全文获取类型
收费全文 | 10019篇 |
免费 | 1204篇 |
国内免费 | 1005篇 |
专业分类
测绘学 | 2055篇 |
大气科学 | 846篇 |
地球物理 | 1800篇 |
地质学 | 3372篇 |
海洋学 | 1759篇 |
天文学 | 20篇 |
综合类 | 893篇 |
自然地理 | 1483篇 |
出版年
2024年 | 47篇 |
2023年 | 110篇 |
2022年 | 227篇 |
2021年 | 338篇 |
2020年 | 391篇 |
2019年 | 400篇 |
2018年 | 309篇 |
2017年 | 483篇 |
2016年 | 459篇 |
2015年 | 479篇 |
2014年 | 650篇 |
2013年 | 717篇 |
2012年 | 619篇 |
2011年 | 671篇 |
2010年 | 560篇 |
2009年 | 588篇 |
2008年 | 666篇 |
2007年 | 686篇 |
2006年 | 625篇 |
2005年 | 485篇 |
2004年 | 421篇 |
2003年 | 330篇 |
2002年 | 344篇 |
2001年 | 291篇 |
2000年 | 223篇 |
1999年 | 221篇 |
1998年 | 172篇 |
1997年 | 142篇 |
1996年 | 106篇 |
1995年 | 97篇 |
1994年 | 85篇 |
1993年 | 75篇 |
1992年 | 50篇 |
1991年 | 29篇 |
1990年 | 31篇 |
1989年 | 24篇 |
1988年 | 23篇 |
1987年 | 14篇 |
1986年 | 7篇 |
1985年 | 10篇 |
1984年 | 12篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1977年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
1.
The ordinary kriging method, a geostatistical interpolation technique, was applied for developing contour maps of design storm depth in northern Taiwan using intensity–duration–frequency (IDF) data. Results of variogram modelling on design storm depths indicate that the design storms can be categorized into two distinct storm types: (i) storms of short duration and high spatial variation and (ii) storms of long duration and less spatial variation. For storms of the first category, the influence range of rainfall depth decreases when the recurrence interval increases, owing to the increasing degree of their spatial independence. However, for storms of the second category, the influence range of rainfall depth does not change significantly and has an average of approximately 72 km. For very extreme events, such as events of short duration and long recurrence interval, we do not recommend usage of the established design storm contours, because most of the interstation distances exceed the influence ranges. Our study concludes that the influence range of the design storm depth is dependent on the design duration and recurrence interval and is a key factor in developing design storm contours. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
2.
In 1903 the Swiss Federal Research Institute WSL started its first forest hydrology measurements with the aim to deliver a sound scientific basis for the implementation of new forest legislation introduced in Switzerland in 1876. This legislation was triggered by several large floods that occurred in Switzerland, for which a major cause was widely seen as the poor condition of forests at that time. Consequently, hydrologic research at WSL first focused on the influence of forests on floods. In the second half of the 20th century, other hydrological issues such as water quality, snow hydrology and sediment transport complemented the hydrologic research at WSL. Some recent results of this work are presented in three papers joining this introductory paper to mark the 100th anniversary of hydrologic research at WSL. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
3.
The supraregional GIS-supported stochastical model, WEKU, for the determination of groundwater residence times in the upper
aquifers of large groundwater provinces is presented. Using a two-dimensional analytical model of groundwater flow, groundwater
residence times are determined within two extreme cases. In the first case, maximal groundwater residence times are calculated,
representing the part of groundwater, that is drained by the main surface water of a groundwater catchment area. In the second
case, minimal groundwater residence times for drainage into the nearest surface water are determined. Using explicit distribution
functions of the input parameters, mean values as well as potential ranges of variations of the groundwater residence times
are derived. The WEKU model has been used for the determination of groundwater residence times throughout Germany. The model
results – mean values and deviations of the groundwater velocity and the maximal and minimal groundwater residence times in
the upper aquifers – are presented by general maps and discussed in detail. It is shown that the groundwater residence times
in the upper aquifer vary regionally, differentiated between less than 1 year and more than 2000 years. Using this information,
the time scales can be specified, until measures to remediate polluted groundwater resources may lead to a substantial groundwater
quality improvement in the different groundwater provinces of Germany. With respect to its supraregional scale of application,
the WEKU model may serve as a useful tool for the supraregional groundwater management on a state, federal or international
level.
Received: 15 August 1995 · Accepted: 15 October 1995 相似文献
4.
5.
6.
ZulkifliNasution MashhorMansor 《湿地科学》2004,2(2):104-109
Situated at an elevation of 905 m above sea level in the Province of North Sumatra, Lake Toba and its surrounding landscapes are regarded as a natural heritage in a certain extent, as a quoted national treasure. Unfortunately degradation of the land and water resources in the watershed along Lake Toba is taking place at an alarming and totally unacceptable rate. The quality of the lake is partly depended on input the quality of the rivers. When compared to the control area the water quality that influenced by the piggeries are highly polluted. It can be concluded that the Salbe River at the downstream of the piggeries has been polluted and apparently it is a serious problem to the catchment area management. It should be noted that the polluted river would influence the water quality of the Lake Toba. Based on calculation, the permissible BOD5 according to B- river standard is 238 mg/L, it means the river still in B standard but the condition and quality are decreasing continuously. Following the Indonesian health standard the permissible BOD is - 461 mg/L. It means BOD in the river should be reduced 461 mg/L or clean program is needed. 相似文献
7.
Heejun Chang 《The Professional geographer》2004,56(2):240-257
This study investigates potential changes in nitrogen and phosphorus loads under a warmer and wetter climate, urban growth, and combined changes in the Conestoga River Basin and its five subbasins in southeastern Pennsylvania. A GIS‐based hydrochemical model was employed for assessing the sensitivity of the basins to the projected changes in 2030. Under the HadCM2 climate change scenario, mean annual nitrogen and phosphorus loads are expected to increase, with great increases in spring but slight decreases in fall primarily because of changes in monthly precipitation. When climate change and urbanization occur concurrently, mean annual nitrogen loads further increase by 50% in the most urbanizing subbasin. Point source nitrogen control could mitigate negative effects of climate and land use changes, reducing mean annual nitrogen loads to the contemporary baseline level. 相似文献
8.
XU Jianming XU Xiangde LIU Yu DING Guoan CHEN Huailiang HU Jiangkai ZHANG Jianchun WU Hao LI Weiliang HE Jinhai YANG Yuanqin WANG Jiahe 《中国科学D辑(英文版)》2005,48(Z2)
Based on analysis of the air pollution observational data at 8 observation sites in Beijing including outer suburbs during the period from September 2004 to March 2005, this paper reveals synchronal and in-phase characteristics in the spatial and temporal variation of air pollutants on a city-proper scale at deferent sites; describes seasonal differences of the pollutant emission influence between the heating and non-heating periods, also significantly local differences of the pollutant emission influence between the urban district and outer suburbs, i.e. the spatial and temporal distribution of air pollutant is closely related with that of the pollutant emission intensity. This study shows that due to complexity of the spatial and temporal distribution of pollution emission sources, the new generation Community Multi-scale Air Quality (CMAQ) model developed by the EPA of USA produced forecasts, as other models did, with a systematic error of significantly lower than observations, albeit the model has better capability than previous models had in predicting the spatial distribution and variation tendency of multi-sort pollutants. The reason might be that the CMAQ adopts average amount of pollutant emission inventory, so that the model is difficult to objectively and finely describe the distribution and variation of pollution emission sources intensity on different spatial and temporal scales in the areas, in which the pollution is to be forecast. In order to correct the systematic prediction error resulting from the average pollutant emission inventory in CMAQ, this study proposes a new way of combining dynamics and statistics and establishes a statistically correcting model CMAQ-MOS for forecasts of regional air quality by utilizing the relationship of CMAQ outputs with corresponding observations, and tests the forecast capability. The investigation of experiments presents that CMAQ-MOS reduces the systematic errors of CMAQ because of the uncertainty of pollution emission inventory and improves the forecast level of air quality. Also this work employed a way of combining point and area forecasting, i.e. taking the products of CMAQ for a center site to forecast air pollution for other sites in vicinity with the scheme of model products "reanalysis" and average over the "area". 相似文献
9.
10.
Alan B. Dixon 《Singapore journal of tropical geography》2008,29(3):341-356
Local community based institutions that coordinate the management of natural resources have been linked to socioecological resilience, adaptation and sustainability within rural livelihood systems throughout the developing world. The resilience and sustainability of related local institutions, however, is influenced by their relationship with external actors and institutions, particularly in facilitating, supporting or hindering local institutional arrangements. From this standpoint, this paper examines the case of local community based institutions involved in wetland management in western Ethiopia. Drawing upon the findings of participatory fieldwork undertaken in eight wetland‐using communities of Illubabor and Western Wellega zones in Oromia Region it is argued that although local institutions do play a key role in coordinating wetland management and sustaining the benefits from wetlands, the sustainability and resilience of the institutions themselves is threatened by a range of factors. Despite their grassroots nature, their effectiveness is influenced by their reliance on local government backstopping that appears to have diminished in recent years, as well as a perceived lack of local government support for collective action over individual rights. 相似文献