Abstract: Arid valley is characterized by fragile and sensitive ecological environment. According to the climatic, vegetation and soil features,arid valley can be divided into three types: arid hot valley,arid warm valley and arid genial valley. The arid valley in the upper reaches of Minjiang River,Sichuan Province,China,belongs to the typical arid warm valley. It is also the center of regional population distribution and the core area of social economy and culture development. So,the spatial definition of arid valley is the basis for ecological and geographical research in the area. However,there is not a mature and complete system to define the range of arid valley. In view of this,firstly,based on the meteorological observation data from 1961 to 2014,as well as DEM,soil type,land use data,etc.,this paper built an index system for diagnosing the characteristics of arid valley. Then,using Mann-Kendal nonparametric test method,the abrupt climate change in the study area was analyzed in nearly 50 years. Finally,the AHP method and linear weighted summation method were adopted to calculate the index for reflecting the characteristics of the arid valley. According to the calculated index,the spatial rang of the arid valley in the upper reaches of Minjiang River were quantitatively defined. What's more,its range variation of the two periods before and after the climate change was analyzed. The results showed as follows:(1)Under current climatic conditions(1982 to 2014),the total length of the arid valley of the upper reaches of Minjiang River was about 151.63 km,and the total area was about 705.62 km²,which was accounting for 2.94% of the administrative area. The arid valley was mainly distributed on the both sides of the main stream and the tributary of Minjiang River.(2)In 1981,the climate changed abruptly in this area. The total area of the arid valley in the period before the abrupt climate change(1961 to 1980)was about 624.02 km². The total area increased 81.61 km² in the next period after the climate change,and the length increased 20.87 km. Compared to the spatial range of two periods, the arid valley broadened its range and expanded to the north in current 30 years.(3)The arid valley in the upper reaches of Minjiang River was mainly covered by shrub woodland and moderate-high grassland. Compared to 1986,the area of farmland and construction land had increased 11.37 km² and 9.86 km² respectively in 2014. Global warming and human activity jointly caused the expanding of the arid valley.(4)Compared to the result of field surveys by Institute of Mountain Hazards and Environment,CAS,the difference of the calculated area of arid valley in this paper was 14.38 km²,and the relative error was 2.00 percent. So,setting up an index system for diagnosing the characteristics of arid valley to define its spatial range was feasible. The result of arid valley range displayed high accuracy. It could provide a new idea and method for defining the range of the arid valley. Furthermore, it laid the foundation data for ecological research in the arid valley area.

Key words: arid valley, spatial definition, range Variation, abrupt climate change, the upper reaches of Minjiang River