全文获取类型
收费全文 | 2545篇 |
免费 | 174篇 |
国内免费 | 487篇 |
专业分类
安全科学 | 79篇 |
废物处理 | 20篇 |
环保管理 | 622篇 |
综合类 | 1195篇 |
基础理论 | 270篇 |
污染及防治 | 113篇 |
评价与监测 | 168篇 |
社会与环境 | 256篇 |
灾害及防治 | 483篇 |
出版年
2024年 | 6篇 |
2023年 | 33篇 |
2022年 | 58篇 |
2021年 | 71篇 |
2020年 | 67篇 |
2019年 | 67篇 |
2018年 | 74篇 |
2017年 | 67篇 |
2016年 | 116篇 |
2015年 | 138篇 |
2014年 | 95篇 |
2013年 | 153篇 |
2012年 | 168篇 |
2011年 | 198篇 |
2010年 | 145篇 |
2009年 | 136篇 |
2008年 | 89篇 |
2007年 | 163篇 |
2006年 | 173篇 |
2005年 | 163篇 |
2004年 | 128篇 |
2003年 | 113篇 |
2002年 | 106篇 |
2001年 | 71篇 |
2000年 | 93篇 |
1999年 | 57篇 |
1998年 | 34篇 |
1997年 | 25篇 |
1996年 | 26篇 |
1995年 | 35篇 |
1994年 | 32篇 |
1993年 | 29篇 |
1992年 | 28篇 |
1991年 | 22篇 |
1990年 | 16篇 |
1989年 | 12篇 |
1988年 | 11篇 |
1987年 | 11篇 |
1986年 | 7篇 |
1985年 | 9篇 |
1984年 | 10篇 |
1983年 | 8篇 |
1982年 | 19篇 |
1981年 | 26篇 |
1980年 | 16篇 |
1979年 | 20篇 |
1978年 | 13篇 |
1977年 | 6篇 |
1975年 | 7篇 |
1972年 | 13篇 |
排序方式: 共有3206条查询结果,搜索用时 109 毫秒
201.
Ton H. Snelder Barry J. F. Biggs 《Journal of the American Water Resources Association》2002,38(5):1225-1239
ABSTRACT: River Environment Classification (REC) is a new system for classifying river environments that is based on climate, topography, geology, and land cover factors that control spatial patterns in river ecosystems. REC builds on existing principles for environmental regionalization and introduces three specific additions to the “ecoregion” approach. First, the REC assumes that ecological patterns are dependent on a range of factors and associated landscape scale processes, some of which may show significant variation within an ecoregion. REC arranges the controlling factors in a hierarchy with each level defining the cause of ecological variation at a given characteristic scale. Second, REC assumes that ecological characteristics of rivers are responses to fluvial (i.e., hydrological and hydraulic) processes. Thus, REC uses a network of channels and associated watersheds to classify specific sections of river. When mapped, REC has the form of a linear mosaic in which classes change in the downstream direction as the integrated characteristics of the watershed change, producing longitudinal spatial patterns that are typical of river ecosystems. Third, REC assigns individual river sections to a class independently and objectively according to criteria that result in a geographically independent framework in which classes may show wide geographic dispersion rather than the geographically dependent schemes that result from the ecoregion approach. REC has been developed to provide a multiscale spatial framework for river management and has been used to map the rivers of New Zealand at a 1:50,000 mapping scale. 相似文献
202.
C. B. Vreugdenhil 《Journal of the American Water Resources Association》2002,38(4):1083-1095
ABSTRACT: Over the last 20 years, our possibilities to model river flows numerically have increased enormously. In this paper, the question is addressed whether a more sophisticated model is always better than a simpler one. Increased detail, both in finer resolution and in physical processes taken into account, has its price in more computer time, higher data need, and perhaps more unknown coefficients to be calibrated. Moreover, uncertainty in actual physical conditions (e.g., bottom roughness), inflow and parameters remains, which may dominate the uncertainty of the results. Also, the questions asked by a decision maker may not always be very precise. For a schematic but relevant example, we show that a better model does not necessarily give more reliable results because some of the basic uncertainties remain. It is concluded that we should use the simplest model that will answer the question as to the accuracy needed, taking into account uncertainties in the data 相似文献
203.
S. K. Jain G. N. Yoganarasimhan S. M. Seth 《Journal of the American Water Resources Association》1992,28(6):1037-1043
ABSTRACT: Many approaches are available for operation of a multipurpose reservoir during flood season; one of them is allocation of storage space for flood control. A methodology to determine a reservoir operation policy based on explicit risk consideration is presented. The objective of the formulation is to maximize the reservoir storage at the end of a flood season while ensuring that the risk of an overflow is within acceptable limits. The Dynamic Programming technique has been used to solve the problem. This approach has been applied to develop operation policies for an existing reservoir. The performance of the policy was evaluated through simulation and was found to be satisfactory. 相似文献
204.
205.
渭、洛河下游受三门峡水库回水淹没及影响,洪灾频繁。本文依据1902年8月惨重洪灾的实例,全面分析了洪水的特殊性和致灾因素,并从流域治理、水库水沙调度、防洪工程建设等方面提出了进一步减灾的措施。 相似文献
206.
E T. Foster T. C. Chen J. P. Newton E. O. Isu 《Journal of the American Water Resources Association》1972,8(5):863-870
ABSTRACT. Theoretical and practical results are summarized for a study to determine optimal water resource allocation in a proposed water conservancy district. The area of this district, which covers several river basins, contains a large number of existing and proposed facilities such as reservoirs and diversions. The operation of all of these facilities was to be determined along with the sizing of the proposed facilities in order to optimize given objective functions. Related efforts in optimal river basin utilization were surveyed, and linear programming was selected as an expedient optimization technique. The problem is formulated by identifying time stages which together constitute a repetitive cycle such as a year. With these stages, it is possible to associate operational and capacity variables with network components, which are branches and nodes. Objective functions are assembled for the component variables. Constraint equations are written in terms of the variables to reflect network nodal continuity, capacity restrictions, and adjudications such as water rights. A numerical example is considered in which the existing and proposed facilities are aggregated to produce a small, tractable number of facilities. This paper examines the example results and suggests future improvements for models of this type. 相似文献
207.
石油类在河流中自净能力的研究 总被引:1,自引:0,他引:1
根据河流的特点,利用示踪剂对河流中石油类污染物的衰减情况进行了分析,建立了污染物扩散模式,确定了河流不同断面的石油通量,求出了石油类污染物的衰减系数。为污染物的排放量和河流环境容量的确定提供了保证。 相似文献
208.
从我国“98”南北洪水看山西防洪问题 总被引:1,自引:0,他引:1
从我国“98”洪灾的惨重经济损失中认为,除了气候因素外,生态环境遭到人为破坏是致灾的主要原因。以此为鉴,结合山西实际,分析了山西防洪中存在的问题及根治洪患的举措 相似文献
209.
210.
区域化变量理论在历史洪涝灾害空间格局重建中的应用──以长江流域1736-1911年洪涝灾害为例 总被引:1,自引:0,他引:1
通过构造一个例子,简介了区域化变量理论的基本原理、计算过程及其在洪涝灾害空间格局重建中的优点。依据区域化变量理论,以长江流域1736~1911年洪涝灾害为实例,重建了长江流域历史时期洪涝灾害的基本空间格局. 相似文献