塔里木河下游植被和景观格局变化及其驱动力分析

利用RS和GIS手段相结合的方法研究了塔里木河下游的植被和景观变化，并分析了由于历史时期塔里木河水流量的变化而导致的地下水位变化、环境因子和人类活动对植被及景观格局的影响。 对环境因子与植被覆盖变化关系的分析结果发现： ⑴在1987年至2004年占研究区总面积26％的景观发生变化；而植被和水体的变化最大；植被的变化中荒漠化和转化为农田是其主要的变化类型。1987年至2004年，仅有占总面积39％的植被没有发生变化，而发生荒漠化和盐碱化的植被仅有少数能够恢复。 ⑵在选取的环境因子中，植被缀块的起始NDVI和植被缀块距河道的距离对植被变化的影响最大，其次为海拔、坡度、缀块形状和缀块距离居民点的距离；而坡向和缀块面积与植被变化的关系较小。发生荒漠化的植被缀块距离河道较远且自身的NDVI较低，表明这些植被缀块的变化主要是受到水资源的限制；而转化为农田的植被缀块自身的NDVI较高且距河道较近，表明人类的开垦活动主要在植被生长较好的地区进行。研究期间内能够保持的植被距河道较近且NDVI较低，这与能够保留的植被主要是较稀疏的胡杨林有关。 ⑶植被缀块距河道的距离在1987年至1999年间与植被的变化显著相关，而在1999年至2004年相关不显著，这表明影响植被变化的影响环境因子随时间变化。 ⑷虽然所选取的环境因子对植被变化的解释力偏低，但在某些时间段对植被变化的解释力可以达到40％。如果增加环境因子，使用梯度分析的方法可以较好的解释环境及其它因子对植被变化的影响。 对地下水位变化对植被及景观时空格局变化的研究结果发现： ⑴在地下水水位剧烈下降的1986年至1999年，植被面积减少30％，距离河道最近的区域减少最多，植被占总面积的比例和植被的减少随距离河道距离的增加而减小；平均NDVI降低；荒漠面积增加4.8％，年均增加1338公顷；盐碱地面积增加。由于生态应急输水工程的实施，1999年至2004年期间沿塔里木河道地下水位回升，植被面积增加48％，距离河道最近的区域面积增加最明显，植被面积增加的增幅随距离河道距离的增加而减小；荒漠面积减少8.3％，年均减少6280公顷；平均NDVI升高；荒漠面积减少；盐碱地面积在某些区段增加，某些区段减少。 ⑵1986年至1999年胡杨林、柽柳灌丛和盐生草甸缀块密度增加，植被趋于破碎化；总体上胡杨林、柽柳灌丛和盐生草甸的缀块形状变得规则，而盐生草甸在某些区段趋于不规则；景观的聚集度在河道左侧增加而右侧减小；景观多样性和均匀性均下降。1999年至2004年，总体上胡杨林、柽柳灌丛和盐生草甸的缀块密度减小，植被趋于整合；胡杨林、柽柳灌丛和盐生草甸缀块形状变得不规则，但盐生草甸在某些区段变得相对规则；景观聚集度指数变化没有明显的规律；景观多样性和均匀性均增加。 ⑶除植被占总面积的比例在沿河道由上段到中断到下段方向上呈现减少的趋势，其它植被及景观特性在沿河道纵向上变化规律不明显；而在沿河道垂直方向上，植被及景观特性的变化表现出与距离河道的距离相关，表明该地区的植被及景观特性是受到地下水控制的。 ⑷在塔里木河实施的生态应急输水工程对当地植被的恢复起到了一定的作用，但主要局限在沿河道附近的区域。对于干旱区植物的保护，合理使用径流水资源，保证水资源的供应畅通是更好和更长远的发展策略。 对人类活动对景观的影响的研究发现： ⑴1987年至1999年，农田面积增加4240公顷，年均增加353公顷；植被面积减少4884公顷，年均减少407公顷。1999年至2004年，农田面积增加4568公顷，年均增加913公顷；植被面积减少8487公顷，年均减少1697公顷，远大于农田面积的增加。 ⑵农田平均缀块面积和农田最大缀块面积从1987年至1999年至2004年增加；聚集度指数在1987年至1999年降低，而1999年至2004年升高。最大植被缀块面积、平均形状指数和聚集指数从1987年至1999年至2004年均下降；其它植被及农田景观指数在1987年至1999年和1999年至2004年间变化不一致。在距河道1km的范围内，农田缀块数、农田面积、最大农田缀块面积、平均缀块面积、平均形状指数以及聚集度指数在从1987年至1999年至2004年一直升高；距河道1km范围内的景观多样性和均匀度指数和平均形状指数在1987年至1999年至2004年增加，而聚集度指数和最大缀块指数在1987年至1999年至2004年一直减小。 ⑶研究结果表明人类活动-主要是开垦农田-导致了当地植被面积的减少，并导致植被破碎化。受到径流来水量减少的影响，人类开垦活动正在逐渐向距离河道近的区域转移，并导致植被向退化的方向发展，河道附近的区域是塔里木河下游植被分布的主要区域，这对该地区植被的维持和保护非常不利。 在塔里木河下游，地下水位的下降和人类活动都导致植被趋于退化，人工输水虽然能够恢复植被，但其效应主要集中在河道附近的地区，对于该生态关键地区植被的保护，合理的使用水资源并适当的进行人类活动才是植被及生态保护的根本。

Land-cover changes in an urban lake watershed in a mega-city, Central China

Urbanization can exert a profound influence on land covers and landscape characteristics. In this study, we characterize the impact of urbanization on land cover and lacustrine landscape and their consequences in a large urban lake watershed, Donghu Lake watershed (the largest urban lake in China), Central China, by using Landsat TM satellite images of three periods of 1987, 1993 and 1999 and ground-based information. We grouped the land covers into six categories: water body, vegetable land, forested land, shrub-grass land, open area and urban land, and calculated patch-related landscape indices to analyze the effects of urbanization on landscape features. We overlaid the land cover maps of the three periods to track the land cover change processes. The results indicated that urban land continuously expanded from 9.1% of the total watershed area in 1987, to 19.4% in 1993, and to 29.6% in 1999. The vegetable land increased from 7.0% in 1987, 11.9% in 1993, to 13.9% in 1999 to sustain the demands of vegetable for increased urban population. Concurrently, continuous reduction of other land cover types occurred between 1987 and 1999: water body decreased from 30.4% to 23.8%, and forested land from 33.6% to 24.3%. We found that the expansion of urban land has at least in part caused a decrease in relatively wild habitats, such as urban forest and lake water area. These alterations had resulted in significant negative environmental consequences, including decline of lakes, deterioration of water and air quality, and loss of biodiversity.

A Novel Inverse Method For Determining The Refractive Indices Of Medium And Dispersed Particles Simultaneously By Turbidity Measurement

The refractive indices of particles and dispersion medium are important parameters in many colloidal experiments using optical techniques, such as turbidity and light scattering measurements. These data are in general wavelength-dependent and may not be available at some wavelengths fitting to the experimental requirement. in this Study we present a novel approach to inversely determine the refractive indices of particles and dispersion medium by examining the consistency of measured extinction cross sections of particles with their theoretical values using a series of trial values of the refractive indices. The colloidal suspension of polystyrene particles dispersed in water was used as an example to demonstrate how this approach works and the data obtained via such a method are compared with those reported in literature, showing a good agreement between both. Furthermore, the factors that affect the accuracy of measurements are discussed. We also present some data of the refractive indices of polystyrene over a range of wavelengths smaller than 400 nm that have been not reported in the available literature. (C) 2008 Elsevier Inc. All rights reserved.

Laser-induced changes on the complex refractive indices of phase-change thin film

The refractive indices of crystalline phase-change films are usually obtained by thermal-induced crystallization. However, this is not accurate, because the crystallization of phase-change film in rewritable optical disks is laser induced. In this study, we use the initializer to crystallize the phase-change films. The dependence of the refractive index n and the extinction coefficient k of the phase-change films on the initialization conditions are studied. Remarkable changes of the refractive indices (especially k) are found when the initialization laser power density is 6.63 mW/mum(2) and the initialization velocity is 4.0 m/s. At the same time, the structure changes of the phase-change films are also studied. This dependence is explained by the structure change of the films. These results are significant in improving the accuracy of optical design and the thermal simulation of phase-change optical disks, as well as in the study of phase-change optical disks at shorter wavelengths. (C) 2003 Society of Photo-Optical Instrumentation Engineers.

Effective medium model for refractive indices of thin films with oblique columnar structure

The refractive indices of thin films, containing dielectric and voids in an oblique columnar structure, are, modelled in the quasi-static limit. The dielectric function is shown to be strongly dependent on the angle of incidence and on the columnar orientation for p-polarized light. This model is applied to model ZnS thin films with oblique columnar structures and the computed results have been given.

Extended effective medium model for refractive indices of thin films with oblique columnar structure

The refractive indices of thin films, containing dielectric and voids in an oblique columnar structure, are modeled by extended effective medium in the quasi-static limit. The dielectric function is shown to be strongly dependent on the angle of incidence and on the columnar orientation for p-polarized light. This model is applied to model ZrO2 thin films with oblique columnar structures and the computed results, with the Maxwell Garnett, the Bragg-Pippard, and the Bruggeman formalisms, have been given. (c) 2004 Elsevier B.V. All rights reserved.