Study On The Ecological Fragility In The Upper Reaches Of Minjiang River Based On PSR Model

The Upper Reaches of Minjiang River is an important ecological barrier of the Upper Yangtze River Basin. Interacted by its topography, climatic conditions and excessive human interference over a long time, the ecosystem in the Upper Reaches of Minjiang River severely degraded. Its ecological environment was extremely fragile. Ecological fragility evaluation in this area could not only help to understand the impact of climate change and human activities on the eco-environment, but also contribute to reveal regional differences of ecological fragility in this mountain area. The achievement could further provide scientific guidance for regional eco-environment restoration and reconstruction, as well as regional sustainable development.First, based on analyzing the principal factors which caused ecological fragile in the upper reaches of Minjiang River, the index system for ecological fragility evaluation was set up adopting ‘pressure- state – response'(PSR) model. Secondly, based on the meteorological data, remote sensing image, landuse, statistical data, and other geographical data, combined with RUSLE model, Information Content model, Hargreaves model and the Spatial Grid method, the spatial database of ecological fragility assessment were established. Then, AHP method was used to calculate the weight, and the linear weighted summation method was used to calculate the ecological fragility index(EFI) in 1996, 2000, 2005, 2010 and 2014 year. Finally, on the basis of calculation results, including coefficient of variation, centroid shift, and transfer matrix, spatial differences and temporal evolution characteristics of ecological fragility in the area were analyzed. The main results were as follows:(1) The mean variation coefficent of EVI in the upper reaches of Minjiang River in 1996, 2000, 2005, 2010 and 2014 was 0.054, the fragility were relatively stable as a whole. The regions with relatively obvious change mainly locaded in the northern of Songpan County, Minjiang River Valley of central Songpan County, northeast of Lixian adjacent to Maoxian, and east Lixian adjacent to Wenchuan County. During the study period, the centroid of ecological fragility migrated gradually towards the southwest, and mainly in latitude direction. The magnitude of centroid offset on different fragility intensities descended as: potential fragile > extreme fragile > mild fragile > moderate fragile > severe fragile.(2) The EVSI in the upper reaches of Minjiang River in 1996, 2000, 2005, 2010 and 2014 were all between 2 to 3. The fragility intensities were mainly mild and moderate, which accounted for more than 80 percent of the total area. The ecological fragility ordered for 5 period as 1996 > 2010 > 2005 > 2000 > 2014. Ecological fragility was gradually improved during the study period overall. The ecosystem was most fragile in 1996. In 2000, about 52.79 percent ecosystem converted to low fragility. From 2000 to 2005, about 23.98 percent ecosystem converted to high-intensity fragile. The fragility was similar in 2005 and 2010. The areas of increased-intensity and weakened-intensity were substantially equal. Compared to the average level for many years, the areas of moderate, severe and extreme fragile regions decreased recently. It reflected that the eco-environment was improved to some extent.(3) Potential fragile area of the Upper Reaches of Minjiang River was mainly distributed in the northwest of Songpan County, Heishui County and Maoxian County. Meanwhile, moderate fragile mainly distributed in the both sides of the valley of the trunk stream and tributaries of Minjiang River. While severe and extreme fragile areas were relatively small, which were mainly distributed in the internal of dry valley in Minjiang River, and near Hongshiba in the northern Songpan County, the junction of northwest Wenchuan County and Lixian, as well as the region of Weiguan Ditch to Rock Dam in Lixian.(4) The ecological fragility difference in the upper reaches of Minjiang River was significantly influenced by topographic and moisture variation. Those areas, which was in low and moderate altitude, and where human activity were centered, were moderate fragile. The area proportion of moderate fragile was accounted for 51.72 and 61.10 respectively in low attitude and middle attitude area. So, the fragility was more severe in low and moderate altitude affected by frequent human disturbance and geological disaster-prone. On the contrary, the regions in higher altitude had small population. Meanwhile, the woodland and grass land were widely distributed there. So, they were less fragile. Moreover, Sub-humid zone in the study area, due to its plentiful water and heat conditions, was mainly moderate fragile, where the mild fragility intensity was accounted for 51.82% area. Meanwhile, Semi-arid zone was more fragile although its fragility was moderate as a whole, because its moderate, severe and extreme fragile areas were accounted for 51.82% area. Furthermore, each county in the upper reaches of Minjiang River had different level of fragility. According to the comparison of ecological fragility, the 5 county were ordered as: Wenchuan>Maoxian>Lixian>Heishui>Songpan.