全文获取类型
收费全文 | 2091篇 |
免费 | 287篇 |
国内免费 | 353篇 |
专业分类
测绘学 | 553篇 |
大气科学 | 311篇 |
地球物理 | 203篇 |
地质学 | 605篇 |
海洋学 | 97篇 |
天文学 | 9篇 |
综合类 | 308篇 |
自然地理 | 645篇 |
出版年
2023年 | 19篇 |
2022年 | 49篇 |
2021年 | 38篇 |
2020年 | 84篇 |
2019年 | 64篇 |
2018年 | 41篇 |
2017年 | 107篇 |
2016年 | 99篇 |
2015年 | 130篇 |
2014年 | 183篇 |
2013年 | 168篇 |
2012年 | 129篇 |
2011年 | 167篇 |
2010年 | 120篇 |
2009年 | 173篇 |
2008年 | 162篇 |
2007年 | 140篇 |
2006年 | 134篇 |
2005年 | 94篇 |
2004年 | 63篇 |
2003年 | 78篇 |
2002年 | 81篇 |
2001年 | 62篇 |
2000年 | 43篇 |
1999年 | 65篇 |
1998年 | 42篇 |
1997年 | 31篇 |
1996年 | 22篇 |
1995年 | 19篇 |
1994年 | 18篇 |
1993年 | 18篇 |
1992年 | 10篇 |
1991年 | 9篇 |
1990年 | 8篇 |
1989年 | 12篇 |
1988年 | 9篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 11篇 |
1984年 | 7篇 |
1983年 | 8篇 |
1982年 | 3篇 |
1954年 | 2篇 |
排序方式: 共有2731条查询结果,搜索用时 218 毫秒
991.
Monitoring 25 years of land cover change dynamics in Africa: A sample based remote sensing approach 总被引:3,自引:1,他引:2
The study examines the changes in sub-Saharan's natural land cover resources for a 25 year period. We assess these changes in four broad land cover classes – forests, natural non-forest vegetation, agriculture and barren – by using high spatial resolution Earth observing satellites. Two sets of sample images, one ‘historical’ targeted at 1975 and a second ‘recent’ targeted at the year 2000, have been selected through a stratified random sampling technique over the study area, targeting a sampling rate of 1% in each of the strata. The results, presented at eco-region level and aggregated at sub-Saharan level, show a 57% increase in agriculture area at the expense of natural vegetation which has itself decreased by 21% over the period, with nearly 5 million hectares forest and non-forest natural vegetation lost per year. The impacts of these changes on the environment on one site and on the socio-economy on the other site are discussed and possible pressures on human well being are highlighted. 相似文献
992.
This study was conducted to collect historical land use information that would help explain the historical patterns in accumulation
of sediment and phosphorus in Lake Pepin documented by Engstrom et al. (J Paleolimnol, this issue). A wide range of historical factors including cropping systems, phosphorus applications from fertilizer and manure, human
and animal populations, river flows and phosphorus discharges from waste water treatment plants were studied using statistical
methods. Results showed that sediment losses from the Minnesota River basin are significantly correlated with historical increases
in river flows, row crop production acreage and basin population. Total phosphorus accumulations in the sediments of Lake
Pepin are significantly correlated with increased phosphorus discharges from metropolitan area wastewater treatment plants,
and increases in row crop acreage and river flows. Total phosphorus inflows to Lake Pepin are significantly correlated with
increases in river flows, row crop acreage and phosphorus fertilizer applied to agricultural lands.
This is one of eight papers dedicated to the “Recent Environmental History of the Upper Mississippi River” published in this
special issue of the Journal of Paleolimnology. D. R. Engstrom served as guest editor of the special issue. 相似文献
993.
994.
Application of Superabsorbent Polymers for Improving the Ecological Chemistry of Degraded or Polluted Lands 总被引:3,自引:0,他引:3
About 3.5 billion ha of land, which amounts to almost 30% of the total solid land of the world, has been degraded by human activities. The ecological restoration of these lands is a major challenge for mankind since they are the only option left for increasing the amount of arable land and producing food for the ever growing worldwide population. One common feature of these degraded lands is the fact that their organic soil matter is degraded also. Rainfall therefore, changes from a blessing to a menace since it is not kept in the soil and therefore causes erosion. A solution for the restoration of these lands could be the application of superabsorbent polymers (SAPs) to these soils. These substances are like ‘artificial humus’ as they are hydrophilic and contain carboxylic groups. This enables them to bind cations and water. They have the following advantages for the restoration of degraded lands. They increase the plant available water in the soil which enables the plants to survive longer under water stress. SAP amendment to soils reduces the evapotranspiration rate of the plants. They induce a significantly higher growth rate in plants growing on SAP amended soil. They bind heavy metals and mitigate their action on plants. They mitigate the effects of salinity. The benefits of SAP amendment to soils substantially outweigh their costs. 相似文献
995.
M. Liberti T. Simoniello M.T. Carone R. Coppola M. D'Emilio M. Macchiato 《Geomorphology》2009,106(3-4):333-343
Soil erosion is the most influential component of land degradation for its strong impacts on both natural and agricultural environments. In order to support effective intervention and recovery policies for eroded areas, monitoring techniques should take into account the space–time variability of the processes involved, and make use of assessed mapping methodologies as baseline criteria for studying the dynamics of landform development. When using multispectral data for mapping eroded areas, low spectral separability is a significant limit in areas with complex features, where soil materials are frequently remixed by surface runoff. Since multispectral satellite images are a valuable data source for multi-temporal analyses of erosion processes at the medium scale, we assessed how accurately a badland area can be identified from LANDSAT TM and ETM data. A protocol for an optimal mapping was built up by testing the performance of different supervised algorithms and input layers (spectral and morphological). Tests were carried out in a well-known badland area of Basilicata, Italy, with an extension of 8000 ha. Results obtained from the use of spectral bands (with and without thermal channel) and principal components returned an overall accuracy ranging from 53% (for classification on first three components) to 72% (for classification on all bands from TM), with low values for the kappa coefficient (0.30–0.50), showing that the spectral information alone are insufficient to accurately identify badland areas. In order to improve mapping, we found that the integration of slopes and aspects derived from a Digital Elevation Model (DEM) can overcome problems inherent to the low separability of spectral signatures. The use of morphological data was tested for different classification algorithms and integration approaches. In discriminating badlands, the better performing algorithm was MLC (Maximum Likelihood Classifier) and the best results were obtained by integrating all seven bands (including TIR) with slope and aspect maps as input within the classifier (A > 0.85 and K 0.75 for both the sensors). We selected such parameters because they play an important role in characterising badlands of study area from a morphological perspective but the proposed approach is also conceptually simple and can be easily exported to other areas. The obtained results support the hypothesis that the combined use of remote sensing imagery and auxiliary morphological data significantly improves the mapping of badlands over large areas with heterogeneous features, thus providing a useful methodology for long-term studies on soil erosion processes. 相似文献
996.
广东省城市规模Zipf法则检验及其影响因素分析 总被引:1,自引:0,他引:1
利用广东省1990年~2007年各城市市区非农业人口数据计算广东省城市规模分布的Zipf指数,得出广东省城市规模分布由分散分布向均衡的位序——规模分布过渡的结论。同时,选取5个解释变量,建立多元回归模型对影响广东省城市规模分布的zipf指数变化的因素进行分析。研究发现,1990年以来,5个解释变量对广东省城市规模分布的影响作用有所变化。1990年~2000年,促使城市规模分布趋于集中的力量强于分散力量。进入2000年以后,集中力量逐渐减弱,与分散力量趋于平衡。 相似文献
997.
本文在MSPAS(Modified Soil-Plant-Atmosphere Scheme)的基础上,引入了一个有效的晴天大气辐射传输方案,建立了一个能在物理上真实地模拟陆气相互作用及其反馈机制的二维模式MLAIM(Modified Land Atmosphere Interaction Model).本文利用HEIFE实验的观测资料对MLAIM的模拟结果进行了检验,对其中不合理的部分进行了分析,指出了在干旱半干旱区陆面过程参数修正的必要性,对干旱半干旱区土壤水分传输以及大气近地面层湍流输送的参数化方案进行了改进.改进后的模式能够较好地模拟夏季连续晴天条件下沙漠的地表能量收支,因此,本文利用MLAIM研究了绿洲对其周围沙漠地表能量收支的影响,并对地表能量收支各分量之间的相互作用进行了分析.结果表明,绿洲向其下风向沙漠的水汽输送是导致其上下风向沙漠间地表能量收支差异的最重要的因子. 相似文献
998.
利用GPS技术监测城市地面沉降,监测基准的合理选取对获得真实的形变结果至关重要.本文详细分析了城市沉降监测网基准的特点,针对传统基准模型的局限性提出了顾及板块运动、基准点稳定性以及其他系统误差影响的拟稳基准模型.该模型考虑了基准点自身的稳定性以及板块运动对高程形变的影响,同时采用系统参数统一各期基线框架以及区域拟稳基准,有效解决了因基准点不稳定性和基线框架不一致对形变结果造成的误差,确保正确形变信息的获取.最后通过六期西安市地面沉降监测网数据试验,验证了该方案的合理性和可靠性. 相似文献
999.
1000.
Non-stationarity of climate drivers and soil-use strongly affects the hydrologic cycle, producing significant inter-annual and multi-decadal fluctuations of river flow regimes. Understanding the temporal trajectories of hydrologic regimes is a key issue for the management of freshwater ecosystems and the security of human water uses. Here, long-term changes in the seasonal flow regime of the Little Piney creek (US) are analyzed with the aid of a stochastic mechanistic approach that expresses analytically the streamflow distribution in terms of a few measurable hydroclimatic parameters, providing a basis for assessing the impact of climate and landscape modifications on water resources. Mean rainfall and streamflow rates exhibit a pronounced inter-annual variability across the last century, though in the absence of clear sustained drifts. Long-term modifications of streamflow regimes across different periods of 2 and 8 years are likewise significant. The stochastic model is able to reasonably reproduce the observed 2-years and 8-years regimes in the Little Piney creek, as well as the corresponding inter-annual variations of streamflow probability density. The study evidences that a flow regime shift occurred in the Little Piney creek during the last century, with erratic regimes typical of the 30s/40s that had been progressively replaced by persistent flow regimes featured by more dumped streamflow fluctuations. Causal drivers of regime shift are identified as the increase of the frequency of events (a byproduct of climate variability) and the decrease of recession rates (induced by a decrease of cultivated lands). The approach developed offers an objective basis for the analysis and prediction of the impact of climate/landscape change on water resources. 相似文献