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   4135篇
  免费   919篇
  国内免费   745篇
测绘学   163篇
大气科学   1110篇
地球物理   1658篇
地质学   1150篇
海洋学   842篇
天文学   28篇
综合类   146篇
自然地理   702篇
  2024年   2篇
  2023年   55篇
  2022年   127篇
  2021年   175篇
  2020年   197篇
  2019年   188篇
  2018年   170篇
  2017年   181篇
  2016年   206篇
  2015年   201篇
  2014年   245篇
  2013年   350篇
  2012年   236篇
  2011年   253篇
  2010年   218篇
  2009年   312篇
  2008年   312篇
  2007年   328篇
  2006年   260篇
  2005年   216篇
  2004年   241篇
  2003年   194篇
  2002年   156篇
  2001年   139篇
  2000年   108篇
  1999年   110篇
  1998年   72篇
  1997年   87篇
  1996年   61篇
  1995年   57篇
  1994年   63篇
  1993年   46篇
  1992年   46篇
  1991年   27篇
  1990年   33篇
  1989年   21篇
  1988年   16篇
  1987年   10篇
  1986年   12篇
  1985年   12篇
  1984年   14篇
  1983年   14篇
  1982年   6篇
  1981年   8篇
  1980年   4篇
  1979年   2篇
  1978年   5篇
  1977年   1篇
  1973年   1篇
  1954年   1篇
排序方式: 共有5799条查询结果,搜索用时 15 毫秒
1.
The 20 km2 Galabre catchment belongs to the French network of critical zone observatories (OZCAR; Gaillardet et al., Vadose Zone Journal, 2018, 17(1), 1–24). It is representative of the sedimentary lithology and meteorological forcing found in Mediterranean and mountainous areas. Due to the presence of highly erodible and sloping badlands on various lithologies, the site was instrumented in 2007 to understand the dynamics of suspended sediments (SS) in such areas. Two meteorological stations including measurements of air temperature, wind speed and direction, air moisture, rainfall intensity, raindrop size and velocity distribution were installed both in the upper and lower part of the catchment. At the catchment outlet, a gauging station records the water level, temperature and turbidity (10 min time-step). Stream water samples are collected automatically to estimate SS concentration-turbidity relationships, allowing quantification of SS fluxes with known uncertainty. The sediment samples are further characterized by measuring their particle size distributions and by applying a low-cost sediment fingerprinting approach using spectrocolorimetric tracers. Thus, the contributions of badlands located on different lithologies to total SS flux are quantified at a high temporal resolution, providing the opportunity to better analyse the links between meteorological forcing variability and watershed hydrosedimentary response. The set of measurements was extended to the dissolved phase in 2017. Both stream water electrical conductivity and major ion concentrations are measured each week and every 3 h during storm events. This extension of measurements to the dissolved phase will allow progress in understanding both the origin of the water during the events and the partitioning between particulate and dissolved fluxes of solutes in the critical zone. All data sets are available at https://doi.osug.fr/public/DRAIXBLEONE_GAL/index.html .  相似文献   
2.
We introduce the freely available web-based Water in an Agricultural Landscape—NUčice Database (WALNUD) dataset that includes both hydrological and meteorological records at the Nučice experimental catchment (0.53 km2), which is representative of an intensively farmed landscape in the Czech Republic. The Nučice experimental catchment was established in 2011 for the observation of rainfall–runoff processes, soil erosion processes, and water balance of a cultivated landscape. The average altitude is 401 m a.s.l., the mean land slope is 3.9%, and the climate is humid continental (mean annual temperature 7.9°C, annual precipitation 630 mm). The catchment is drained by an artificially straightened stream and consists of three fields covering over 95% of the area which are managed by two different farmers. The typical crops are winter wheat, rapeseed, and alfalfa. The installed equipment includes a standard meteorological station, several rain gauges distributed across the basin, and a flume with an H-type facing that is used to monitor stream discharge, water turbidity, and basic water quality indicators. Additionally, the groundwater level and soil water content at various depths near the stream are recorded. Recently, large-scale soil moisture monitoring efforts have been introduced with the installation of two cosmic-ray neutron sensors for soil moisture monitoring. The datasets consist of observed variables (e.g. measured precipitation, air temperature, stream discharge, and soil moisture) and are available online for public use. The cross-seasonal, open access datasets at this small-scale agricultural catchment will benefit not only hydrologists but also local farmers.  相似文献   
3.
The drought of summer 2018, which affected much of Northern Europe, resulted in low river flows, biodiversity loss and threats to water supplies. In some regions, like the Scottish Highlands, the summer drought followed two consecutive, anomalously dry, winter periods. Here, we examine how the drought, and its antecedent conditions, affected soil moisture, groundwater storage, and low flows in the Bruntland Burn; a sub-catchment of the Girnock Burn long-term observatory in the Scottish Cairngorm Mountains. Fifty years of rainfall-runoff observations and long-term modelling studies in the Girnock provided unique contextualisation of this extreme event in relation to more usual summer storage dynamics. Whilst summer precipitation in 2018 was only 63% of the long-term mean, soil moisture storage across much of the catchment were less than half of their summer average and seasonal groundwater levels were 0.5 m lower than normal. Hydrometric and isotopic observations showed that ~100 mm of river flows during the summer (May-Sept) were sustained almost entirely by groundwater drainage, representing ~30% of evapotranspiration that occurred over the same period. A key reason that the summer drought was so severe was because the preceding two winters were also dry and failed to adequately replenish catchment soil moisture and groundwater stores. As a result, the drought had the biggest catchment storage deficits for over a decade, and likely since 1975–1976. Despite this, recovery was rapid in autumn/winter 2018, with soil and groundwater stores returning to normal winter values, along with stream flows. The study emphasizes how long-term data from experimental sites are key to understanding the non-linear flux-storage interactions in catchments and the “memory effects” that govern the evolution of, and recovery from, droughts. This is invaluable both in terms of (a) giving insights into hydrological behaviours that will become more common water resource management problems in the future under climate change and (b) providing extreme data to challenge hydrological models.  相似文献   
4.
Dissolved pollutants in stormwater are a main contributor to water pollution in urban environments. However, many existing transport models are semi-empirical and only consider one-dimensional flows, which limit their predictive capacity. Combining the shallow water and the advection–diffusion equations, a two-dimensional physically based model is developed for dissolved pollutant transport by adopting the concept of a ‘control layer’. A series of laboratory experiments has been conducted to validate the proposed model, taking into account the effects of buildings and intermittent rainfalls. The predictions are found to be in good agreement with experimental observations, which supports the assumption that the depth of the control layer is constant. Based on the validated model, a parametric study is conducted, focusing on the characteristics of the pollutant distribution and transport rate over the depth. The hyetograph, including the intensity, duration and intermittency, of rainfall event has a significant influence on the pollutant transport rates. The depth of the control layer, rainfall intensity, surface roughness and area length are dominant factors that affect the dissolved pollutant transport. Finally, several perspectives of the new pollutant transport model are discussed. This study contributes to an in-depth understanding of the dissolved pollutant transport processes on impermeable surfaces and urban stormwater management.  相似文献   
5.
西北太平洋副热带高压(以下简称副高)是影响中国气候的大尺度环流系统,为了进一步了解副高对中国气候的影响,本文利用站点观测资料和大气环流再分析资料,通过资料诊断分析和数值模拟方法,探讨了6月副高东西变动对中国南部降水的影响,以及影响副高东西变动的前期海洋因子.结果表明副高东西变动对中国西南和华南地区降水的影响明显不同:副高偏东有利于降水西南偏多而华南偏少,偏西则降水变化刚好相反.其原因与副高东西变化引起的环流差异有关,华南降水与副高东(西)变动时西太平洋地区副高西北侧的东北(西南)风异常以及东亚中低纬度地区异常经向波列的变化直接有关,而西南降水异常不仅与副高东西变动在东南亚地区引起的纬向风异常有关,与青藏高原大地形动力作用对副高北侧异常纬向风的变化也有十分密切的联系.此外,副高东西变动时影响西南和华南地区的水汽来源不同,影响西南的水汽主要来源于赤道印度洋80°E附近越赤道气流,而影响华南的水汽主要来源于副高南侧偏东气流从西北太平洋地区输送的水汽.进一步分析发现前期冬春季热带西北太平洋和赤道西太平洋海温变化的偶极差异与后期初夏副高东西变动有密切联系,冬春季西北太平洋暖海温和赤道西太平洋冷海温变化有利于后期初夏副高偏西,相反则有利于副高偏东,数值模拟结果在一定程度证实了资料诊断分析结果.  相似文献   
6.
Within a wave-exposed mangrove forest, novel field observations are presented, comparing millimeter-scale turbulent water velocity fluctuations with contemporaneous subtidal bed elevation changes. High-resolution velocity and bed level measurements were collected from the unvegetated mudflat, at the mangrove forest fringe, and within the forest interior over multiple tidal cycles (flood–ebb) during a 2-week period. Measurements demonstrated that the spatial variability in vegetation density is a control on sediment transport at sub-meter scales. Scour around single and dense clusters of pneumatophores was predicted by a standard hydraulic engineering equation for wave-induced scour around regular cylinders, when the cylinder diameter in the equations was replaced with the representative diameter of the dense pneumatophore clusters. Waves were dissipated as they propagated into the forest, but dissipation at infragravity periods (> 30 s) was observed to be less than dissipation at shorter periods (< 30 s), consistent with the predictions of a simple model. Cross-wavelet analysis revealed that infragravity-frequency fluctuations in the bed level were occasionally coherent with velocity, possibly indicating scour upstream of dense pneumatophore patches when infragravity waves reinforced tidal currents. Consequently, infragravity waves were a likely driver of sediment transport within the mangrove forest. Near-bed turbulent kinetic energy, estimated from the turbulent dissipation rate, was also correlated with bed level changes. Specifically, within the mangrove forest and over the unvegetated mudflat, high-energy events were associated with erosion or near-zero bed level change, whereas low-energy events were associated with accretion. In contrast, no single relationship between bed level changes and mean current velocity was applicable across both vegetated and unvegetated regions. These observations support the theory that sediment mobilization scales with turbulent energy, rather than mean velocity, a distinction that becomes important when vegetation controls the development of turbulence.  相似文献   
7.
In this article we apply the CASCADE network-scale sediment connectivity model to the Vjosa River in Albania. The Vjosa is one of the last unimpaired braided rivers in Europe and, at the same time, a data scarce environment, which limits our ability to model how this pristine river might respond to future human disturbance. To initialize the model, we use remotely sensed data and modeled hydrology from a regional model. We perform a reach-by-reach optimization of surface grain size distribution (GSD) and bedload transport capacity to ensure equilibrium conditions throughout the network. In order to account for the various sources of uncertainty in the calculation of transport capacity, we performed a global sensitivity analysis. The modeled GSD distributions generated by the sensitivity analysis generally match the six GSDs measured at different locations within the network. The modeled bedload sediment fluxes increase systematically downstream, and annual fluxes at the outlet of the Vjosa are well within an order of magnitude of fluxes derived from previous estimates of the annual suspended sediment load. We then use the modeled sediment fluxes as input to a set of theoretically derived functions that successfully discriminate between multi-thread and single-thread channel patterns. This finding provides additional validation of the model results by showing a clear connection between modeled sediment concentrations and observed river morphology. Finally, we observe that a reduction in sediment flux of about 50% (e.g., due to dams) would likely cause existing braided reaches to shift toward single thread morphology. The proposed method is widely applicable and opens a new avenue for application of network-scale sediment models that aid in the exploration of river stability to changes in water and sediment fluxes.  相似文献   
8.
To enhance the utilization efficiency of farmland irrigation water and reduce the leakage of water conveyance channels, the leakage process of channels was simulated dynamically. The simulated results were compared with data measured in laboratory experiments, and the performance of the model was evaluated. The results indicated that the simulated values of the model were consistent with the observation values, and the R2 values varied between 0.91 and 0.99. In addition, based on the laboratory experiments, a water supply system (Mariotte bottles) and soil box were built using plexiglass. Three influencing factors, namely, the channel form, soil texture and channel cross-sectional area, were varied to observe and calculate the resulting cumulative infiltration amount, infiltration rate and wetting front migration distance. HYDRUS-3D software was used to solve the three-dimensional soil water movement equation under different initial conditions. The results demonstrated that the U-shaped channel was more effective than the trapezoidal channel in increasing the utilization efficiency of the water resources. A U-shaped channel with a small channel cross-sectional area should be adopted and the soil particle size should be prioritized in the construction of water conveyance channels for farmlands. The simulation results were in agreement with the observed results, which indicates that HYDRUS-3D is a reliable tool that can accurately simulate the soil moisture movement in water conveyance channels. The research results can provide a reference for the design and operation of farmland irrigation systems.  相似文献   
9.
Satellite-tracked Lagrangian drifters are used to investigate the transport pathways of near-surface water around the Luzon Strait. Particular attention is paid to the intrusion of Pacific water into the South China Sea(SCS).Results from drifter observations suggest that except for the Kuroshio water, other Pacific water that carried by zonal jets, Ekman currents or eddies, can also intrude into the SCS. Motivated by this origin problem of the intrusion water, numerous simulated trajectories are constructed by altimeter-based velocities. Quantitative estimates from simulated trajectories suggest that the contribution of other Pacific water to the total intrusion flux in the Luzon Strait is approximately 13% on average, much smaller than that of Kuroshio water. Even so, over multiple years and many individual intrusion events, the contribution from other Pacific water is quite considerable. The interannual signal in the intrusion flux of these Pacific water might be closely related to variations in a wintertime westward current and eddy activities east of the Luzon Strait. We also found that Ekman drift could significantly contribute to the intrusion of Pacific water and could affect the spreading of intrusion water in the SCS. A case study of an eddy-related intrusion is presented to show the detailed processes of the intrusion of Pacific water and the eddy-Kuroshio interaction.  相似文献   
10.
本文首先提出了河流泥沙输移过程中泥沙中的钙镁矿物溶蚀消耗水体中的CO2并具有碳汇功能的观点。基于前人长江干流从源头到入海口和支流2003~2007年期间4次河流悬移质泥沙的化学元素组成和矿物组成资料,分析悬移质中CaO、MgO含量和方解石、白云石含量变化特征,定量计算了这些取样点悬移质泥沙的CO2总碳汇能力和非永久性、永久性碳汇能力,分析了不同碳汇能力沿程变化规律及其原因。碳汇计算结果表明:寸滩—大通河段1956~2000年期间泥沙输移过程中钙镁矿物溶蚀产生的总碳汇量、非永久性和永久性碳汇量分别为2572万t/a、1700万t/a和872万t/a。由于输沙量减少,寸滩站—大通站河段的总碳汇量、非永久性和永久性碳汇量2006~2019年期间较1956~2000年期间相应分别减少了1852万t、1224万t和872万t。三峡水库年均淤积量1. 145亿t,损失总碳汇量675. 6万t,相当于三峡电站减排二氧化碳8580万t的7. 9%。全球河流入海年输沙量126. 1亿t,以寸滩- 吴淞口河段碳汇功能0. 060 t/t计,总碳汇量7. 57亿t相当于全球岩石风化碳汇总量10. 56亿t CO2的71. 6%。河流泥沙输移过程中钙镁矿物溶蚀的碳汇量具有重要的作用,其溶蚀速率大于原地风化。  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