全文获取类型
收费全文 | 338篇 |
免费 | 51篇 |
国内免费 | 7篇 |
专业分类
测绘学 | 21篇 |
大气科学 | 25篇 |
地球物理 | 193篇 |
地质学 | 85篇 |
海洋学 | 11篇 |
天文学 | 1篇 |
综合类 | 2篇 |
自然地理 | 58篇 |
出版年
2024年 | 1篇 |
2023年 | 9篇 |
2022年 | 8篇 |
2021年 | 7篇 |
2020年 | 19篇 |
2019年 | 19篇 |
2018年 | 17篇 |
2017年 | 29篇 |
2016年 | 17篇 |
2015年 | 18篇 |
2014年 | 17篇 |
2013年 | 44篇 |
2012年 | 15篇 |
2011年 | 13篇 |
2010年 | 17篇 |
2009年 | 18篇 |
2008年 | 15篇 |
2007年 | 9篇 |
2006年 | 12篇 |
2005年 | 13篇 |
2004年 | 7篇 |
2003年 | 7篇 |
2002年 | 13篇 |
2001年 | 4篇 |
2000年 | 9篇 |
1999年 | 9篇 |
1998年 | 7篇 |
1997年 | 8篇 |
1996年 | 3篇 |
1995年 | 3篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有396条查询结果,搜索用时 296 毫秒
1.
2.
Soil sediment samples of 10 layers with a spacing of 10 cm each were collected in different floodplain zones adjacent to Huolin
River in the Xianghai Nature Reserve, and contents of total N, total P and organic matters were analyzed. The results showed
that contents of total N, total P and organic matters were generally decreasing with the increase of distance from sample
locations to the river channel, and contents of the three items were generally higher in the upper soil layer than that in
the lower soil layer. The content variations displayed how flooding functions influenced nutrient matter content variations
in floodplain soils since the flood inundation frequencies of the sample locations varied. The correlation analysis displayed
that there were remarkable relativities between total N, total P and organic matters within definite spatial distance from
the Huolin River channel. 相似文献
3.
4.
A maximum difference conjoint (MDC) model was part of a questionnaire toassess floodplain residents' preferences for the outcomes of water level managementinside an enclosed embankment system on the left bank of the Dhaleswari River, calledthe Compartmentalization Pilot Project (CPP)-Tangail. The outcomes were described as(a) varied flood depths in the rice field, (b) incidence of flooding on the courtyard andinside homes, (c) changes in floodplain and culture fisheries, and (d) varied conditionsof drainage congestion. Following a standard survey, the respondents from inside andoutside a completed section of the compartment (Cluster 1b) were shown profiles of these flood management outcomes and were asked to indicate for each profile the one most preferred and the one least preferred item. The results of the study indicated that the respondents had a clear preference for preventing flooding of their homes and courtyards and for an ideal water depth of 2 ft in the aman rice fields. At the same time, they also showed a strong concern about malfunctioning of sluices and to a lesser extent about the changes in the fish habitat. The successful application of the MDC as an approach to model tradeoffs among rural residents of Bangladesh shows that a relatively complex quantitative survey method, incorporating choice cards as pictograms, can be applied successfully even in a developing country. 相似文献
5.
This study presents the evolution of agreements between the governments of Canadaand Québec on flood damage reduction. In Québec, the implementation of a regulation about building in floodplains came about in 1983–1984. Today, this regulation takes the form of a policy called ``Policy of shores, littoral and floodplain protection'. Municipalities must adopt rules that concur with the principles of this policy.The Chaudiére River basin was selected for analysis of urban developmentduring the period following the application of building rules in flood-risk areas.Despite the ban on building in the strong current zone (0–20 year return periodflood zone), many buildings, essentially residential, have been erected in thiszone. These new constructions generally account for a low percentage of thetotal property value in the 0–100 year flooding area, but are legal since theyare connected to a water and sewage network that existed prior to the officialfloodplain regulation.Flood damage along the Chaudiére River will tend to increase for two reasons.Firstly, while respecting the policy mentioned above, sites are still available infloodplains for future development. Secondly, no structural flood protection workshave been erected in view of the fact they are only marginally profitable from anbenefit-cost point of view. 相似文献
6.
Pollen and charcoal records from two large, shallow lakes reveal that throughout most of the past 50,000 yr Noel Kempff Mercado National Park, in northeastern lowland Bolivia (southwestern Amazon Basin), was predominantly covered by savannas and seasonally dry semideciduous forests. Lowered atmospheric CO2 concentrations, in combination with a longer dry season, caused expansion of dry forests and savannas during the last glacial period, especially at the last glacial maximum. These ecosystems persisted until the mid-Holocene, although they underwent significant species reassortment. Forest communities containing a mixture of evergreen and semideciduous species began to expand between 6000 and 3000 14C yr B.P. Humid evergreen rain forests expanded to cover most of the area within the past 2000 14C yr B.P., coincident with a reduction in fire frequencies. Comparisons between modern pollen spectra and vegetation reveal that the Moraceae-dominated rain forest pollen spectra likely have a regional source area at least 2-3 km beyond the lake shore, whereas the grass- and sedge-dominated savanna pollen spectra likely have a predominantly local source area. The Holocene vegetation changes are consistent with independent paleoprecipitation records from the Bolivian Altiplano and paleovegetation records from other parts of southwestern Amazonia. The progressive expansion in rain forests through the Holocene can be largely attributed to enhanced convective activity over Amazonia, due to greater seasonality of insolation in the Southern Hemisphere tropics driven by the precession cycle according to the Milankovitch Astronomical Theory. 相似文献
7.
Cynthia S. Simmons 《GeoJournal》2005,64(4):307-317
Brazil possesses a long history of violent struggle for land, and its most recent phase is occurring predominantly in the
Amazon Basin. Consequently, this paper attempts to territorialize land conflict in the Brazilian Amazon, and in so doing,
to illuminate the place-specific intersection of historic social, political, and economic circumstance that created a violent
landscape in the so-called “South of Pará.” The paper’s premise is that such conflict can be best viewed as resulting from
a dialectic between general social processes operating across spatial scales, which create necessary conditions for conflict,
and place-specific historical circumstances that transform necessary into sufficient conditions. The paper presents a framework
integrating the theory of contentious politics and literatures addressing violence associated with the Amazonian frontier
and with resource scarcity (and abundance). The discussion and theoretical application deconstruct the environmental, cognitive,
and relational mechanisms that created violence in the South of Pará, and set the stage for the Eldorado dos Carajás massacre in April of 1996. 相似文献
8.
Macropores are subsurface connected void spaces caused by processes such as fracture of soils, micro‐erosion, and fauna burrows. They are common near streams (e.g. hyporheic and riparian zones) and may act as preferential flow paths between surface and groundwaters, affecting hydrologic and biogeochemical processes. We tested the hydrologic function of macropores by constructing an artificial macropore within the saturated zone of a meander bend (open macropore, ‘OM’) and later filling its upstream end (partially filled macropore, ‘PFM’). For each treatment, we injected saline tracer at an upgradient monitoring well within the meander and monitored downgradient hydraulics and tracer transport. Pressure transducers in monitoring wells indicated hydraulic gradients within the meander were 32% higher perpendicular to and 6% higher parallel to the macropore for the OM than for the PFM. Additionally, hydraulic conductivities measured via falling head tests were 29 to 550 times higher along the macropore than in nearby sediment. We used electrical conductivity probes in wells and electrical resistivity imaging to track solute transport. Transport velocities through the meander were on average 9 and 21% higher (per temporal moment analysis and observed tracer peak, respectively) for the OM than for the PFM. Furthermore, temporal moments of tracer breakthrough analysis indicated downgradient longitudinal dispersion and breakthrough tracer curve tailing were on average 234% and 182% higher for the OM, respectively. This suggests the OM enabled solute transport at overall shorter timescales than the matrix but also increased tailing. Our results demonstrate the importance of macropores to meander bend hydrology and solute transport. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
9.
Intensity of in‐lake processes in floodplain lakes within the Bug River zone of fluvial activity
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)
The paper presents a new approach to calculating the erosion and deposition values of floodplain lake basins, the erosion–deposition index (EDI). The EDI is a sum of the basin geometry indices (BGIs), which can be calculated for a separate cross section of the lake. The distribution of processes within the basin was investigated in two selected floodplain lakes with the use of BGIs. Field research was carried out in the Bug River valley from 1 November 2006 to 31 October 2011. The highest erosion was observed in the lakes located close to the parent river. Deposition processes were observed in lakes with high inflow of groundwater. The results showed that EDI values of 48 out of the 71 floodplain lakes ranged from ?0.2 to 0.2. Spatial distribution of erosion and deposition processes within the lake basins resulted from a velocity of water inflowing or flowing through the basin. This was observed especially in contrafluent–confluent lake. Inflow of rivers water via upstream crevasse occurred later than via downstream one, but energy of flowing water was higher, which favoured erosion of this part of the lake basin. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
10.
M. González‐Sanchis J. Murillo A. Cabezas J. E. Vermaat F. A. Comín P. García‐Navarro 《水文研究》2015,29(3):384-394
Phosphorus (P) is one of the major limiting nutrient in many freshwater ecosystems. During the last decade, attention has been focused on the fluxes of suspended sediment and particulate P through freshwater drainage systems because of severe eutrophication effects in aquatic ecosystems. Hence, the analysis and prediction of phosphorus and sediment dynamics constitute an important element for ecological conservation and restoration of freshwater ecosystems. In that sense, the development of a suitable prediction model is justified, and the present work is devoted to the validation and application of a predictive soluble reactive phosphorus (SRP) uptake and sedimentation models, to a real riparian system of the middle Ebro river floodplain. Both models are coupled to a fully distributed two‐dimensional shallow‐water flow numerical model. The SRP uptake model is validated using data from three field experiments. The model predictions show a good accuracy for SRP concentration, where the linear regressions between measured and calculated values of the three experiments were significant (r2 ≥ 0.62; p ≤ 0.05), and a Nash–Sutcliffe coefficient (E) that ranged from 0.54 to 0.62. The sedimentation model is validated using field data collected during two real flooding events within the same river reach. The comparison between calculated and measured sediment depositions showed a significant linear regression (p ≤ 0.05; r2 = 0.97) and an E that ranged from 0.63 to 0.78. Subsequently, the complete model that includes flow dynamics, solute transport, SRP uptake and sedimentation is used to simulate and analyse floodplain sediment deposition, river nutrient contribution and SRP uptake. According to this analysis, the main SRP uptake process appears to be the sediment sorption. The analysis also reveals the presence of a lateral gradient of hydrological connectivity that decreases with distance from the river and controls the river matter contribution to the floodplain. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献