Research On Evolution And Driving Factors Of Ecological Environment In Dunhuang City

Under the dual influences of climate change and human activities, the ecological environment of arid oasis in Northwest China is in further deterioration, and the most prominent problem is water resources used over-exploitation by human activities. The vulnerability of environment change induced by human activities increases significantly, the increase in population, over dependence on limited resources and global wanning are all mean that human society and ecological environment will be more sensitive and vulnerable than ever. The climate is extremely arid and ecology is very fragile of Dunhuang City in Gansu province. In recent years, with the continued population growth and arable land expansion, water over-consumption and significant environmental degradation have become serious threats to the existence of the famous human cultural heritage Mogao Grottoes, the nature scenic Mingsha Hill and Crescent Moon Spring, which aroused wide attention at home and abroad. So. this paper took Dunhuang City as a study area, and used Landsat-5 TM remote sensing images in 1987,1990,1996.2000 and 2007, climate observations data from 1954 to 2008, socio-economic statistical data from 1987 to 2007 as the data sources, and by comprehensive application of field survey, "3S" technology, ecological spatial analysis, model simulation and other methods, the ecological environment change process and driving mechanism are analyzed in the last 20 years from different angles and levels, and the way to environmental protection and restoration, the model for test zone of sustainable development are proposed, which can provide basis for the rational use of water-land resources and ecological protection. The basic results and understandings from the thesis are as follows:Firstly, analysis on the general characteristics of land use/cover change in Dunhuang City. Dunhuang oasis is mainly distributed along the river, and in the area where the elevation under 1200m (98.4% of total area), slope less than 1°(61.01% of total area). Over the past 20 years, the unique landscape pattern, oasis surrounded by desert, has not changed in Dunhuang City. For the eight types of land use/cover, non-vegetation area take the largest proportion(above 85%). Secondly, analysis on the temporal and spatial characteristics of land use/cover change in Dunhuang City. From temporal aspect. the spatial extent of vegetation-coverage was shrinking continuously, ecological environment degraded significantly. From spatial aspect, vegetation coverage in the edge region of crop land increased, crop land area expanded outwardly; water-wet land, shrub land, high coverage grassland, moderate coverage grassland and low coverage grassland in other regions shrinked from edge to central, and no-vegetation area showed an opposite trend, which is expanded mainly from edge to outward..Thirdly, analysis on the changing ways, direction, scale and speed of land use/cover types in Dunhuang City. The changing ways of land use/cover are different, which showed a feature that the unidirectional change is the main way, gradually change and retrograde change are fewer relatively. Also, there are big differences in changing scale and speed of land use/cover, part of grassland and non-vegetation area transformed into the cultivation areas, part of water-wet land and shrub land changed into the grassland, and the grassland from high coverage to medium one, low one and non-vegetation land in successive steps. The comprehensive dynamic degree of land use from 1987 to 1996 and 1996 to 2007 are 0.56% and 0.66% respectively, which showed that land use/cover change of the latter phases is more significant than the previous phase.Fourthly, scenario simulation of land-use pattern in Dunhuang City. Based on Markov process model and CLUE-S model, the simulation and prediction are conducted on the land use pattern of Dunhuang City, the results show high accuracy. By modifying the transition probability matrix among different land use/cover types, we set four scenarios types:natural development type, ecological protection type, economic development type and comprehensive development type. It can be seen that from different scenarios, the spatial pattern of land use/cover in study area shows a great difference. Comprehensive development scenario is an ideal development way, because it can make up the above shortcomings, and make more comprehensive consideration in the needs of ecological environment restoration and economic development. In addition, the simulation results are good.Fifthly, analysis on the change of ecological environment factors in Dunhuang City. According to actual quadrat biomass measured and its corresponding fitting curves of land use area, total ground biomass was estimated. The results demonstrate that the total ground biomass decreased by 170856.52t, with an annual average reduction of 5.8%o. Among these, biomass of natural oasis shows a net decrease, while the biomass of artificial oasis shows a net increase,. There is a strong correspondence relationship between the decreasing of total biomass and the decreasing of natural oasis biomass. Based on the size of the biomass and the actual function, the total service value of ecosystem was estimated. The value decreases 43127.44×104 yuan from 1987 to 2007, and average annual decreases 2156.37×104 yuan. With the direct use value increased, the indirect use value reduces quickly. It means that the rapid growth of the value of artificial ecosystems resulted from high loss of the value of natural ecosystems, and showes that human activity has a significant impact on regional sustainable development.Sixthly, analysis on the driving mechanism of ecological environment change in Dunhuang City. During the time scale in this study, the factors that caused ecological environment change in Dunhuang City are mainly come from natural and human aspects. On the background of global climate change, although increased temperature has a certain effect on vegetation growth, land reclamation, irrigation, tourism and other human activities increase the demand for water resources, and diverted ecological water, which are the main reasons for vegetation and natural eco-system degradation. The principal component and stepwise regression analysis show that human factors are the mainly driving factors which caused ecological environment change, especially from the final test results of population growth, agricultural production, technological progress and living standard, they can be come down to the impact on water resources.Seventhly, the influence of human activities on the environment change in Dunhuang City. From the quantity ratio relationship between two land use/cover types which substitute for each other, we can see that crop land has a clearly substitute relationship, increasing of 1.33km2 shrub land at least can substitute 42.57 km2 low coverage grassland at most.Water consumption of natural vegetation decreases with the increased water consumption of human activities, and the total water consumption scale is about 6.3×108m3.Water consumption of natural vegetation decreases from 3.0727×10 m3 to 2.17×108m3 during this period (nearly 30%), cumulative reduction is 9.5017×108m3 for the past 20 years. Water consumption of human activities increases from 3.3157×108m3 to 4.093×108m3 during this period, cumulative addition is 5.9155×108m3 for the past 20 years.From the estimation based on soil-water balance principle, it can be seen that in the conventional surface irrigation conditions, the suitable oasis in study area is 653.45km2,and in water-saving irrigation conditions, the suitable oasis is 909.65km2, which is much smaller than the current actual area.Eighthly, Based on the current severe situation of ecological environment in Dunhuang City. in order to achieve the coordinated and sustainable development among society, ecology and economy, ecological environment need to be protected and restored from the following aspects, sustainable utilization of water resources, ecological restoration and reconstruction, industrial restructuring. system assurance, ecological planning and so on. Meanwhile, sustainable development model of mutual dependence and mutual restriction should be established, including appropriate population growth, circular economy, resource conservation and efficient, eco-environmental protection and cultural heritage protection, which can achieve the faster and better development of experimental zone.