全文获取类型
收费全文 | 1196篇 |
免费 | 51篇 |
国内免费 | 172篇 |
专业分类
林业 | 79篇 |
农学 | 30篇 |
基础科学 | 419篇 |
438篇 | |
综合类 | 303篇 |
农作物 | 27篇 |
水产渔业 | 20篇 |
畜牧兽医 | 38篇 |
园艺 | 9篇 |
植物保护 | 56篇 |
出版年
2024年 | 3篇 |
2023年 | 19篇 |
2022年 | 25篇 |
2021年 | 38篇 |
2020年 | 31篇 |
2019年 | 36篇 |
2018年 | 20篇 |
2017年 | 33篇 |
2016年 | 61篇 |
2015年 | 56篇 |
2014年 | 52篇 |
2013年 | 74篇 |
2012年 | 96篇 |
2011年 | 99篇 |
2010年 | 78篇 |
2009年 | 73篇 |
2008年 | 78篇 |
2007年 | 75篇 |
2006年 | 71篇 |
2005年 | 50篇 |
2004年 | 40篇 |
2003年 | 47篇 |
2002年 | 31篇 |
2001年 | 29篇 |
2000年 | 26篇 |
1999年 | 27篇 |
1998年 | 21篇 |
1997年 | 17篇 |
1996年 | 19篇 |
1995年 | 18篇 |
1994年 | 8篇 |
1993年 | 26篇 |
1992年 | 13篇 |
1991年 | 7篇 |
1990年 | 7篇 |
1989年 | 7篇 |
1988年 | 4篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有1419条查询结果,搜索用时 0 毫秒
41.
灌区土壤盐渍化发展模拟预测与对策研究 总被引:8,自引:0,他引:8
以秦王川灌区观测和调查资料为依据,采用目前广为实用的MODFLOW地下水模拟软件,对灌区地下水动态进行了预测和分析,并对地下水质的发展趋势作出评价。灌溉实施以来,秦王川灌区地下水位逐年上升。未来10年内盐沼区面积扩大约为28 km2,并且将继续增加,地下水埋深0~2 m的面积达31.4 km2,占灌区面积的比例约为6%。未来20年地下水埋深0~2 m的面积达47 km2,占灌区面积的比例为10%左右。根据地下水位的变化特征,对控制土壤盐渍化发展的因素和对策进行了分析,提出以明沟排水为主,加大灌区下游地下水排泄能力;以渠灌为主,适度发展井灌,大力推行田间节水灌溉技术等对策。 相似文献
42.
分析了排灌企业加强货币资金内部控制的必要性.提出了排灌企业加强货币资金内部控制的目标和主要方法.第一,建立严格的货币资金管理制度,着重对于现金和银行存款日常收支过程中最有可能出现的漏洞提出防范措施.第二,建立严格的岗位分工制度,强调经办货币资金收付的不相容各岗位分离,形成相互牵制.第三,建立严格的授权、审批制度.第四,建立严格的会计电算化内控制度,明确会计主管、系统管理员、凭证录入人员的职责权限. 相似文献
43.
灌溉试验站规划的有关问题 总被引:1,自引:0,他引:1
根据全国的气候、农业、水利及经济发展水平等区划,合理地选择各省的中心站及重点试验站,并对其进行科学地规划设计,对于总体把握我国灌溉试验基础数据尤为重要。针对目前正在全国进行的灌溉试验站网规划,对省级中心站及重点站的课题规划、试验站的总体布置及设施设备规划和做好本次规划的其他有关问题进行了探讨。 相似文献
44.
45.
Strategies for reducing subsurface drainage in irrigated agriculture through improved irrigation 总被引:1,自引:0,他引:1
The traditional approach ofinstalling subsurface drainage systems tosolve shallow ground water problems is notfeasible along the west side of the SanJoaquin Valley of California because of thelack of drain water disposal methods thatare economical, technically feasible, andenvironmentally friendly. Thus, optionssuch as drainage reduction through improvedirrigation and drain water reuse are beingexamined as methods for coping with thesubsurface drainage problem. This paperdiscusses options for reducing subsurfacedrainage through improved irrigationpractices. Options are discussed forimproving irrigation system design such asupgrading existing irrigation methods andconverting to systems with higher potentialirrigation efficiencies. Methods forimproving water management are alsopresented. Case studies on upgradingexisting systems or converting to otherirrigation methods are presented along with study results of the effect of variouspolicies on reducing subsurface drainage. 相似文献
46.
Highly productive, irrigated agriculturecan be found in California's Central Valleymade up of the Sacramento, San Joaquin, andTulare Lake basins. High water tablesthroughout much of the San Joaquin andTulare Lake portions of the Valley threatenthis highly productive region. Due totopographic and environmental constraintsmuch of the region is currently withoutdrainage. In 1990 State and Federalgovernment agencies combined to produce areport that outlined possible steps to dealwith the drainage issue. These stepsincluded: 1) Source control (practices toreduce the amount of drainage water); 2)Drainage reuse; 3) Evaporation systems; 4)Land retirement (cease irrigation); 5)Groundwater management; and 6) Discharge tothe San Joaquin River. General backgroundinformation to the history and hydrology ofthe Valley as well as a discussion of thefeasibility and constraints of providingdrainage by discharging drain water to theSan Joaquin River are presented. Inaddition a general discussion of thetechnical and political limitations ofproviding drainage in the River arediscussed. 相似文献
47.
Nigel W.T. Quinn 《Agricultural Water Management》2009,96(3):484-492
Many perceive the implementation of environmental regulatory policy, especially concerning non-point source pollution from irrigated agriculture, as being less efficient in the United States than in many other countries. This is partly a result of the stakeholder involvement process but is also a reflection of the inability to make effective use of Environmental Decision Support Systems (EDSS) to facilitate technical information exchange with stakeholders and to provide a forum for innovative ideas for controlling non-point source pollutant loading. This paper describes one of the success stories where a standardized Environmental Protection Agency (EPA) methodology was modified to better suit regulation of a trace element in agricultural subsurface drainage and information technology was developed to help guide stakeholders, provide assurances to the public and encourage innovation while improving compliance with State water quality objectives. The geographic focus of the paper is the western San Joaquin Valley where, in 1985, evapo-concentration of selenium in agricultural subsurface drainage water, diverted into large ponds within a federal wildlife refuge, caused teratogenecity in waterfowl embryos and in other sensitive wildlife species. The fallout from this environmental disaster was a concerted attempt by State and Federal water agencies to regulate non-point source loads of the trace element selenium. The complexity of selenium hydrogeochemistry, the difficulty and expense of selenium concentration monitoring and political discord between agricultural and environmental interests created challenges to the regulation process. Innovative policy and institutional constructs, supported by environmental monitoring and the web-based data management and dissemination systems, provided essential decision support, created opportunities for adaptive management and ultimately contributed to project success. The paper provides a retrospective on the contentious planning process and offers suggestions as to how the technical and institutional issues could have been resolved faster through early adoption of some of the core principles of sound EDSS design. 相似文献
48.
Lambert K. Smedema Safwat Abdel-Dayem Walter J. Ochs 《Irrigation and Drainage Systems》2000,14(3):223-235
This paper explores the role of drainage as aninstrument for agricultural and rural development andthe related drainage development forces and processes.Five specific roles of drainage are distinguished:foodproduction, agricultural intensification anddiversification, sustainable irrigated land use, ruraldevelopment and environmental protection. Specialattention is given to the drainage development needsof the developing countries. It is argued that whileat early stages of agricultural development, drainagedevelopment is generally driven by ongoingagricultural development, at later stages ofdevelopment these roles often reverse withagricultural development risking to become stagnatedwhen drainage is not improved. These relationshipsshould be taken into account in the design of drainagedevelopment programs and projects. It is emphasizedthat improved drainage can contribute to establishinga more diversified, competitive and sustainable typeof agriculture, enhance sanitary and public healthconditions and generally contribute to ruraldevelopment, rural well being and poverty alleviation.Drainage development in the developing countries ishowever often severely constrained by the lack of supporting public policies, institutional frameworksand professional cadres. Overcoming these constraintsshould be given high priority on the national andinternational drainage development agendas. 相似文献
49.
50.