Study On Variations Of The Hydro-thermal Characteristics Of Frozen Under Different Vegetation Types In Qilian Mountain

Qilian Mountain is located in the intersection of the two natural areas of the northwest inland arid area and the Qinghai Tibet Plateau.It is an important ecological security barrier in the northwest.The changes of water resources and ecological environment have an important impact on the distribution of water resources and the balance of ecological environment in the Northwest Cold Region.Therefore,it is of great significance to understand the characteristics of soil water and heat change and soil freeze-thaw under different vegetation types for the rational utilization and distribution of Qilian Mountains and waters resources and the protection of Qilian mountain ecological environment.Based on the data of soil temperature,soil humidity and soil freeze-thaw from October 2015 to September 2017,the changes of soil temperature and humidity and soil freeze-thaw in the main distribution areas of the root system of Picea crassifolia forest,shrub and grassland in the same small environment were studied,and the soil was analyzed the main conclusions are as follows:1)The interannual change of soil temperature of 0-60 cm under different vegetation types shows periodic change,and the change curve is sine curve,which is basically consistent with the change trend of temperature and solar radiation;the interannual change of soil humidity also shows periodic change,and the change curve is "U" curve.With the increase of soil depth,the amplitude of soil temperature and humidity gradually decreases,and the phase change has a certain lag.With the increase of soil depth,the amplitude of daily change of soil temperature gradually decreases.There was no significant diurnal change in soil moisture,but only in 0-10 cm layer of grassland,the diurnal change of soil moisture could reach 1.1 %.2)Picea crassifolia forest and shrub began to freeze in late October and thaw in April;grassland began to freeze in late November and thaw around March.There are differences in soil freezing rate,Picea crassifolia forest > grassland > shrub is the slowest;there are also differences in ablation rate,grassland > Picea crassifolia forest > shrub.The soil freezing and thawing of Picea crassifolia forest and shrub showed oneway freezing and one-way ablation,while that of grassland showed one-way freezing and two-way ablation3)The fitting analysis of soil temperature and moisture showed that the fitting effect of exponential curve was the best for Picea crassifolia forest,and polynomial curve for shrub land and grassland.There is a good power function fitting relationship between the unfrozen water content and the absolute value of the negative temperature of the soil in Picea crassifolia forest and shrub land,and the polynomial fitting is the best.