首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   697篇
  免费   28篇
  国内免费   17篇
电工技术   1篇
综合类   39篇
化学工业   14篇
机械仪表   10篇
建筑科学   32篇
能源动力   25篇
轻工业   26篇
水利工程   326篇
石油天然气   3篇
无线电   3篇
一般工业技术   7篇
冶金工业   205篇
自动化技术   51篇
  2024年   1篇
  2023年   5篇
  2022年   9篇
  2021年   7篇
  2020年   6篇
  2019年   9篇
  2018年   8篇
  2017年   9篇
  2016年   22篇
  2015年   20篇
  2014年   26篇
  2013年   30篇
  2012年   23篇
  2011年   47篇
  2010年   60篇
  2009年   63篇
  2008年   41篇
  2007年   51篇
  2006年   48篇
  2005年   47篇
  2004年   35篇
  2003年   47篇
  2002年   25篇
  2001年   28篇
  2000年   14篇
  1999年   12篇
  1998年   7篇
  1997年   4篇
  1996年   5篇
  1995年   1篇
  1994年   3篇
  1993年   3篇
  1992年   8篇
  1991年   2篇
  1990年   6篇
  1989年   3篇
  1988年   2篇
  1987年   1篇
  1985年   2篇
  1984年   1篇
  1983年   1篇
排序方式: 共有742条查询结果,搜索用时 15 毫秒
111.
The Egyptian Ministry of Water Resources and Irrigation (MWRI) has expressed concern about the current high cost of the Irrigation Improvement Project (IIP) implemented in the “old lands” of the lower reaches of the Nile River. Because the IIP is financed through fees paid by the direct beneficiaries of the project, i.e., irrigation water users, these same Egyptian agricultural producers are especially sensitive to the higher than expected costs that the project has incurred to date. The objective of the current study is to evaluate whether this cost reduction is associated with any deficiency in irrigation performance. This Monitoring and Evaluation (M&;E) study aims to answer questions such as: Does reducing pump capacity and increased pump operation affect the farmers negatively? And, does the use of electricity instead of diesel fuel affect the farmers positively? Six indicators were used to compare new design criteria in the W10 and the IIP1 irrigation system designs, water savings, irrigation cost, irrigation time, night irrigation, and land savings. The W10 project design achieved the minimum actual operating cost per unit of area and per unit of water compared to the IIP1 project design on both Meet Yazied and El-Mesk Canals. The total cost per unit of area in the W10 project design is lower than IIP1 by 19.33% and 24.92% on the Meet Yazied and El-Mesk Canals, respectively. The total cost per unit of water in the W10 design is also lower than IIP1 on the Meet Yazied and El-Mesk Canals. Average irrigation time for rice and cotton crops cultivated in the W10 area are higher than average irrigation time for these crops cultivated in the IIP1 area for all locations on the Mesqa (head, middle and tail), because of increased pump operation in the W10 area (16 to 20 h d???1). The number of irrigation events at the head of the Sefsafa Canal in the W10 area is higher than on the Meet Yazied and El-Mesk Canals in the IIP1 areas because of the increased pump operation in the W10 area.  相似文献   
112.
Water use patterns are not distributed evenly over space and time. Determining the amount of water used within a region, as well as the various ways in which water is used is important for making adequate and sustainable water management policies and determining future water availability. We examined differences in spatial trends in Oregon freshwater use (total, municipal, and agricultural water withdrawals), by county, between the years 1985 and 2005. We also explored biophysical and socioeconomic factors that explain spatial patterns using Moran’s I, local index of spatial autocorrelation (LISA), and spatial regression models. There was a moderate positive spatial autocorrelation among counties that had similar total and irrigation withdrawals. LISA analysis identified hot spots between certain arid agricultural counties in the southeastern Oregon and cold spots between certain humid northwestern counties, including within the Portland metro area. Annual precipitation and income are negatively associated with total water withdrawals. Summer temperature and farm size is positively associated with irrigation water withdrawals, while net cash return and income are negatively associated with irrigation water withdrawals. When compared to ordinary least square regression models, spatial error models that take into account spatial dependence provide a more comprehensive explanation of the variations of water use, suggesting that water resource planning and management should incorporate spatial and neighborhood effects to effective manage limited natural resources.  相似文献   
113.
黄河三角州典型灌区泥沙资源优化配置的原则与方法   总被引:1,自引:0,他引:1  
采用实测资料分析与理论探讨相结合的方法,对小开河灌区泥沙资源优化配置的原则与方法进行了深入的分析和评价,研究结果表明,小开河灌区泥沙处理应转变观念,结合灌区的实际情况,注重泥沙的合理性和有效性,因地制宜的处理引黄泥沙。小开河灌区泥沙资源优化配置的方法:①浑水灌溉、输沙入田;②以挖待沉,整理开发土地资源;③利用引黄泥沙生产建筑材料。  相似文献   
114.
根据可持续发展理论并借鉴现代企业核心能力理念,阐明了灌区持续发展能力的内涵,对灌区工程配套建设及现代化调控能力、市场化运转及协调能力、机制创新及规范化运作能力和以人为本的灌区服务能力进行了系统分析。结合皂河灌区实践探讨了灌区持续发展能力建设的途径,科学评价了灌区能力建设的效果。对促进大型灌区科学、高效管理体制和良性运行机制的建立,提高大型灌区的可持续发展水平,具有重要的示范和推广作用。  相似文献   
115.
近年来国内外学者对农田水利的研究集中在制度、组织合作等方面。综述了集体行动与农田水利,集体行动、制度对我国农田水利建设的影响,指出目前研究的不足:缺少对个案的深入细致调查研究。  相似文献   
116.
按照生态农业工程理论和抗旱节水工程技术要求,对棉花转基因抗旱品种、不同密度、不同基肥N、P的施用量进行田间水分胁迫综合试验.经在关中灌区3年试验研究结果表明:密度以每公顷种植7.5万株比种植4.5、6万株分别增产671.854、83.30 kg,增产57.49%和41.36%.品种陕402(早熟)、陕576(中早熟)分别比中棉所41(中熟)增产16.62%和16.17%.施磷酸二铵比尿素增产,每公顷施1502、25 kg分别增产11.35%和10.53%.抗旱节水农业工程育种和农艺节水工程技术结合,不仅可降低生产成本44%~47%,还可提高产量和品质,增收1 050~5 000元/hm2,同时节约了水资源,保护了农业生态环境.  相似文献   
117.
在潦河灌区西潦北干五支渠安装了远程测控智能平板闸门装置,开展渠道自动量水监测试验,分析验证远程测控智能闸门装置的实际应用效果。结果表明:该技术能实现灌区明渠输配水的远程智能化控制、下泄水流量数据实时监测、统计,测流误差综合精度小于5%,测流精度和可靠度较好,该系统适用于灌区小型明渠输水控制,为灌区量水设施改造工作提供了有效手段。  相似文献   
118.
This paper presents a new method to design pressurized branched irrigation networks. This method is called recursive design and is based on application of the problem-solving technique known as backtracking to the problem of the optimum design of pressurized branched irrigation networks with a known delivery piezometric head (pipe-sizing). Recursive design is a heuristic optimizer, like genetic algorithms, and has been implemented in a fast, versatile computer application. After presenting and precisely defining the design problem, the writers review the theoretical foundations of some of the main existing design methods: maximum velocity, recommended velocity, Mougnie velocity, constant hydraulic slope, Lagrange multipliers, linear programming, Labye’s method, and genetic algorithms. Next, the writers explain what recursive design consists of and apply its methodology in detail to a simple network. In the results section, the solutions obtained by recursive design are compared with those obtained by the other design methods, giving satisfactory results. For example, in an analyzed standard network, genetic algorithms take more than 20?minutes to offer a solution, whereas recursive design offers a cheaper solution with less than 3?seconds of computation time.  相似文献   
119.
以GIS技术支持下的江西省农田节水灌溉系统为研究对象,阐述了系统研究的主要内容、方法和技术框架,并对GIS技术在节水灌溉中应用的特点及优势进行了分析。  相似文献   
120.
Optimum land and water allocation to different crops grown in different regions of an irrigation scheme is a complex process, especially when these irrigation schemes are characterized by different soils and environment and by a large network of canals. At the same time if the water supply in the irrigation schemes is limited, there is a need to allocate water both efficiently and equitably. This paper describes the approach to include both productivity (efficiency) and equity in the allocation process and to develop the allocation plans for optimum productivity and/or maximum equity for such irrigation schemes. The approach presented in this paper considers the different dimensions of equity such as water distribution over the season, water distribution during each irrigation, and benefits generated. It also includes distribution and conveyance losses while allocating water equitably to different allocation units. This paper explains the approach with the help of the area and water allocation model which uses the simulation–optimization technique for optimum allocation of land and water resources to different crops grown in different allocation units of the irrigation scheme.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号