全文获取类型
收费全文 | 4611篇 |
免费 | 557篇 |
国内免费 | 1496篇 |
专业分类
安全科学 | 1523篇 |
废物处理 | 32篇 |
环保管理 | 409篇 |
综合类 | 2723篇 |
基础理论 | 781篇 |
环境理论 | 1篇 |
污染及防治 | 266篇 |
评价与监测 | 268篇 |
社会与环境 | 140篇 |
灾害及防治 | 521篇 |
出版年
2024年 | 57篇 |
2023年 | 205篇 |
2022年 | 299篇 |
2021年 | 361篇 |
2020年 | 321篇 |
2019年 | 328篇 |
2018年 | 255篇 |
2017年 | 302篇 |
2016年 | 328篇 |
2015年 | 356篇 |
2014年 | 321篇 |
2013年 | 435篇 |
2012年 | 439篇 |
2011年 | 467篇 |
2010年 | 297篇 |
2009年 | 309篇 |
2008年 | 213篇 |
2007年 | 268篇 |
2006年 | 283篇 |
2005年 | 181篇 |
2004年 | 111篇 |
2003年 | 99篇 |
2002年 | 89篇 |
2001年 | 50篇 |
2000年 | 56篇 |
1999年 | 48篇 |
1998年 | 32篇 |
1997年 | 29篇 |
1996年 | 24篇 |
1995年 | 18篇 |
1994年 | 20篇 |
1993年 | 16篇 |
1992年 | 10篇 |
1991年 | 7篇 |
1990年 | 5篇 |
1989年 | 3篇 |
1988年 | 5篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1971年 | 1篇 |
1970年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有6664条查询结果,搜索用时 15 毫秒
261.
DUNCAN S. WILSON‡ MARGO A. STODDARD† MATTHEW G. BETTS KLAUS J. PUETTMANN 《Conservation biology》2009,23(4):982-991
Abstract: Species conservation risk assessments require accurate, probabilistic, and biologically meaningful maps of population distribution. In patchy populations, the reasons for discontinuities are not often well understood. We tested a novel approach to habitat modeling in which methods of small area estimation were used within a hierarchical Bayesian framework. Amphibian occurrence was modeled with logistic regression that included third-order drainages as hierarchical effects to account for patchy populations. Models including the random drainage effects adequately represented species occurrences in patchy populations of 4 amphibian species in the Oregon Coast Range (U.S.A.). Amphibian surveys from other locations within the same drainage were used to calibrate local drainage-scale effects. Cross-validation showed that prediction errors for calibrated models were 77% to 86% lower than comparable regionally constructed models, depending on species. When calibration data were unavailable, small area and regional models performed similarly, although poorly. Small area estimation models complement wildlife ecology and habitat studies, and can help managers develop a regional picture of the conservation status for relatively rare species. 相似文献
262.
Assessing ecological risk on a regional scale 总被引:17,自引:0,他引:17
Carolyn T. Hunsaker Robin L. Graham Glenn W. Suter II Robert V. O'Neill Lawrence W. Barnthouse Robert H. Gardner 《Environmental management》1990,14(3):325-332
Society needs a quantitative and systematic way to estimate and compare the impacts of environmental problems that affect
large geographic areas. This paper presents an approach for regional risk assessment that combines regional assessment methods
and landscape ecology theory with an existing framework for ecological risk assessment. Risk assessment evaluates the effects
of an environmental change on a valued natural resource and interprets the significance of those effects in light of the uncertainties
identified in each component of the assessment process. Unique and important issues for regional risk assessment are emphasized;
these include the definition of the disturbance scenario, the assessment boundary definition, and the spatial heterogeneity
of the landscape.
Although the research described in this article has been funded wholly or in part by the United States Environmental Protection
Agency (EPA) through Interagency Agreement Number DW89932112-01-2 to the U.S. Department of Energy, it has not been subjected
to EPA review and therefore does not necessarily reflect the views of EPA and no official endorsement should be inferred. 相似文献
263.
ABSTRACT: The probability distributions of annual peak flows used in flood risk analysis quantify the risk that a design flood will be exceeded. But the parameters of these distributions are themselves to a degree uncertain and this uncertainty increases the risk that the flood protection provided will in fact prove to be inadequate. The increase in flood risk due to parameter uncertainty is small when a fairly long record of data is available and the annual flood peaks are serially independent, which is the standard assumption in flood frequency analysis. But standard tests for serial independence are insensitive to the type of grouping of high and low values in a time series, which is measured by the Hurst coefficient. This grouping increases the parameter uncertainty considerably. A study of 49 annual peak flow series for Canadian rivers shows that many have a high Hurst coefficient. The corresponding increase in flood risk due to parameter uncertainty is shown to be substantial even for rivers with a long record, and therefore should not be neglected. The paper presents a method of rationally combining parameter uncertainty due to serial correlation, and the stochastic variability of peak flows in a single risk assessment. In addition, a relatively simple time series model that is capable of reproducing the observed serial correlation of flood peaks is presented. 相似文献
265.
Thanakorn Uan-On Otto J. Helweg 《Journal of the American Water Resources Association》1988,24(2):261-268
ABSTRACT: The Nonlinear Risk-Benefit (NRB) Algorithm includes risk as one of the objectives in a multiple-objective optimization problem. The NRB Algorithm is derived by extending the Surrogate Worth Trade-Off method to quadratic programming. This category of problem is common in water resources planning and design, especially multipurpose reservoir systems. Consequently, an example is given using the algorithm for optimally operating a multipurpose reservoir. 相似文献
266.
降低企业库存风险问题研究 总被引:4,自引:1,他引:4
大多数企业都面临着库存风险问题 ,库存过多 ,造成积压 ,不仅占用大量资金和场地 ,加重企业的利息负担 ,而且长期存放会使物品陈旧过时 ,失去原有的价值和使用价值。如何降低库存风险 ,使库存经常处于合理水平 ,是每个企业都十分关心的问题。笔者分析了库存管理中存在的问题 ,提出需求预测和安全库存的确定 ,是降低库存风险的两个重要前提 ,同时讨论了安全库存的计算方法 ,以及降低库存风险的主要措施 相似文献
267.
268.
G. Grasa V. Navarro O. Rubio J. A. Pea J. Santamaria 《Journal of Loss Prevention in the Process Industries》2002,15(6):525-530
Opinion polls were conducted to assess the level of risk perceived by the citizens in communities of different sizes where chemical industries were present. The aim of the study was to relate the risk perceived to variables such as the nature of risks, the specific information received regarding the industrial activity and the economic impact of this activity in the community. 相似文献
269.
Shie‐Yui Liong Chandrasekaran Sivapragasam 《Journal of the American Water Resources Association》2002,38(1):173-186
ABSTRACT: Machine learning techniques are finding more and more applications in the field of forecasting. A novel regression technique, called Support Vector Machine (SVM), based on the statistical learning theory is explored in this study. SVM is based on the principle of Structural Risk Minimization as opposed to the principle of Empirical Risk Minimization espoused by conventional regression techniques. The flood data at Dhaka, Bangladesh, are used in this study to demonstrate the forecasting capabilities of SVM. The result is compared with that of Artificial Neural Network (ANN) based model for one‐lead day to seven‐lead day forecasting. The improvements in maximum predicted water level errors by SVM over ANN for four‐lead day to seven‐lead day are 9.6 cm, 22.6 cm, 4.9 cm and 15.7 cm, respectively. The result shows that the prediction accuracy of SVM is at least as good as and in some cases (particularly at higher lead days) actually better than that of ANN, yet it offers advantages over many of the limitations of ANN, for example in arriving at ANN's optimal network architecture and choosing useful training set. Thus, SVM appears to be a very promising prediction tool. 相似文献
270.
William L. Bathke R. J. Freund J. R. Conner 《Journal of the American Water Resources Association》1970,6(4):476-482
In this paper, a procedure for analyzing a water resource system with special emphasis on evaluation of acceptable economic risk due to occasional failures to deliver water is proposed. The basic methodology includes the development of a simple mathematical model which describes the physical hydrologic and economic characteristics of a single reservoir irrigation and city water supply system and an evaluation of economic benefits of the system with full and partial deliveries of water. The system is simulated for various combinations of decision variables (system magnitudes) and an optimum design is obtained by response surface technology. Emphasis is placed on the basic model and methodology although, in order to introduce some realism, the procedure is applied to data based on the existing reservoir system on the South Concho River in West Central Texas. 相似文献