全文获取类型
收费全文 | 6854篇 |
免费 | 1425篇 |
国内免费 | 2017篇 |
专业分类
测绘学 | 596篇 |
大气科学 | 1417篇 |
地球物理 | 1690篇 |
地质学 | 3616篇 |
海洋学 | 987篇 |
天文学 | 321篇 |
综合类 | 718篇 |
自然地理 | 951篇 |
出版年
2024年 | 52篇 |
2023年 | 128篇 |
2022年 | 411篇 |
2021年 | 434篇 |
2020年 | 361篇 |
2019年 | 474篇 |
2018年 | 475篇 |
2017年 | 427篇 |
2016年 | 497篇 |
2015年 | 446篇 |
2014年 | 502篇 |
2013年 | 465篇 |
2012年 | 469篇 |
2011年 | 447篇 |
2010年 | 442篇 |
2009年 | 394篇 |
2008年 | 385篇 |
2007年 | 333篇 |
2006年 | 282篇 |
2005年 | 226篇 |
2004年 | 193篇 |
2003年 | 216篇 |
2002年 | 290篇 |
2001年 | 257篇 |
2000年 | 198篇 |
1999年 | 240篇 |
1998年 | 202篇 |
1997年 | 172篇 |
1996年 | 139篇 |
1995年 | 128篇 |
1994年 | 97篇 |
1993年 | 112篇 |
1992年 | 84篇 |
1991年 | 52篇 |
1990年 | 43篇 |
1989年 | 40篇 |
1988年 | 46篇 |
1987年 | 23篇 |
1986年 | 26篇 |
1985年 | 21篇 |
1984年 | 18篇 |
1983年 | 9篇 |
1982年 | 6篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1976年 | 2篇 |
1958年 | 7篇 |
1954年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
171.
172.
173.
174.
Accepting the concept of standardization introduced by the standardized precipitation index, similar methodologies have been developed to construct some other standardized drought indices such as the standardized precipitation evapotranspiration index (SPEI). In this study, the authors provided deep insight into the SPEI and recognized potential deficiencies/limitations in relating to the climatic water balance it used. By coupling another well‐known Palmer drought severity index (PDSI), we proposed a new standardized Palmer drought index (SPDI) through a moisture departure probabilistic approach, which allows multi‐scalar calculation for accurate temporal and spatial comparison of the hydro‐meteorological conditions of different locations. Using datasets of monthly precipitation, temperature and soil available water capacity, the moisture deficit/surplus was calculated at multiple temporal scales, and a couple of techniques were adopted to adjust corresponding time series to a generalized extreme value distribution out of several candidates. Results of the historical records (1900–2012) for diverse climates by multiple indices showed that the SPDI was highly consistent and correlated with the SPEI and self‐calibrated PDSI at most analysed time scales. Furthermore, a simple experiment of hypothetical temperature and/or precipitation change scenarios also verified the effectiveness of this newly derived SPDI in response to climate change impacts. Being more robust and preferable in spatial consistency and comparability as well as combining the simplicity of calculation with sufficient accounting of the physical nature of water supply and demand relating to droughts, the SPDI is promising to serve as a competent reference and an alternative for drought assessment and monitoring. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
175.
176.
177.
178.
179.
An analytical study on three‐dimensional versus two‐dimensional water table‐induced flow patterns in a Tóthian basin
下载免费PDF全文
![点击此处可从《水文研究》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Jun‐Zhi Wang Xiao‐Wei Jiang Zhi‐Yuan Zhang Li Wan Xu‐Sheng Wang Hailong Li 《水文研究》2017,31(22):4006-4018
Although it has been increasingly acknowledged that groundwater flow pattern is complicated in the three‐dimensional (3‐D) domain, two‐dimensional (2‐D) water table‐induced flow models are still widely used to delineate basin‐scale groundwater circulation. However, the validity of 2‐D cross‐sectional flow field induced by water table has been seldom examined. Here, we derive the analytical solution of 3‐D water table‐induced hydraulic head in a Tóthian basin and then examine the validity of 2‐D cross‐sectional models by comparing the flow fields of selected cross sections calculated by the 2‐D cross‐sectional model with those by the 3‐D model, which represents the “true” cases. For cross sections in the recharge or discharge area of the 3‐D basin, even if head difference is not significant, the 2‐D cross‐sectional models result in flow patterns absolutely different from the true ones. For the cross section following the principal direction of groundwater flow, although 2‐D cross‐sectional models would overestimate the penetrating depth of local flow systems and underestimate the recharge/discharge flux, the flow pattern from the cross‐sectional model is similar to the true one and could be close enough to the true one by adjusting the decay exponent and anisotropy ratio of permeability. Consequently, to determine whether a 2‐D cross‐sectional model is applicable, a comparison of hydraulic head difference between 2‐D and 3‐D solutions is not enough. Instead, the similarity of flow pattern should be considered to determine whether a cross‐sectional model is applicable. This study improves understanding of groundwater flow induced by more natural water table undulations in the 3‐D domain and the limitations of 2‐D models accounting for cross‐sectional water table undulation only. 相似文献
180.
Spatial heterogeneity is ubiquitous in nature, which may significantly affect the soil hydraulic property curves. The models of a closed‐form functional relationship of soil hydraulic property curves (e.g. VG model or exponential model) are valid at point or local scale based on a point‐scale hydrological process, but how do scale effects of heterogeneity have an influence on the parameters of these models when the models are used in a larger scale process? This paper uses a two‐dimensional variably saturated flow and solute transport finite element model (VSAFT2) to simulate variations of pressure and moisture content in the soil flume under a constant head boundary condition. By changing different numerical simulation block sizes, a quantitative evaluation of parameter variations in the VG model, resulting from the scale effects, is presented. Results show that the parameters of soil hydraulic properties are independent of scale in homogeneous media. Parameters of α and n in homogeneous media, which are estimated by using the unsaturated hydraulic conductivity curve (UHC) or the soil water retention curve (WRC), are identical. Variations of local heterogeneities strongly affect the soil hydraulic properties, and the scale affects the results of the parameter estimations when numerical experiments are conducted. Furthermore, the discrepancy of each curve becomes considerable when moisture content becomes closer to a dry situation. Parameters estimated by UHC are totally different from the ones estimated by WRC. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献