额济纳天然绿洲景观演化驱动因子分析

额济纳天然绿洲位于我国西北干旱内陆河流域黑河流域下游 ,近年来 ,随着黑河中、上游地区下泄地表径流量的减少 ,额济纳天然绿洲景观退化严重。基于研究区近十几年来的景观演化总体特征 ,分别从气候因素、水文因素、人类干扰因素等几个主要的景观变化驱动因子入手 ,探讨其演化机制、揭示其演化规律。研究结果表明 :黑河流域中、上游下泄地表径流量的锐减而导致研究区地下水水位下降、地下水矿化度升高以及水质恶化是额济纳天然绿洲景观发生退化的根本原因 ,绿洲人类活动强度的增加、绿洲关键区域的超载和过牧以及蒸发度指数的升高、湿润系数的下降均加剧了研究区景观退化的程度。

Analysis of landscape change drivers in the Ejina natural oasis

The Ejina natural oasis landscape lies in the lower reaches of the Heihe river basin in the arid inland regions of northwestern China. With decreasing water resources from the upper reaches of the Heihe river basin, many urgent ecological and environmental issues in the Ejina natural oasis have become more pressing in recent years. Using remote sensing techniques and GIS, based on Landsat Thematic Mapper data in 1987 and 1994 and Landsat Enhanced Thematic Mapper Plus data in 2001, the changing patterns of the Ejina natural oasis landscape showed that: (1) During the past 15 years, the water area and main oasis area sharply decreased. The oasis desertification process is more serious at present than before. (2) The most intense conversion among different landscape types was occurring in the transitional zone between oasis landscapes and desert landscapes. (3) As results of the Ejina natural oasis landscape change, the landscape patch complexity and heterogeneity were significantly reduced. Moreover, the landscape texture became increasing coarse and the landscape diversity and stability were decreased. Based on the general characteristics of the Ejina natural oasis landscape during the past decade, some key driving factors, including climate change indicators, hydrological indicators and human activities indicators, were analyzed. The climate change indicators consisted of evaporation degree, wetness coefficient and dryness degree, which were calculated from the meteorologic data of precipitation, air temperature, evaporation and humidity from 1957 to 2001. The hydrological indicators were the annual change of surface water quantity from 1947 to 2001, the spatio-temporal change of groundwater level between 1980s and 2000s, and the change of groundwater quality in recent years. The human activities indicators were changes in total population, livestock quantity and farmland area in the past 50 years. Results show that: (1) Increased evaporation degree, decreased wetness coefficient and enhanced dryness degree in the past decade, especially in recent years, all accelerated the desertification of the Ejina natural oasis landscape; (2) Significantly reduced surface water quantity from the upper reaches of the Heihe river basin, which simultaneously led to descending groundwater level, ascending groundwater mineralization degree and soil saltness content, and badly deteriorating water quality, was the key driver for the degradation of the Ejina natural oasis landscape; and (3) Increased intensity of human activities induced by rising population and livestock in recent decades, overgrazing livestock in the fragile natural oasis regions, and frequently reclaiming and abandoning farmland owing to unbalanced distribution of limited water resources were also important driving factors for the Ejina natural oasis landscape degradation in past decades. Furthermore, owing to the ecological fragilities and the difficulties of ecological restoration for the natural oasis landscapes in arid regions, the authors suggest that research on the natural oasis landscape health assessment and landscape ecological patterns are essential fields in the future for protecting regional landscape health and maintaining regional ecological safety.