实地调查和试验的基础上,依据生态经济学的理论和方法,对洞庭湖湿地因资源退化引起的一系列自然灾害和生态灾害所造成的湿地生态功能损失进行了价值损益评估,由此得出湿地资源退化对湖区经济可持续发展所造成的损失;并指出只有恢复洞庭湖区湿地生态环境,保护好洞庭湖区湿地资源,才能实现洞庭湖区湿地资源的可持续利用,保证湖区经济的可持续发展。 相似文献
487.
在遥感和地理信息系统技术支持下,利用湿地调查资料、地图和遥感影像数据,获得1956~2016年期间9个时期湖南省的湿地数据;采用景观分析方法,系统地分析了9个时期湖南省的湿地分布及其变化特征和湿地景观格局变化及其影响因素。研究结果表明,与1956年相比,2016年湖南省的湿地面积减少了955.4 km^2,其中,永久性淡水湖泊和河流面积明显减少,水产养殖场面积明显增加;湿地的景观结构也相应地发生了改变,1986年湖南省的湿地的破碎化程度最严重;1956~2016年期间,湿地景观类型分布不均衡程度在不断加剧;永久性河流和永久性淡水湖泊是湖南省的优势湿地景观类型,且永久性淡水湖泊的最大斑块指数都远大于其它类型湿地;围垦和退耕还湿活动是影响湿地变化的主要因素。 相似文献
488.
Snow hydrology of a constructed watershed in the Athabasca oil sands region,Alberta, Canada
下载免费PDF全文
![点击此处可从《水文研究》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Wetlands are now being integrated into oil sands mining landscape closure design plans. These wetland ecosystems will be constructed within a regional sub‐humid climate where snowfall represents ~25% of annual precipitation. However, few studies focus on the distribution of snow and, hence, the storage of winter precipitation in reclaimed oil sands landscapes. In this study, the distribution, ablation and fate of snowmelt waters are quantified within a constructed watershed in a post‐mining oil sands environment. Basin‐averaged peak SWE was 106 mm, with no significant difference between reclaimed slopes with vegetation and those that were sparsely vegetated or bare. Snow depth was greatest and more variable near the toe of slopes and became progressively shallower towards the crest. Snow ablation started first on the vegetated slope, which also exhibited the maximum observed ablation rates. This enhanced melt was attributed to increased absorption of short‐wave radiation by vegetation stems and branches. Recharge to reclaimed slopes and a constructed aquifer during the snowmelt period was minimal, as the presence of ground frost minimized infiltration. Accordingly, substantial surface run‐off was observed from all reclaimed slopes, despite being designed to reduce run‐off and increase water storage. This could result in increased flashiness of downstream watercourses during the spring freshet that receive run‐off from post‐mining landscapes where large reclaimed slopes are prolific. Run‐off ratios for the reclaimed slopes were between 0.7 and 0.9. Thus, it is essential to consider snow dynamics when designing landscape‐scale constructed ecosystems. This research demonstrates that the snowmelt period hydrology within reclaimed landscapes is fundamentally different from that reported for natural settings and represents one of the first studies on snow dynamics in constructed watershed systems in the post‐mined oil sands landscape. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
489.
Polar Bear Pass is a large High Arctic low‐gradient wetland (100 km2) bordered by low‐lying hills which are notched by a series of v‐shaped valleys. The spring and summer hydrology of two High Arctic hillslope‐wetland catchments, a first‐order stream, 0·2 km2 Landing Strip Creek (LSC) and a larger second‐order basin, 4·2 km2 Windy Creek (WC), is described here. A water balance framework was employed in 2008 to examine the movement of water from upland reaches into the low‐lying wetland. Snowcover was low in both basins (<50 mm in water equivalent units), but they both exhibited nival‐type regimes. After the main snowmelt season ended, runoff ceased in the smaller catchment (LSC), but not at the larger basin (WC) which continued to flow throughout the summer. Both basins responded to summer rains in different ways. At LSC, late‐summer continuous streamflow occurred only when rainfall satisfied the large soil moisture deficit in the upper bowl‐shaped zone of the basin. At WC, the presence of thinly thawed, ice‐rich polygonal terrain within the stream channel and in the upper reaches of the catchment likely limited infiltration in these near‐stream zones and enhanced runoff in response to both moderate and high rainfall. Subsequently, seasonal runoff ratios differed between the two sites (0·19 vs 0·68) as did the seasonal storage + residual (+16 vs ?50 mm). This suggests that the post‐snowmelt season runoff response to summer precipitation is very much modified by the unique basin characteristics (soil‐type, vegetation, ground ice) and their location within each stream order type. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
490.
Jonathan M. Malzone Ethan G. Sweet Addison C. Bell Gilbert L. Minzenberger 《水文研究》2020,34(4):1089-1100
Geographically isolated wetlands (GIWs) are commonly reported as having hardpan or low hydraulic conductivity units underneath that produce perched groundwater, which can sustain surface water levels independently of regional aquifer fluctuations. Despite the potential of GIW-perched aquifer systems to provide important hydrological and ecological functions such as groundwater storage and native amphibian habitat, little research has studied the hydrologic controls and dynamics of these systems. We compared several ridge-top depressional GIW-perched groundwater systems to investigate the role of watershed morphology on hydroregime and groundwater-surface water interaction. Ridge-top depressional wetlands in the Daniel Boone National Forest, Kentucky were chosen because they offer natural controls such as lack of apparent connection to surface water bodies, similar climate, and similar soils. Three wetlands with different topographic slopes and hillslope structures were mapped to distinguish key geomorphic parameters and monitored to characterize groundwater-surface water interaction. Wetlands with soil hummocks and low upland slopes transitioned from infiltration to groundwater discharge conditions in the spring and during storm events. The magnitude and duration of this transition fell along a continuum, where higher topographic slopes and steeper uplands produced comparably smaller and shorter head reversals. This demonstrates that ridge-top GIW-perched groundwater systems are largely sensitive to the runoff-recharge relationship in the upland area which can produce significant groundwater storage on a small-scale. 相似文献