全文获取类型
收费全文 | 152篇 |
免费 | 15篇 |
国内免费 | 3篇 |
专业分类
测绘学 | 3篇 |
大气科学 | 13篇 |
地球物理 | 43篇 |
地质学 | 61篇 |
海洋学 | 23篇 |
天文学 | 9篇 |
自然地理 | 18篇 |
出版年
2024年 | 1篇 |
2021年 | 5篇 |
2020年 | 4篇 |
2019年 | 8篇 |
2018年 | 4篇 |
2017年 | 13篇 |
2016年 | 11篇 |
2015年 | 7篇 |
2014年 | 9篇 |
2013年 | 10篇 |
2012年 | 5篇 |
2011年 | 16篇 |
2010年 | 9篇 |
2009年 | 7篇 |
2008年 | 7篇 |
2007年 | 10篇 |
2006年 | 6篇 |
2005年 | 5篇 |
2004年 | 4篇 |
2003年 | 4篇 |
2002年 | 4篇 |
2001年 | 4篇 |
2000年 | 3篇 |
1999年 | 1篇 |
1998年 | 1篇 |
1996年 | 1篇 |
1993年 | 1篇 |
1987年 | 2篇 |
1981年 | 1篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1961年 | 1篇 |
1949年 | 1篇 |
排序方式: 共有170条查询结果,搜索用时 15 毫秒
81.
82.
Portland, Oregon, is renowned as a paradigmatic “sustainable city.” Yet, despite popular conceptions of the city as a progressive ecotopia and the accolades of planners seeking to emulate its innovations, Portland’s sustainability successes are inequitably distributed. Drawing on census data, popular media, newspaper archives, city planning documents, and secondary source histories, we attempt to elucidate the structural origins of Portland’s “uneven development,” exploring how and why the urban core of this paragon of sustainability has become more White and affluent while its outer eastside has become more diverse and poor. We explain how a “sustainability fix”—in this case, green investment in the city’s core—ultimately contributed to the demarcation of racialized poverty along 82nd Avenue, a major north–south arterial marking the boundary of East Portland. Our account of structural processes taking place at multiple scales contributes to a growing body of literature on eco-gentrification and displacement and inner-ring suburban change while empirically demonstrating how Portland’s advances in sustainability have come at the cost of East Portland’s devaluation. Our “30,000 foot” perspective reveals systemic patterns that might then guide more fine-grained analyses of particular political-socio-cultural processes, while providing cautionary insights into current efforts to extend the city’s sustainability initiatives using the same green development model. 相似文献
83.
Water Level Declines in the High Plains Aquifer: Predevelopment to Resource Senescence 总被引:1,自引:0,他引:1
下载免费PDF全文
![点击此处可从《Ground water》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A large imbalance between recharge and water withdrawal has caused vital regions of the High Plains Aquifer (HPA) to experience significant declines in storage. A new predevelopment map coupled with a synthesis of annual water levels demonstrates that aquifer storage has declined by approximately 410 km3 since the 1930s, a 15% larger decline than previous estimates. If current rates of decline continue, much of the Southern High Plains and parts of the Central High Plains will have insufficient water for irrigation within the next 20 to 30 years, whereas most of the Northern High Plains will experience little change in storage. In the western parts of the Central and northern part of the Southern High Plains, saturated thickness has locally declined by more than 50%, and is currently declining at rates of 10% to 20% of initial thickness per decade. The most agriculturally productive portions of the High Plains will not support irrigated production within a matter of decades without significant changes in management. 相似文献
84.
85.
Ronald E. Stewart Daniel Betancourt James B. Davies Deborah Harford Yaheli Klein Robert Lannigan Linda Mortsch Erin O’Connell Kathy Tang Paul H. Whitfield 《Natural Hazards》2017,87(2):791-815
Using studies of places where heat waves are common, projected changes in heat waves in Metro Vancouver are assessed from a multi-disciplinary perspective with respect to the potential impacts of the physical change on the people and infrastructure with the intention of being better prepared for future events. Trends in maximum temperature parameters for Metro Vancouver for the past 75 years are generally not statistically significant; however, projections for 2041–2060 and 2081–2100 suggest that the region will experience such events more frequently in the future due to climate change. While Metro Vancouver, British Columbia (BC) generally does not typically experience heat waves it was strongly affected by a major heat event in July 2009, with temperature records being broken at Vancouver (≥31 °C) on the coast and at Abbotsford (≥36 °C) 65-km inland. A lack of sea breeze during this event meant that there was no cooling effect, and land surface temperatures over the downtown area approached 40 °C and excess deaths occurred. Many victims were either in the 65–74 age category, the vulnerable poor, or people with mental health issues. Because these events are rare, many buildings lack air-conditioning, and residents of Metro Vancouver under-anticipate their vulnerability. The costs of health-related impacts outweighed those related to higher electricity usage. 相似文献
86.
Volusia Blue Spring (VBS) is the largest spring along the St. Johns River in Florida (USA) and the spring pool is refuge for hundreds of manatees during winter months. However, the water quality of the spring flow has been degraded due to urbanization in the past few decades. A three-dimensional contaminant fate and transport model, utilizing MODFLOW-2000 and MT3DMS, was developed to simulate boron transport in the Upper Florida Aquifer, which sustains the VBS spring discharge. The VBS model relied on information and data related to natural water features, rainfall, land use, water use, treated wastewater discharge, septic tank effluent flows, and fertilizers as inputs to simulate boron transport. The model was calibrated against field-observed water levels, spring discharge, and analysis of boron in water samples. The calibrated VBS model yielded a root-mean-square-error value of 1.8 m for the head and 17.7 μg/L for boron concentrations within the springshed. Model results show that anthropogenic boron from surrounding urbanized areas contributes to the boron found at Volusia Blue Spring. 相似文献
87.
Peter Griggs 《The Australian geographer》2001,32(3):359-376
The late nineteenth and early twentieth centuries saw the emergence of large single-product organisations whose expansion was stimulated by a desire to control a substantial part of a particular product market. An Australian example is the Colonial Sugar Refining Company Ltd (CSR). By 1915, the company accounted for 90 per cent of refined sugar production in Australia. CSR had achieved this control over the Australian refined sugar market by using underselling, strategies that denied its competitors outlets for their refined sugars and amalgamations with rivals. After 1890, however, CSR was interested in sharing the Australian market for refined sugar with its rivals by forming spheres of influence in marketing areas or adopting production quotas, although the Queensland National Bank was the only organisation to successfully negotiate annual market share agreements with CSR in the 1900s. These agreements ensured the survival of its Millaquin Refinery. Other competitors eventually exited the industry. In 1915, the Commonwealth government institutionalised this duopoly in the sugar refining sector by agreements with CSR and the Queensland government, and allowed it to continue until deregulation of the Australian sugar refining sector in the early 1990s. 相似文献
88.
Hungry Valley, and arid basin situated within the Transverse Ranges of Southern California, has been utilized by off-road
vehicles (ORVs) since 1971. Loss of vegetation, severe soil erosion and gullying, alluvial fan formation, and increased sediment
discharge are the direct results of ORV activity in the valley. These hydrologic-geologic effects have been documented and
monitored utilizing sequential aerial photographs, ground surveys, and sediment transport measurements. Despite the increased
usage of ORVs on public lands, and the well-documented short-term physical environmental impacts, little data exists on erosion
or sediment transport rates in disturbed vs. adjacent undisturbed areas, the ability of natural vegetation to re-establish
itself, or the feasibility of site rehabilitation techniques or methods. 相似文献
89.
An average of 230,000 cubic meters of sand is provided to the beaches of northern Monterey Bay each year by littoral transport
from upcoast and from local river input. Two jetties constructed as part of a small craft harbor interrupted the littoral
flow of sand and significantly altered the area's natural coastal processes. A wide protective beach immediately formed upcoast
against a formerly retreating beach cliff. Sand now filling the harbor mouth each winter has led to expensive yearly dredging
as well as seasonally or permanently depleted downcoast beaches. Seacliff retreat, always a problem in the area, is caused
primarily by surf attack of weaker stratigraphic units and erosion along joint sets and faults, causing collapse of the bluffs.
The seasonal loss of protective beaches has led to a two- to three-fold increase in the rate of downcoast cliff retreat following
harbor construction except where protective rip-rap has been emplaced by property owners. 相似文献
90.
Process-based snowmelt modeling: does it require more input data than temperature-index modeling? 总被引:4,自引:0,他引:4
M. Todd Walter Erin S. Brooks Donald K. McCool Larry G. King Myron Molnau Jan Boll 《Journal of Hydrology》2005,300(1-4):65-75
Modeling snow hydrology for cold regions remains a problematic aspect of many hydro-environmental models. Temperature-index methods are commonly used and are routinely justified under the auspices that process-based models require too many input data. To test this claim, we used a physical, process-based model to simulate snowmelt at four locations across the conterminous US using energy components estimated from measured daily maximum and minimum temperature, i.e. using only the same data required for temperature-index models. The results showed good agreement between observed and predicted snow water equivalents, average R2>0.9. We duplicated the simulations using a simple temperature-index model best fitted to the data and results were poorer, R2<0.8. At one site we applied the process-based model without substantial parameter estimation, and there were no significant (=0.05) differences between these results and those obtained using temperature-estimated parameters, despite relatively poorly predicted specific energy budget components (R2<0.8). These results encourage the use of mechanistic snowmelt modeling approaches in hydrological models, especially in distributed hydrological models for which landscape snow distribution may be controlled by spatially distributed components of the environmental energy budget. 相似文献