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   91篇
  免费   1篇
  国内免费   1篇
测绘学   7篇
地球物理   14篇
地质学   25篇
海洋学   16篇
天文学   14篇
自然地理   17篇
  2020年   1篇
  2019年   1篇
  2018年   2篇
  2017年   1篇
  2016年   3篇
  2015年   2篇
  2014年   5篇
  2013年   9篇
  2012年   2篇
  2011年   3篇
  2010年   6篇
  2009年   1篇
  2008年   7篇
  2007年   2篇
  2006年   2篇
  2005年   2篇
  2004年   4篇
  2003年   3篇
  2002年   1篇
  2001年   4篇
  2000年   5篇
  1999年   1篇
  1998年   2篇
  1997年   4篇
  1995年   1篇
  1994年   2篇
  1993年   2篇
  1992年   2篇
  1991年   1篇
  1987年   2篇
  1984年   3篇
  1982年   2篇
  1980年   1篇
  1979年   1篇
  1975年   1篇
  1974年   1篇
  1961年   1篇
排序方式: 共有93条查询结果,搜索用时 15 毫秒
71.
An unpredicted sudden outburst flood from Sólheimajökull, Southern Iceland, in July 1999 may herald a major subglacial volcanic eruption beneath Mýrdalsjökull ice cap.  相似文献   
72.
Geografisk Tidsskrift, Danish Journal of Geography 108(1):121–136, 2008

SnowModel, a physically-based snow evolution modeling system that includes four submodels—MicroMet, EnBal, SnowPack, and SnowTran-3D—was used to simulate eight full-year (1998/99 through 2005/06) evolutions of snow accumulation, blowing snow sublimation, evaporation, snow and ice surface melt, runoff, and mass changes on the entire Mittivakkat Glacier (31 km2) in southeast Greenland. Meteorological observations from two meteorological stations inside the glacier catchment were used as model input, and glaciological mass balance observations were used for model calibration (1998/99 through 2001/02) and validation (2002/03 through 2005/06) of winter snow simulations. As confirmed by observations, the spatially modeled end-of-winter snow water equivalent (SWE) accumulation increased with elevation up to 700–800 m a.s.l. in response to elevation, topography, and dominating wind direction, and maximum snow deposition occurred on the lee side of the ridge east and south of the glacier. Simulated end-of-summer cumulative runoff decreased with elevation and minimum runoff occurred on the shadowed side of the ridge east and south of the glacier. The modeled test period averaged annual mass balance was 65 mm w. eq. y?1 or ~8% more than the observed. For the simulation period, the glacier net mass balance varies from -199 to -1,834 mm w.eq. y?1, averaging -900 (±470) mm w.eq.y?1. The glacier averaged annual modeled precipitation ranged from 1,299 to 1,613 mm w.eq. y?1, evaporation and sublimation from 206 to 289 mm w.eq., and runoff from 1,531 to 2,869 mm w.eq. y?1. The model simulated Mittivakkat Glacier net loss of900 mm w.eq. y?1 contributes approximately 42% to the average simulated runoff of 2,140 mm w.eq. y?1, indicating a mean specific runoff of 67.8 l s?1 km?2.  相似文献   
73.
74.
This paper describes glaciological features observed on the surface of the large outlet glacier Kuannersuit Glacier on Disko Island, West Greenland, during its initial quiescent phase after a major surge event occurred in 1995–98 in order to classify the glaciomorphological surge system. Focus is on surge features either associated with active surging such as a propagating surge front and serac formation or formed as a consequence of surging such as looped medial moraines and potholes; and on hydrological features related to the evolution of the glacier drainage system such as the supraglacial routing, moulin types and chasms. A chronological glaciomorphological model of the initial quiescent phase of Kuannersuit Glacier, which can be regarded as representative for large outlet surge‐type glaciers with terrestrial terminal regime and long quiescent phase, is proposed.  相似文献   
75.
During the restructuring of the Vietnamese economy in 1986, the Communist party of Vietnam (CPV) took the decision to leave central planning and build a market economy in Vietnam. In order to promote rural development and deal with deforestation, poverty and declining agricultural profitability, the government implemented the forestland allocation policies (FLA) and started a process of decentralization. This research note examines the impact of FLA policies on the agricultural practices of an upland community in central Vietnam and discusses the effects of increased tenure security. It is shown that the promotion of a sedentary production system has altered the sustainability of swidden agriculture leading to declining profitability and that a transition towards new crops and specialized crop cultivation has compensated for the loss. Furthermore, it is argued that the FLA policies have increased the villagers’ incentive to invest, their access to credit and the forest area of the village, but that the CPV has been the main driver of these changes rather than the increased tenure security.  相似文献   
76.
The Marine Living Resources Act (MLRA), which was enacted in 1998, is the primary legislation addressing South Africa's marine fisheries. In June 2012, another important instrument, the Policy for the Small Scale Fisheries Sector (SSFP) in South Africa, was adopted to rectify the exclusion of many small-scale fishers from access to resources, that had resulted from weaknesses in the MLRA. This paper assesses the MLRA, aspects of the SSFP and selected other subsidiary policies and regulations in relation to current best-practices, especially the extent to which they support an ecosystem approach to fisheries (EAF). The study concludes that there are some serious gaps and shortcomings in the MLRA that should be addressed. These include, among others, the need to: (i) revise the MLRA so as to incorporate requirements for open and transparent management and governance; (ii) entrench the principles of co-management, emphasised in the SSFP, for all fisheries; and (iii) include a legal requirement for detailed management plans for all fisheries. Despite these shortcomings, there has been considerable progress in implementation of EAF, at least in the country's bigger fisheries. However, this should not be seen as a justification for avoiding or delaying a revision of the MLRA to bring it into line with modern best-practices as encompassed by EAF.  相似文献   
77.
This study describes the hydrogeochemistry and distributions of As in groundwater from a newly investigated area of Burkina Faso. Groundwaters have been sampled from hand-pumped boreholes and dug wells close to the town of Ouahigouya in northern Burkina Faso. Although most analysed groundwaters have As concentrations of less than 10 μg L−1, they have a large range from <0.5 to 1630 μg L−1. The highest concentrations are found in borehole waters; all dug wells analysed in this study have As concentrations of <10 μg L−1. Skin disorders (melanosis, keratosis and more rare skin tumour) have been identified among the populations in three villages in northern Burkina Faso, two within the study area. Although detailed epidemiological studies have not been carried out, similarities with documented symptoms in other parts of the world suggest that these are likely to be linked to high concentrations of As in drinking water. The high-As groundwaters observed derive from zones of Au mineralisation in Birimian (Lower Proterozoic) volcano-sedimentary rocks, the Au occurring in vein structures along with quartz and altered sulphide minerals (pyrite, chalcopyrite, arsenopyrite). However, the spatial variability in As concentrations in the mineralised zones is large and the degree of testing both laterally and with depth so far is limited. Hence, concentrations are difficult to predict on a local scale. From available data, the groundwater appears to be mainly oxic and the dissolved As occurs almost entirely as As(V) although concentrations are highest in groundwaters with dissolved-O2 concentrations <2 mg L−1. The source is likely to be the oxidised sulphide minerals and secondary Fe oxides in the mineralised zones. Positive correlations are observed between dissolved As and both Mo and W which are also believed to be derived from ore minerals and oxides in the mineralised zones. The discovery of high As concentrations in some groundwaters from the Birimian rocks of northern Burkina Faso reiterates the need for reconnaissance surveys in mineralised areas of crystalline basement.  相似文献   
78.
This study evaluates interannual variations and trends in growing season daily temperature sum and daily precipitation sum in Finland during 1961–2011, and their connections to well known atmospheric circulation patterns. Changes in summer (June–August) climate partially explain changes in growing season daily temperature sum and daily precipitation sum over Finland, which naturally decreased from south to north. On a national scale, growing season warmed and became wetter during 1961–2011, as growing season daily temperature sum and daily precipitation sum significantly (p < 0.05) increased by 5.01 ± 3.17°C year–1 and 1.39 ± 0.91 mm year–1, respectively. The East Atlantic pattern was the most influential atmospheric circulation pattern for variations in growing season daily temperature sum (rho = 0.40) across Finland and the East Atlantic/West Russia pattern was most influential for growing season daily precipitation sum variability (rho = –0.54). There were significant (p < 0.05) increasing trends in growing season daily temperature sum and daily precipitation sum throughout Finland during 1961–2011. Increased growing season daily temperature sum was mainly observed in northern, central, western, eastern and coastal areas of south‐western Finland. This warming was positively associated with the East Atlantic pattern in the north, centre and south, but negatively associated with the East Atlantic/West Russia pattern in eastern Finland. Increased GSP mostly occurred in southern, eastern, western, central, northern and north‐western Finland. These wetting trends were positively correlated with the East Atlantic pattern in the north and negatively correlated with the Polar pattern in the south and the East Atlantic/West Russia pattern in the east, west, centre and north‐east of Finland. The overall agroclimatic year‐to‐year variability in Finland between 1961 and 2011 was mostly linked to variations in the East Atlantic and East Atlantic/West Russia patterns.  相似文献   
79.
Geografisk Tidsskrift, Danish Journal of Geography 107(2):29–44, 2007

The literature on livelihoods in rural Africa has for the past decade documented and discussed the growing importance of non-farm income for rural livelihoods and often linked this to an increasing divorce of rural livelihoods from farming and land. This paper challenges the view that the increasing importance of income earned on non-farm activities is a direct expression of processes of deagrari-anisation. Drawing on empirical evidence from recent research in the Ghanaian cocoa frontier, the paper shows that a growing importance of income from non-farm activities is evident, but that incomefrom cocoa is still the determining factor for most households' income and thereby for the demand for non-farm goods and investment in the non-farm sector. By identifying two overall factors explaining the level of income diversification, the paper questions how generally applicable processes of deagrarianisation are in the Ghanaian case. Firstly, the actual crop cultivated and its price and marketing possibilities are of great importance for a farmer's choice to engage and stay in the agricultural sector or to diversify into non-farm activities. Secondly, different dynamics are evident between settlements dominated by either migrant or indigenous households, mainly due to unequal access to land. The paper argues that what on a general scale may look like processes of deagrarianisation during the last decade to a large extent can be explained by migrants without access to land who engage full-time in the nonfarm sector and by cocoa farmers diversifying their incomes, but not leaving a deteriorated agriculture.  相似文献   
80.
An Empirical Orthogonal Function (EOF) variance analysis was performed to map in detail the spatiotemporal variability in individual stake mass-balances (ba) on Mittivakkat Gletscher (MG) – in a region where at present five out of ~20.000 glaciers have mass-balance observations. The EOF analysis suggested that observed ba was summarized by two modes: EOF1 and EOF2 represented 80% (significant) and 6% (insignificant) of the explained variance, respectively. EOF1 captured a decline in ba that was uniformly distributed in space at all stakes. The decline was correlated with albedo observations and air temperature observations from nearby stations. EOF2, however, described variations in ba that were heterogeneously distributed among stakes and associated with local slope and aspect. Low elevation stakes (~<400 m a.s.l.) showed relatively negative (out of phase) correlation and higher elevated stakes relatively positive (in phase) eigenvector correlation values with EOF2. Such relatively negative and positive eigenvector correlation values were present where the constituted of exposed glacier ice or snow cover, respectively. The results from this study show how EOF analyses can provide information on spatiotemporal patterns of glacier mass-balance. Understanding such detailed variabilities in mass-balance on a Greenlandic glacier is of interest because a fifth of the Arctic contribution from glaciers and ice caps to sea-level rise originates from Greenland.  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