全文获取类型
收费全文 | 1751篇 |
免费 | 78篇 |
国内免费 | 179篇 |
专业分类
林业 | 79篇 |
农学 | 62篇 |
基础科学 | 149篇 |
643篇 | |
综合类 | 509篇 |
农作物 | 6篇 |
水产渔业 | 71篇 |
畜牧兽医 | 54篇 |
园艺 | 7篇 |
植物保护 | 428篇 |
出版年
2024年 | 39篇 |
2023年 | 86篇 |
2022年 | 129篇 |
2021年 | 95篇 |
2020年 | 76篇 |
2019年 | 99篇 |
2018年 | 62篇 |
2017年 | 114篇 |
2016年 | 128篇 |
2015年 | 94篇 |
2014年 | 95篇 |
2013年 | 109篇 |
2012年 | 122篇 |
2011年 | 140篇 |
2010年 | 124篇 |
2009年 | 93篇 |
2008年 | 77篇 |
2007年 | 67篇 |
2006年 | 49篇 |
2005年 | 52篇 |
2004年 | 27篇 |
2003年 | 20篇 |
2002年 | 14篇 |
2001年 | 6篇 |
2000年 | 9篇 |
1999年 | 6篇 |
1998年 | 13篇 |
1997年 | 13篇 |
1996年 | 11篇 |
1995年 | 3篇 |
1994年 | 8篇 |
1993年 | 6篇 |
1992年 | 8篇 |
1991年 | 7篇 |
1990年 | 5篇 |
1989年 | 2篇 |
排序方式: 共有2008条查询结果,搜索用时 0 毫秒
141.
Based on the sample data on the concentrations of nutrient elements and heavy metals in the sediment collected from Baiyangdian
Lake (BL) in June 2004, the pollution level and the distribution of N, P, organic matter (OM), and the heavy metals Cu, Zn,
Cd and Pb in water body sediment were analyzed. The pollution trends and current loads between BL and Taihu Lake (TL) were
compared using relevant data from BL obtained in 1982 and 1994 and from TL in 1988 and 2000. The findings showed an ever-increasing
trend of pollution year by year both in the sediment of BL and TL, although N and P pollution was somehow controlled at the
end of the 1990s. The pollution situation in the BL area indicated that the downriver regions were more seriously polluted
than the upriver regions, and the most heavily polluted sites were the outlet of the Fuhe River, the original wastewater reservoir
of Tanghe River, and the densely populated Chunshui and Wangjiazhai villages. For the TL, the northern lakeside regions were
found to be more seriously polluted than the southern regions, although the center of the lake was still relatively pollution
free. The most heavily polluted spots were Wuli Lake, Meiliang Bay near Changzhou, Wuxi and other neighboring cities. While
the Cd pollution in BL was greater than that in TL, there were not many differences in the average loads of Cu, Zn and Pb.
This paper discusses the alternative measures to deal with the different pollution levels in these two lakes. 相似文献
142.
农村聚落空心化问题探讨——以太原盆地东南部为例 总被引:5,自引:1,他引:5
在实地考察的基础上 ,分析了农村聚落空心化的因素 ,主要归纳为人口和家庭因素、社会经济与收入、交通条件、制度因素和文化因素等 ,指出了农村聚落空心化造成的问题 ,并提出健全用地制度和提高人口素质是遏制农村聚落空心化的有效途径 相似文献
143.
四川盆地低山丘陵区柏木低效林改造效应研究 总被引:2,自引:0,他引:2
柏木低效林是四川盆地丘陵区一种典型的低效林类型。对四川省阆中市哑口乡低效柏木人工林设计了4种间伐强度(对照、21.6%的弱度间伐、49.4%的强度间伐和71.3%的极强度间伐)的低改试验。于5年后对试验地进行了林分生长量调查,并采用Shannon-Wiener多样性指数P、ielou群落均匀度指数和Simpson生态优势度指数对低改后林下植被的多样性进行了对比研究。结果表明:抚育间伐对树高生长无显著影响(P>0.05);不同间伐强度对林分平均胸径生长影响显著(P<0.01),其生长量随着间伐强度的增强而增加,但保留密度2 490和1 560株/hm2的2个处理间的差异不显著(P>0.05);林分低改后,增加了林下植被的Shannon-Wiener多样性指数和Pielou群落均匀度指数,减低了Simpson生态优势度指数。因此,作为水土保持林,可对柏林低效林进行50%的抚育间伐,即保留密度在2 500株/hm2左右。 相似文献
144.
The downstream impacts of increasing water consumption in the upstream rain-fed areas of the Karkheh Basin, Iran are simulated using the semi-distributed SWAT model. Three scenarios are tested at subbasin and basin levels: converting rain-fed areas to irrigation agriculture (S1), improving soil water availability through rainwater harvesting (S2), and a combination of both (S3). The results of these scenarios were compared against the baseline period 1988-2000. The S1 scenario shows a 10% reduction in mean annual flow at the basin level, varying from 8-15% across the subbasins. The reductions in mean monthly flows are in the range of 1-56% at the basin level, with June witnessing the highest flow reduction. Flow reductions are comparatively higher in the upstream parts of the basin, as a result of a relatively higher potential of developing rain-fed areas coupled with comparatively lower amount of available runoff. The impacts of S2 are generally small with reductions of 2-5% and 1-9% in mean annual and mean monthly flows, respectively. The results of S3 are in general similar to those of S1. Although the estimated annual flow reductions remain well within the available water resources development potential, measures needs to be taken to avoid excessive flow reductions in May, June and July. It is recommended that only a limited agricultural area should be converted from rain-fed to irrigated agriculture (about 0.1 million ha), and should practice supplementary irrigation. The supplies should also be augmented through developing additional water storage. Adopting such measures is extremely important for the upper subbasins Gamasiab and Qarasou where comparatively higher flow reductions were estimated. 相似文献
145.
146.
石羊河流域主要作物的需水量及需水规律的研究 总被引:3,自引:0,他引:3
试验于2008年在甘肃武威进行,采用水量平衡法在充分灌溉的条件下研究了石羊河流域春小麦、春玉米、制种油葵、色素辣椒、小麦/玉米带田等5种作物的需水量及需水规律,并根据作物生育期需水规律提出了适宜的灌溉制度.试验结果表明,春小麦全生育期需水量为480.5 mm,需水关键期为拔节期和抽穗开花期;春玉米需水量为569.0 m... 相似文献
147.
In order to assess its potential for estimating soil redistribution rates, the naturally occurring fallout radionuclide 210Pbex has been used in parallel with 137Cs, derived from the atmospheric testing of nuclear weapon testing in the 1950s to 1970s, to estimate rates of soil redistribution on a sloping field with traditional erosion control measures located near Jiajia Village, Jianyang County, in the Sichuan Hilly Basin of China. The local 210Pbex reference inventory of 12,860 Bq m− 2 is higher than those reported for many other areas of the world and may reflect the influence of cloudy weather in preventing 210Pb released to the atmosphere across the local region moving up into the upper troposphere, where is would be more widely dispersed. The mean 210Pbex and 137Cs inventories measured in cores collected from the upper part of the field with an average slope of 10° were 8028 Bq m− 2 and 993 Bq m− 2, respectively, and the equivalent values for the lower part of the field, where the slopes are steeper (20°) were 11,388 Bq m− 2 and 1299 Bq m− 2. The pattern of post-fallout 210Pbex and 137Cs redistribution on the sloping field reflects not only the effects of water erosion and redistribution by tillage, but also the local traditional practice of “Tiaoshamiantu”, whereby sediment trapped in the ditches is returned to the fields by the farmer. The estimates of annual rates of soil loss provided by the 210Pbex measurement are closely comparable with those derived from the 137Cs measurements and are consistent with existing knowledge for the study area. The results obtained from this study confirm the potential for using 210Pbex measurement to estimate soil erosion rates over medium-term timescale of 50–100 years. By combining the estimates of erosion rates provided by the 210Pbex and 137Cs measurements, the weighted mean net soil loss was estimated to be 48.7 t ha− 1 year− 1 from the upper subfield and 16.9 t ha− 1 year− 1 from the lower subfield. These rates are considerably lower than the erosion rates obtained from runoff plot measurements in the local area. It is suggested that the traditional erosion control practices and the practice of “Tiaoshamiantu” have a significant effect in reducing soil loss and conserving valuable cultivated soil on sloping fields in the Sichuan Hilly Basin. 相似文献
148.
小江流域土地利用变化及其趋势分析 总被引:1,自引:0,他引:1
运用遥感和地理信息系统技术,提取了小江流域1987,1995年和2000年土地利用空间数据,分析了1987~2000年间小江流域土地利用类型变化的幅度、速度以及类型之间相互转移,并应用马尔柯夫概率模型对小江流域2015年土地利用格局进行了预测。结果表明:13年来小江流域耕地、林地以及城乡工矿用地面积均显著增加,其中耕地增加最为显著,由1987年的345.1km^2增加到2000年的398.7km^2。小江流域土地利用变化速度很快,年均变化速度达0.49%;对单一土地利用类型来说,城乡工矿用地的年均变化速度最大,年变化率达3.52%,其次为耕地和水域。增加的耕地主要来自草地、林地和水域;林地中除部分转化为耕地外,由12.64%的林地退化成草地;草地和未利用地基本保持不变。到2015年,小江流域耕地、林地、水域和城乡工矿用地面积和比例将会增加,草地和未利用地面积和比例将会下降。 相似文献
149.
为研究衡阳盆地农村水资源系统脆弱性动态演变问题,采用GIS/RS方法,以衡阳盆地为研究对象,构建由多年平均降水量、干旱指数(7—9月)、坡度指数、土壤蓄水能力指数、植被覆盖指数、土地利用指数、水源可获得性指数和人类活动指数8个指标组成的评价指标体系,对1990—2010年衡阳盆地农村水资源系统脆弱性进行定量评价,构建模型对其脆弱性指数的时空演变特征进行分析。结果表明:1)衡阳盆地农村水资源系统脆弱性近20年内整体呈现"中高周低,北强南弱"空间分布之势。2)研究期内,脆弱性整体有所增强,空间分异明显,脆弱性增强区域呈条带状南北展布,脆弱性减弱区域则呈团聚状分布在东西两翼。从各时段的变化来看,脆弱性变化大体呈"首尾增长、中间下降"的规律。3)从全时段脆弱性指数变化来看,衡山、衡阳和衡东3县农村水资源脆弱性增强明显,耒阳和祁东2县(市)则以脆弱性减弱为主,常宁和衡南2县(市)各种变化类型相对均衡。4)脆弱性指数变化的空间分异特征大致可以分为4种类型:波动变化型,先减弱、后强势增长型,先增强、后强势减弱型和先强势减弱、后恢复性增长型。5)从不同时间变化趋势来看,脆弱性以波动变化为主要类型,其次是未变化类型,单调增加、单调减少两种变化类型非常少。可见,近20年来,衡阳盆地水资源脆弱性强度整体有所增强,脆弱性指数具有明显的时空分异特征。加强水资源系统的科学管理成为有关部门的紧迫任务。 相似文献
150.