Special characteristics and long-term trend of aridity index in the west part of the Tianshan Mountains during recent 50 years

Based on the precipitation air temperature and water surface evaporation data observed at meteorological stations in 9 counties in the west part of the Tianshan Mountains during recent 50 years, a spatial distribution and temporal trend of aridity index values were analyzed by applying the aridity index calculation method. The results show that, this region can be classified as a semi-arid-semi-humid climatic zone according to two different calculation specifications which are based on administration division and watershed division. Within the region, the values of annual aridity index have remarkable regional difference and high inter-annual variation. Besides, frequency for five classes of aridity index occurred in different administrative divisions or watersheds are calculated and they are obviously different. The values of annual aridity index decrease generally during the past 50 years, especially the summer values. Significant reduction of evaporation from water surface is found to be the major influencing factor.