Research On The Characteristics Of Soil Moisture In Southwest Karst Area

Based on 10 cm,20 cm and 50 cm layer soil moisture data,daily temperature,precipitation data and NCEP-R2 reanalysis data of 31 stations in southwest karst area from 1991 to 2013,the temporal and spatial evolution and distribution characteristics of soil moisture in different layers over the years were studied by using linear trend analysis,EOF method,M-K trend test method,sliding T test and Morlet wavelet analysis.A brief analysis of the circulation situation in the regular period is made.The main results are as follows:(1)From 1991 to 2013,the annual average soil moisture of 10 cm ranged from 60.1 to 62.5%.The soil moisture of 20 cm and 50 cm layers ranged from 72.5 to 82.5% and 75.1 to 87.5% respectively.Soil moisture increased with depth.The soil moisture of Guangxi and Western Yunnan increased most with depth,while that of Panxi Plateau did not increase significantly with depth.(2)In the mean season of 1991-2013,the soil moisture of each layer in spring and winter was generally "high in the East and low in the west".The soil moisture of Panxi Plateau,central Yunnan and southern Guizhou were 38.1-50.0%,46.1-60.0% and 45.1-65.0% respectively.The highest soil moisture of Eastern Guangxi and Eastern Guizhou was more than 70%.In summer and autumn,the distribution of "low in the East and high in the west" was observed,while that of Western Sichuan and Yunnan Plateau was in summer and autumn.Significant increase to 75.1-80.0%,Guizhou relatively low altitude areas dropped to 60.1-70.0%.(3)Interannual variations of soil moisture at 10 cm,20cm and 50 cm have obvious spatial distribution characteristics of "rising from south to falling from north",and the climate tendency variations are-3.10-6.60%/10 a,-5.10-6.41%/10 a and-2.07-7.21%/10 a respectively.Guizhou and Chongqing are the main areas of soil moisture decline,while Guangxi,Yunnan and southern Sichuan are the main areas of soil moisture increase.The independent significant modal results of EOF spatial decomposition correspondingly confirm the same.Main characteristics of interannual variation of soil moisture in different layers.(4)The annual variation of soil moisture at 10 cm and 20 cm is lower than that at 50 cm on the whole.1991-2005 is the main rising and fluctuating period,followed by fluctuating and declining period.The annual variation of soil moisture at 50 cm is more stable and the fluctuating trend is less than 10 cm and 20 cm layer.Soil moisture in each season shows an increasing trend in varying degrees from 1990 s to 2000,except in autumn after 2000.The average soil moisture in spring and summer remained basically unchanged throughout the study period,slightly increased in autumn,and increased most significantly in winter.The overall soil moisture was the highest in summer and autumn,and lower in spring and winter.(5)The results of the interannual mutation test of soil moisture in each layer showed that the sites with rising mutation were 18,17 and 14,and the stations with decreasing mutation were 13,14 and 17,respectively.The mutation types were basically similar,but the number of mutations in different layers was different.The decrease mutation of soil moisture in Guizhou was the main one below 10-50 cm,while the rise mutation was the main one in Guangxi and Yunnan.Sichuan and Chongqing accounted for half of the increase and half of the decrease,and the occurrence nodes of the mutation were mainly concentrated between 1998 and 2005.(6)The periods of soil moisture in each layer are mainly 1-10,11-20,21-23 years.The main periods of soil moisture in each layer are different,10 cm layer is 10,8,13 stations,20 cm layer and 50 cm layer are 5,11,15 and 3,12 and 16 stations,respectively.This indicates that the period of soil moisture in deep layer is dominated by long period,while that in shallow layer is little different.(7)Lower soil moisture often accompanied by persistent high temperatures and less rainfall.The atmospheric circulation shows that the location of the high-level South Asia High and subtropical anticyclone extend eastward and westward than conventional year,and the upper part of Southwest of Southwest Karst Region is controlled by high pressure,the vertical transport of water vapor in the lower layer is inhibited.The warm and humid water vapor transport in the bay of Bengal and the South China Sea is weaker than that in the whole year,and it is difficult for the cold and warm air in the lower layer to meet in the Southwest Karst Region.Such a vertical and deep circulation situation in the high and low levels leads to the extremely hot and rainless weather in Southwest Karst Region,which leads to the soil moisture can not be supplemented enough and is in a state of loss of moisture,which leads to a decline.