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   430篇
  免费   46篇
  国内免费   34篇
测绘学   8篇
大气科学   22篇
地球物理   118篇
地质学   270篇
海洋学   18篇
天文学   23篇
综合类   28篇
自然地理   23篇
  2023年   8篇
  2022年   19篇
  2021年   45篇
  2020年   33篇
  2019年   25篇
  2018年   80篇
  2017年   43篇
  2016年   60篇
  2015年   37篇
  2014年   30篇
  2013年   41篇
  2012年   14篇
  2011年   20篇
  2010年   8篇
  2009年   12篇
  2008年   8篇
  2007年   2篇
  2006年   3篇
  2005年   3篇
  2004年   2篇
  2003年   2篇
  2002年   2篇
  2001年   3篇
  2000年   2篇
  1999年   1篇
  1998年   2篇
  1996年   2篇
  1995年   2篇
  1986年   1篇
排序方式: 共有510条查询结果,搜索用时 15 毫秒
141.
The Mutis-Timau Forest Complex,one of the remaining mountainous tropical forest areas in Timor Island,eastern Indonesia that covers an area of 31,984 ha,tends to decrease gradually.Efforts to secure mountain forest functions and counteract the negative impact of declining forest areas are often constrained by data uncertainty on factors contributing to deforestation.For this reason,this study attempts to develop models of deforestation and predict future deforestation in the Mutis-Timau Forest Complex.We constructed models of deforestation that describe the relationship between deforestation and factors contributing to deforestation using spatial statistical models.In this model,we used the deforestation data for the 1987-2017 period obtained from a previous study as dependent variables and the potential causes of deforestation generated from Geographic Information System spatial analysis as independent variables.Using the probability of deforestation derived from the model,we predicted future deforestation under two different scenarios,namely,business-as-usual(as the reference scenario)and reducing emission fromdeforestation and forest degradation.Our findings showed that a positive relationship exists between probability of deforestation,distance to the settlement,and population density variables,whereas a negative relationship exists between likelihood of deforestation,elevation,slope,distance to the road,distance to the savanna,and forest management unit variables.During the 2017-2030 period,under the business-as-usual scenario,the Mutis-Timau Forest Complex will lose 1327.65 ha in forest area with an annual deforestation rate of 0.54%.Meanwhile,under the reducing emission from deforestation and forest degradation scenario,the overall forest loss was estimated to be 1237.11 ha with an annual deforestation rate of 0.50%.The predicted area of avoided deforestation in 2017-2030 under the reducing emission from deforestation and forest degradation scenario was 90.54 ha.Such data and information are important for the Mutis-Timau Forest Complex authority in prioritizing actions for combating deforestation and designing appropriate forest-related policies and supporting data for reducing emission from deforestation and forest degradation programme or other incentive schemes in reducing deforestation.  相似文献   
142.
This paper presents an assessment of the Soil and Water Assessment Tool(SWAT) on a glaciated(Qugaqie) and a non-glaciated(Niyaqu) subbasin of the Nam Co Lake. The Nam Co Lake is located in the southern Tibetan Plateau, two subbasins having catchment areas of 59 km~2 and 388 km~2, respectively. The scores of examined evaluation indices(i.e., R~2, NSE, and PBIAS) established that the performance of the SWAT model was better on the monthly scale compared to the daily scale. The respective monthly values of R~2, NSE, and PBIAS were 0.94, 0.97, and 0.50 for the calibration period while 0.92, 0.88, and -8.80 for the validation period. Glacier melt contribution in the study domain was simulated by using the SWAT model in conjunction with the Degree Day Melt(DDM) approach. The conjunction of DDM with the SWAT Model ensued improved results during both calibration(R~2=0.96, NSE=0.95, and PBIAS=-13.49) and validation (R~2=0.97, NSE=0.96, and PBIAS=-2.87) periods on the monthly time scale. Average contribution(in percentage) of water balance components to the total streamflow of Niyaqu and Qugaqie subbasins was evaluated. We found that the major portion(99.45%) of the streamflow in the Niyaqu subbasin was generated by snowmelt or rainfall surface runoff(SURF_Q), followed by groundwater(GW_Q, 0.47%), and lateral(LAT_Q, 0.06%) flows. Conversely, in the Qugaqie subbasin, major contributor to the streamflow(79.63%) was glacier melt(GLC_Q), followed by SURF_Q(20.14%), GW_Q(0.13%), and LAT_Q(0.089%). The contribution of GLC_Q was the highest(86.79%) in July and lowest(69.95%) in September. This study concludes that the performance of the SWAT model in glaciated catchment is weak without considering glacier component in modeling; however, it performs reasonably well in non-glaciated catchment. Furthermore, the temperature index approach with elevation bands is viable in those catchments where streamflows are driven by snowmelt. Therefore, it is recommended to use the SWAT Model in conjunction with DDM or energy base model to simulate the glacier melt contribution to the total streamflow. This study might be helpful in quantification and better management of water resources in data scarce glaciated regions.  相似文献   
143.
A total of 160 barramundi's(Lates calcarifer Bloch,1790)sampled from four rivers(Tentulia,Balaswar,Bakkhali,and Andarmanik)along the southern coastal region of Bangladesh were investigated in terms of morphometric characters to reveal the intraspecific variation.Twenty-five morphometric measurements were extracted using the conventional method and subjected to multivariate analyses(i.e.,principal component analysis(PCA),discriminate function analysis(DFA),cluster analysis(CA))to distinguish individuals from different rivers.The result demonstrated that twenty-two out of 2 5 measurements was statistically significant(Univariate ANOVA)among all four populations.PCA analy sis of morphometric characters resulted in two principal components,PC I and PCII which accounted for 79.25%and 4.28%of the total data variance.PC I-PC Ⅱ plot explained 83.5 3%of total variance differentiated the population of L.calcarifer into two groups.Discriminate analysis correctly classified about 88.1%of the examined fish into the four areas.The UPGMA dendrogram showed that Bakkhali populations were the most morphologically different populations in comparison to other populations,while Andarmanik and Balaswar populations were very close to each other.The strong morphometric variation between Bakkhali and Tentulia,Andarmanik and Balaswar was observed in the present study,suggested the evidence of the separate stock population of barramundi in these locations,which might require distinct stock management strategies for resource sustainability in the waters of southern Bangladesh.However,if these findings are supported by further molecular markers and geometric morphometry,this would be a strong indication of different stocks of this population in the four rivers of southern Bangladesh.  相似文献   
144.
Landsat-5 Thematic Mapper (TM) dataset have been used to estimate salinity in the coastal area of Hong Kong. Four adjacent Landsat TM images were used in this study, which was atmospherically corrected using the Second Simulation of the Satellite Signal in the Solar Spectrum (6S) radiative transfer code. The atmospherically corrected images were further used to develop models for salinity using Ordinary Least Square (OLS) regression and Geographically Weighted Regression (GWR) based on in situ data of October 2009. Results show that the coefficient of determination (R2) of 0.42 between the OLS estimated and in situ measured salinity is much lower than that of the GWR model, which is two times higher (R2 = 0.86). It indicates that the GWR model has more ability than the OLS regression model to predict salinity and show its spatial heterogeneity better. It was observed that the salinity was high in Deep Bay (north-western part of Hong Kong) which might be due to the industrial waste disposal, whereas the salinity was estimated to be constant (32 practical salinity units) towards the open sea.  相似文献   
145.
Belt and Road Initiative (BRI) is a Chinese national strategy which calls for cooperative economic, political and cultural exchange at the global level along the ancient Silk Road. The overwhelming natural hazards located along the belt and road bring great challenges to the success of BRI. In this framework, a 5-year international program was launched to address issues related to hazards assessment and disaster risk reduction (DRR). The first workshop of this program was held in Beijing with international experts from over 15 countries. Risk conditions on Belt and Road Countries (BRCs) have been shared and science and technology advancements on DRR have been disseminated during the workshop. Under this program, six task forces have been setup to carry out collaborative research works and three prioritized study areas have been established. This workshop announced the launching of this program which involved partners from different countries including Pakistan, Nepal, Russia, Italy, United Kingdom, Sri Lanka and Tajikistan. The program adopted the objectives of Sendai Framework for Disaster Risk Reduction 2015–2030 and United Nation Sustainable Development Goals 2030 and was implemented to assess disaster risk in BRCs and to propose suitable measures for disaster control which can be appropriate both for an individual country and for specific sites. This paper deals with the outcomes of the workshop and points out opportunities for the near future international cooperation on this matter.  相似文献   
146.
The Porcupine Basin is a Mesozoic failed rift located in the North Atlantic margin, SW of Ireland, in which a postrift phase of extensional faulting and reactivation of synrift faults occurred during the Mid–Late Eocene. Fault zones are known to act as either conduits or barriers for fluid flow and to contribute to overpressure. Yet, little is known about the distribution of fluids and their relation to the tectono‐stratigraphic architecture of the Porcupine Basin. One way to tackle this aspect is by assessing seismic (Vp) and petrophysical (e.g., porosity) properties of the basin stratigraphy. Here, we use for the first time in the Porcupine Basin 10‐km‐long‐streamer data to perform traveltime tomography of first arrivals and retrieve the 2D Vp structure of the postrift sequence along a ~130‐km‐long EW profile across the northern Porcupine Basin. A new Vp–density relationship is derived from the exploration wells tied to the seismic line to estimate density and bulk porosity of the Cenozoic postrift sequence from the tomographic result. The Vp model covers the shallowest 4 km of the basin and reveals a steeper vertical velocity gradient in the centre of the basin than in the flanks. This variation together with a relatively thick Neogene and Quaternary sediment accumulation in the centre of the basin suggests higher overburden pressure and compaction compared to the margins, implying fluid flow towards the edges of the basin driven by differential compaction. The Vp model also reveals two prominent subvertical low‐velocity bodies on the western margin of the basin. The tomographic model in combination with the time‐migrated seismic section shows that whereas the first anomaly spatially coincides with the western basin‐bounding fault, the second body occurs within the hangingwall of the fault, where no major faulting is observed. Porosity estimates suggest that this latter anomaly indicates pore overpressure of sandier Early–Mid Eocene units. Lithological well control together with fault displacement analysis suggests that the western basin‐bounding fault can act as a hydraulic barrier for fluids migrating from the centre of the basin towards its flanks, favouring fluid compartmentalization and overpressure of sandier units of its hangingwall.  相似文献   
147.
海草床是海岸带最富生产力的生态系统之一,支撑着各种各样的伴生生物。热带的印度和太平洋地区被认为拥有海草植物种类多样性最高,且分布面积最广,然而,这个区域的海草床大型底栖生物我们知之甚少。为了填补认知的空白,我们在该区域开展了一项生态调查,旨在描述该区热带海草床大型底栖生物的丰度和多样性,以及确定大型底栖生物丰度、物种丰富度和群落结构是否明显存在断面内的站间变化和样地间变化。2014年5月和2015年10月我们分别在北苏拉威西省东海岸和西海岸开展野外工作,使用柱状取样器采集海草床大型底栖生物样品。所得样品共计鉴定14大类149种底栖生物,种类最为丰富的类别为多毛类(56种,占26%的总个体数),十足类(20种,占9%的总个体数)和端足类(18种,占35%的总个体数)。东、西海岸海草床大型底栖生物表现出不同的空间分布模式。在东海岸,同一断面的大型底栖生物和端足类的丰度存在显著的站间差异;而在西海岸,大型底栖生物和多毛类的种类丰富度和丰度都表现出明显的站间变化,这可能归结于同一断面底质不均所造成。单因素ANOSIM以及MDS排序表明了北苏拉威西省东海岸和西海岸海草床大型底栖生物群落结构存在显著不同,正好对应于将海草床分成两大类型的栖息地,即西海岸的红树林-海草床-珊瑚连续体和东海岸的海草床-珊瑚连续体。与在热带海区开展的其他研究相比,本研究的大型底栖生物丰度和多样性处于中等水平。东、西海岸海草床大型底栖生物群落存在显著区别,其原因可能源于多方面,包括了沉积物模式,海草床结构和时间变化。  相似文献   
148.
Natural Resources Research - This study aims to evaluate the use of Tigris River sediments as abrasives for polishing marble surfaces to achieve a usable form as floor tiles, facing stones and...  相似文献   
149.
Muhammad  Ahmad  Külahcı  Fatih  Akram  Pishtiwan 《Natural Hazards》2020,104(1):979-996
Natural Hazards - Time series studies depend mostly on stochastic models for radon seasonal, annual or temporal variability explanations. Others solve radon transport steady state equation...  相似文献   
150.
Semi-arid and dry sub-humid areas (especially in the tropics) are characterized by high inter-annual and intra-seasonal rainfall variability. Agriculture, which employs the bulk of the rapidly increasing populations, is largely rain-fed, low-input based and highly resource dependent. Recent spates of drought have, therefore, exacerbated the now-too-familiar specter of famine and starvation in these areas with glaring examples being the recurring episodes in sub-Saharan Africa since the great Sahel drought of 1969–1973. A great need for accurate and timely hazard forecast products in aid of agriculture thus exists.Several schemes are currently employed by various agencies around the globe in this direction. There does remain, however, a gap between product provision and user expectations. This paper examines this gap suggesting a five-point framework within which it can be addressed as an action agenda for the climate science community. The paper posits that changes are possible to existing methodologies (related to three of these points), which, within the context of current science, can greatly enhance the utility of forecast products for agriculture in marginal areas. The remaining two points have, however, been identified as requiring additional applied research and necessary pointers for addressing these issues are provided. First is the need for appropriate impact-related indicators for intra-seasonal and interannual rainfall variability that are easy to compute, amenable to forecasting and follow closely the experiences of farmers in marginal areas. The second is a consideration of appropriate forecast information formatting and communication medium that guarantee effective feedback between forecast producers and users. Specific examples of the status quo and of work currently underway are cited from southern Africa – a region currently attracting international attention as a result of recent droughts and the threat of famine.  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