全文获取类型
收费全文 | 2656篇 |
免费 | 280篇 |
国内免费 | 193篇 |
专业分类
林业 | 184篇 |
农学 | 155篇 |
基础科学 | 216篇 |
715篇 | |
综合类 | 816篇 |
农作物 | 78篇 |
水产渔业 | 189篇 |
畜牧兽医 | 598篇 |
园艺 | 58篇 |
植物保护 | 120篇 |
出版年
2024年 | 12篇 |
2023年 | 64篇 |
2022年 | 70篇 |
2021年 | 103篇 |
2020年 | 108篇 |
2019年 | 122篇 |
2018年 | 69篇 |
2017年 | 144篇 |
2016年 | 129篇 |
2015年 | 114篇 |
2014年 | 126篇 |
2013年 | 144篇 |
2012年 | 226篇 |
2011年 | 195篇 |
2010年 | 147篇 |
2009年 | 166篇 |
2008年 | 114篇 |
2007年 | 148篇 |
2006年 | 157篇 |
2005年 | 107篇 |
2004年 | 113篇 |
2003年 | 68篇 |
2002年 | 69篇 |
2001年 | 62篇 |
2000年 | 39篇 |
1999年 | 46篇 |
1998年 | 45篇 |
1997年 | 29篇 |
1996年 | 29篇 |
1995年 | 29篇 |
1994年 | 48篇 |
1993年 | 23篇 |
1992年 | 19篇 |
1991年 | 14篇 |
1990年 | 13篇 |
1989年 | 4篇 |
1988年 | 2篇 |
1987年 | 5篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1973年 | 1篇 |
1962年 | 1篇 |
1955年 | 2篇 |
排序方式: 共有3129条查询结果,搜索用时 31 毫秒
181.
182.
《Land Degradation \u0026amp; Development》2017,28(7):2207-2221
Relatively few studies have so far investigated the hydrological impacts of urbanization in Mediterranean catchments, and particularly in peri‐urban catchments experiencing relatively rapid and large changes in their land‐use mosaic. This study uses data‐based model simulations to investigate such impacts, with the Ribeira dos Covões catchment in Portugal as a concrete Mediterranean peri‐urban catchment example. We distinguish the impacts of urbanization from those of climatic change on the water flux partitioning and connectivity in the catchment over the period 1958–2013. Decrease in precipitation over this period has primarily driven decreases in annual runoff and actual evapotranspiration, while the urbanization development has primarily changed the relative flux partitioning and connectivity pattern in the catchment. The relative contribution of overland flow to annual and seasonal runoff has increased, keeping the absolute overland flow more or less intact, while the baseflow contribution to the stream network has decreased. Methodologically, the present simulation approach provides a relevant means for distinguishing main drivers of change in hydrological flux partitioning and connectivity under concurrent urbanization and climatic changes. © 2017 The Authors. Land Degradation & Development Published by John Wiley & Sons Ltd. 相似文献
183.
以女儿寨小流域为研究对象,着重研究了植被恢复与重建过程中小流域系统的降雨和水沙变化特征,并对植被恢复过程中的总体减沙水平做了分析。结果表明:①受大气环流的影响,小流域年际降雨量差异较大;小流域降雨年内时间分布极为不均,集中性很大,主要集中在汛期(4-8月),但年内降雨量的月间分布趋势基本一致。②小流域迳流年际变化很大,迳流年内分配不均。迳流集中是该小流域迳流变化的一个显著特征:小流域的植被恢复与重建对洪峰的出现和洪峰流量具有明显的延缓和削减作用。③径流(Qi)主要来源于降水(Pi),二者呈明显的一元线型关系:Pi=115.7678+2.7386Qi。④小流域的年输沙模数与年迳流量的变化规律基本一致,小流域年内产沙也高度集中,且与降水、迳流关系密切,小流域的土壤侵蚀主要由汛期的强降水引起。⑤随着小流域大面积、高标准的生态环境建设,1990年至2004年间小流域的输沙模数呈锐减之势,植被恢复与重建起到了显著的减沙效应。 相似文献
184.
In India, small farmers have generally found that the growing of eucalypts as a cash crop provides poor returns, and many have reverted to growing annual crops. Farmgate prices have been much lower than retail prices, due to the existence of middlemen, and retail prices have fallen to lower levels than expected as markets have become saturated. This article describes the situation in eastern Gujarant, where farmers sold their eucalypts directly to buyers at prices close to those in organised wood markets. An important reason for this unusual situation is the absence in Gujarat of controls on eucalyptus sales, which has helped producers to avoid dependence on rent-seeking intermediaries. The survey shows that growing eucalyptus can be a profitable activity for small farmers, given the right circumstances. 相似文献
185.
186.
LIU Huifang* WEI Tianxing ZHU QingkeSoil Water Conservation Combat Desertification Laboratory Beijing Forestry University Beijing P.R.China 《中国林业科技(英文版)》2004,3(2):71-76
1 INTRODUCTIONThe Loess Plateau is the most drastic soil erosion regionin China, with the most severe water erosion. Thedistribution of types and intensity of erosion not onlyhave regional diversity but also have perpendicularvariation. In the small watershed, there are differencesin erosion characters among its upper, middle and lowerreaches. It is very significant for taking rationalmeasures of soil and water conservation and forpreventing soil and water loss to analyze the relationbe… 相似文献
187.
文章以乌海市平沟煤矸石山上平台和边坡两种不同小地形上的土壤为研究对象,通过野外调查取样和室内分析化验,研究煤矸石山两种小地形对土壤养分的恢复情况。结果表明,土壤有机质、全N、碱解N含量在两种不同小地形上表现为边坡>平台,土壤P素、K素含量表现为平台>边坡,总体而言,边坡上土壤养分恢复效果比平台好。平台和边坡上土壤养分含量在不同土层深度中均表现为:0~20 cm层>20~40 cm层。通过参考土壤养分分级与丰缺度标准,目前土壤养分丰缺度主要呈现出缺或极缺状态。 相似文献
188.
第二代杉木林养分动态研究 总被引:3,自引:0,他引:3
根据连续定位观测所取得的数据,对4个小集水区第二代10年生杉木人工林生态系统中的养分循环进行了研究.结果表明.集水区由降水输入的N、P、K、Ca、Mg等营养元素总计为59.285kg/hm2a,径流输出为38.938kg/hm2a,净积累量为20.347kg/hm2a.与该林分7年生时相比,净积累量提高了7.2%,说明集水区第二代杉木林生态恢复处于进展的变化过程. 相似文献
189.
Classifying tree planters and non planters in a subsistence farming system using a discriminant analytical approach 总被引:1,自引:0,他引:1
Since the 1970s tree planting has been promoted to meet the multipurpose needs of subsistence farmers and to arrest deforestation in many developing countries. Financial support and extension systems were developed as tree growing outside the forest became more important. The limited success of the forest extension approaches used in the 1980s has been attributed to the failure to ascertain householders' priorities and attitudes to tree growing. Although the broader theoretical paradigm of factors influencing householders' planting of trees has been discussed household and regional level analyses are rare. This study used discriminant analysis to assess the extent to which social and economic factors affect smallholder farmer tree planting in Orissa, eastern India. The validity of the predictive model to define the different groups was tested by assessing the accuracy of classification. The significance of the factors was probed using the identified variables in the discriminant functions developed for smallholder farmers in the coastal and inland zones of Orissa. The results contradicted some common assumptions that only large landowners with a substantial income are innovators. The likelihood of adopting agroforestry is dependent on the progressive attitude of farmers, membership of village organisations, their wealth status and, more importantly, their perceived risk concerning agricultural production. This approach enables tree planters and non-planters to be characterised and hence give better targetting of planning and social forestry programmes.This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献
190.
以女儿寨小流域为研究对象,着重研究了植被恢复重建过程中小流域系统的小气候变化特征。随着小流域内的植被恢复与重建,小流域内最高气温约降低了1~4℃,尤其以夏、秋季更为突出,约降低了3~4℃;最低气温提高了1~2℃,夏季较其它季节更为显著一些;森林不能降低小流域的日平均气温,但能略微增加秋冬季的平均气温0.1~0.5℃;平均地面温度降低了0.9~1.3℃,秋冬季的地面平均温度略微增加0.1~0.5℃;年降水量增加了77~115mm;年蒸发量降低105~175mm。小流域小气候环境发生了较大的变化,小气候质量在不断的提高。 相似文献