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   796篇
  免费   44篇
  国内免费   136篇
安全科学   8篇
废物处理   6篇
环保管理   142篇
综合类   436篇
基础理论   187篇
污染及防治   15篇
评价与监测   58篇
社会与环境   113篇
灾害及防治   11篇
  2024年   7篇
  2023年   14篇
  2022年   39篇
  2021年   20篇
  2020年   23篇
  2019年   23篇
  2018年   20篇
  2017年   23篇
  2016年   35篇
  2015年   30篇
  2014年   39篇
  2013年   56篇
  2012年   43篇
  2011年   58篇
  2010年   57篇
  2009年   46篇
  2008年   49篇
  2007年   39篇
  2006年   53篇
  2005年   44篇
  2004年   30篇
  2003年   38篇
  2002年   29篇
  2001年   21篇
  2000年   21篇
  1999年   23篇
  1998年   10篇
  1997年   9篇
  1996年   7篇
  1995年   10篇
  1994年   7篇
  1993年   5篇
  1992年   5篇
  1991年   4篇
  1990年   6篇
  1989年   2篇
  1988年   3篇
  1987年   3篇
  1986年   4篇
  1985年   3篇
  1983年   3篇
  1982年   1篇
  1981年   1篇
  1980年   3篇
  1979年   2篇
  1978年   1篇
  1977年   3篇
  1974年   1篇
  1972年   2篇
  1971年   1篇
排序方式: 共有976条查询结果,搜索用时 31 毫秒
931.
ABSTRACT: A canopy reflectance model is incorporated into a routine for simulating water and energy flows in the soil-plant-atmosphere system. The reflectance model is structured tocalculate canopy albedo throughout each simulation period and to determine spectral reflectances at a specified time during the day. Spectral vegetation indices are then calculated from the reflectances and related to the evapotranspiration and thermal response of the canopy. The canopy reflectance model is also used to establish the photo-sytheticaily active radiation load at various depths in the canopy. Stomatal resistances are calculated using these radiation values and integrated to give the minimum canopy resistance. Actual canopy resistance is obtained by adjusting minimum canopy resistance for environmental stresses such as leaf water potential and leaf temperature. Using data for a soybean canopy, canopy evapotranspiration and temperatures are simulated for a range of leaf area index values and compared with the corresponding spectral vegetation indices. The resuits indicate that the normalized difference spectral index has an inverse linear relationship with canopy temperature, concurring with results obtained from satellite observations. The possibility of using a spectral vegetation index and thermal observations together to parameterize surface moisture availability for evapotranspiration is considered.  相似文献   
932.
A technique is presented for developing an isohyet map for the Hualapai Valley, a closed hydrologic basin of about 315 square miles in the northwestern Great Basin in Nevada. In this basin there is practically no climatic data, and in the northwest Great Basin there are too few stations for determination of rainfall on a detailed basis. Using a vegetational typing to represent a range in elevation and precipitation, an initial mean annual rainfall is determined for selected points on a grid pattern. This rainfall is then modified by using topographic parameters of slope, orientation, exposure, and rainfall shadow effect. The resulting point determinations of mean annual rainfall are then smoothed using a trend surface analysis, and an isohyetal map is drawn from the smoothed points. The technique provides an estimated accuracy of one inch of mean annual precipitation and one mile of resolution on isohyets.  相似文献   
933.
Precipitation, soil moisture, runoff, and vegetation were measured on two, 5- to 10-acre, big sagebrush (Artemisia tridentata) watersheds and two, equally small, beardless bluebunch wheatgrass (Agrophron inerme) watersheds that were converted from big sagebrush in 1967. The watersheds are located near Wolcott, Colorado, at an elevation of 7,200 feet, and are mantled with 2 to 3 feet of silty clay soils. Annual precipitation was about 13.5 inches; about 9 inches occurred as rain or snow from April through October and about 4.5 inches accumulated as a snowpack from November through March. Evapotranspiration was about 2 inches greater in 1968 and 1 inch greater in 1969 from the sagebrush watersheds than from the grass watersheds. With a mature stand of grass in 1970 and 1971 the differences in evapotranspiration were within the range of differences measured during the 3-year calibration period when all four watersheds were sagebrush. Water use was similar in the top 1 foot of soil but slightly more water was used by the grass in the 1- to 2-foot zone and more water being used by the sagebrush below 2 feet. Soil-water potential data indicated that only the big sagebrush used a small amount of water from the fractured shale at depths below 40 inches. Sagebrush used more water in August and September than the grass.  相似文献   
934.
/ Three large rivers in northern Finland, the Kemijoki, Iijoki, and Oulujoki rivers, were dammed for hydropower generation in the 1940s-1960s. Due to differences in environmental conditions, these impoundments require detailed study to produce guidelines for fisheries management.Water quality, hydrology, vegetation, and geomorphology data of 16 impoundments were gathered. Shoreline land-use data were derived from maps, and fish assemblage data were collected by exploratory fishing and from the annual fishery statistics. The relations among environmental variables were studied, and a classification of the impoundments was developed by hierarchical cluster analysis. Consequently, three impoundment groups with different environmental characteristics were formed. Significant differences among impoundment groups were also detected in fish yield. We conclude that the variation in environmental conditions, together with differences in fish communities in the impoundments were important enough to justify the claim that impoundments of different types require different management strategies. KEY WORDS: Impoundments; River regulation; Fish assemblage; Water quality; Aquatic vegetation; Finland  相似文献   
935.
Organochlorine pesticide(OCP) concentrations in tree bark from the upper Yellow River region were determined. Total OCP concentrations ranged from 2.7 to 82 ng/g dw, with a mean of 20 ng/g dw. Concentrations of total(Σ) DDTs(0.49–37 ng/g dw), HCHs(0.55–4.5 ng/g dw), and HCB(0.1–1.0 ng/g dw) were much higher than the other OCPs and accounted for 89% of the ΣOCP concentrations. p,p′-DDT was the dominant member of the DDT pesticide group and-HCH was the dominant HCH isomer. The p,p′-DDT/p,p′-DDE and-HCH/-HCH ratios suggested that there were recent DDT and-HCH releases. OCP concentrations in the air were estimated from the tree bark, and the estimated median ΣDDTs, ΣHCHs, and HCB concentrations in the air were 0.09, 0.14, and 0.024 pg/m3, respectively. The relationship between total OCP concentrations and socioeconomic and natural environment indices were assessed using multiple linear regression analysis, and a regression equation including all these factors was obtained. Population density and tertiary industry were the two dominant factors that appeared to affect OCP concentrations in the upper Yellow River region.  相似文献   
936.
以印江槽谷型喀斯特石漠化地区11种植被类型(人工纯林、人工混交林、天然纯林、天然混交林、疏林、竹林、经果林、灌木林、石山地、宜林地、草地)为研究对象,分析了植被碳储量的空间分布格局,并对区域植被固碳速率、碳储量进行了估算,并预测了理论最大固碳潜力。结果表明:印江研究区植被碳储量空间分布为,乔木层(25.06 t/hm2) > 灌木层(3.51 t/hm2) > 草本层(1.10 t/hm2),其平均固碳速率为10.63 t/(hm2·a),植被碳储量为172.23×103 t,植被理论最大固碳潜力为94.02 t/hm2。研究结果对于评价和估计印江槽谷型喀斯特石漠化地区森林的碳汇功能,以及提高碳储量有重要意义。  相似文献   
937.
为了揭示退耕还林(草)、封山育林和石漠化综合治理等生态恢复工程驱动下喀斯特地区的植被覆盖度的时空变化情况,以黔中为研究区,利用2000年、2005年和2010年三期的MODIS影像数据,通过NDVI像元二分模型法,对研究区植被覆盖进行了动态变化评估。结果表明:2000年到2010年,极高植被覆盖度面积持续增长,高植被覆盖度面积持续减少,极低植被覆盖度、低植被覆盖度和中等植被覆盖度面积先增后减,植被覆盖总体呈现转好趋势;此外,植被覆盖景观斑块数减小,平均斑块面积增大,香农多样性指数减小,表明这一时期内植被覆盖景观的破碎化程度逐渐降低,生态恢复工程取得良好实效。  相似文献   
938.
基于MODIS EVI的重庆植被覆盖变化的地形效应   总被引:12,自引:0,他引:12  
基于2000—2015年的MODIS EVI数据,采用趋势分析结合地形差异修正,分析了重庆市植被覆盖变化在高程、坡度和坡向上的空间分布差异。结果表明:1)近16 a来,重庆植被以低覆盖度和中覆盖度为主,低覆盖度呈下降趋势,劣覆盖度、中覆盖度和高覆盖度呈上升趋势;2)研究区植被显著减少趋势占3.84%,基本不变占81.12%,显著增加占15.04%,植被变化总体上呈恢复趋势;3)在地势低(400 m)、坡度小(6°)的区域,植被覆盖度低,但变化趋势显著;在400~1 200 m、6°~15°的区域,植被变化趋势分布出现由优势到非优势或由非优势到优势的转折;4)在地势高(1 200 m)、坡度大(15°)的区域,植被覆盖度高,变化趋势较弱,但是在高程大于1 500 m、坡度大于25°的区域也存在植被减少的现象;5)不同坡向上,除了在平地区域植被变化趋势较显著外,其余坡向差异不明显。  相似文献   
939.
矿区不同植被复垦模式对土壤细菌群落结构的影响   总被引:4,自引:2,他引:2  
以安太堡露天煤矿复垦区为研究对象,用PCR-DGGE技术分析不同复垦植被(榆树、落叶松、杏树、云杉和刺槐)及复垦年限(15年和20年)对土壤细菌的影响.土壤细菌多样性分析结果表明:复垦20年组,榆树最高,杏树最低,其余3个植被无显著差异;复垦15年组,云杉显著高于刺槐;刺槐随复垦年限延长,其土壤细菌多样性显著增高,而云杉却反之.相似性系数分析、聚类分析和PCA均显示,相同复垦年限的土样细菌群落结构相似性高.相关性分析表明细菌多样性指数和土壤pH显著正相关.优势和差异条带测序鉴定出Nitrospira、Sphingomonas、Arthrobacter、Brachybacterium、Rhizobium以及Mesorhizobium等或参与氮循环、或降解多环芳烃及杂环有机物的细菌属.本研究说明榆树和云杉有利于土壤细菌多样性的恢复;复垦区土壤的优势菌群多为有利于污染土壤的生态修复和肥力恢复的功能细菌属.  相似文献   
940.
地形和植被会改变水分在土壤中的运移,进而影响土壤中硝态氮(NO_3~--N)的分布,并可能导致对水体污染的差异.在黄土高塬沟壑区黑河流域选取3个样点,采集刺槐林和草地在不同坡位(上、中和下坡位)的6 m深土样,分析了坡位和植被对NO_3~--N迁移的影响,并初步评估了其对地表水及地下水污染的潜在风险.不同坡位及植被条件下,土壤中硝态氮均没有出现累积,在表层土壤达到最大值后逐渐减小.2种植被下NO_3~--N达到稳定时的深度约为200 cm,稳定浓度均为下坡位上坡位中坡位,但在同一坡位的稳定浓度均有草地高于刺槐林的特点,说明坡位及植被覆盖类型均会影响NO_3~--N在土壤中的分布.整个流域地表水NO-3含量枯水期及汛期分别为(6.90±2.10)mg·L~(-1)和(5.84±2.86)mg·L~(-1),而坡地表层土壤(0~20 cm)中可移动态NO_3~-为(29.55±6.59)mg·L~(-1),明显大于地表水中的浓度,很有可能随径流流失造成地表水氮素污染.地下水枯水期和汛期的NO_3~-含量分别为(24.61±23.72)mg·L~(-1)和(15.70±10.78)mg·L~(-1),而坡地深层土壤(200 cm)中NO-3为(0.78±0.16)mg·L~(-1),由于浓度较低,对地下水造成污染的可能性较小.  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