Study on the dynamic change of soil moisture in karst area：A case of Huaxi district in Guiyang City

Huaxi district is located in the south of Guiyang City in Guizhou Province. It bestrides an area between E106°27′-106°52′and N26°11′-26°34′, with a land area of 958.56 km2 and a population size of 4.9×105. Soils are predominated by yellow and lime soils. The area is characterized by humid subtropical monsoonal plateau climate, with the average annual temperature of 14.9 ℃, the average annual rainfall of 1，178.3  mm, annual sunshine time of about 1，278 h, the average annual relative humidity of 78% and frost-free period of about 270 d. 94% of the landmass is karst landform, represented by low hills and lowlands. In order to explore the dynamic change of soil moisture with rainfall, we used Excel 2003, SPSS software and the cluster analysis method to analyze the relationship between the soil moisture and precipitation in the study area. Results show that， (1) The vertical change in soil moisture content is significant at depth in 2013 and 2015, which can be divided into active layer (0-10 cm), sub-active layer (10-30 cm) and relatively stable layer (30-100 cm); (2) The dynamic change of soil moisture is mainly affected by rainfall and evaportranspiration, which can be accurately reflected by the above relationship at the 10-30 cm deep soil layer; (3) The dynamic change of soil moisture has an obvious character of seasonal fluctuation, of which the spring and winter can be identified as the soil water recharge period, while the soil moisture consumption takes place in summer and autumn. The main reasons for the change of soil moisture in karst area are the result of the interaction of rainfall and soil evaporation, plant transpiration and vegetation consumption.