Variations in Soil Moisture at Different Time Scales of BJ Site on the Central Tibetan Plateau

The temporal and spatial variations in soil moisture play an important role in the water and energy cycle on the Tibetan Plateau.Based on the observed high resolution hourly soil moisture data(4-210 cm) obtained by GAME-Tibet at BJ site on the central Tibetan Plateau from 1 January 2001 to 31 December 2005,the diurnal,seasonal and interannual variation characteristics in soil moisture are analysed.The results are as follows.(1) The average diurnal variation in soil moisture at 4 cm soil depth is evident.However,the average diurnal variation in soil moisture at 20-210 cm soil depth is extremely weak.The average diurnal amplitude(ADA) of soil moisture decreases gradually with the increase of soil depth.But the ADA of soil moisture at 210 cm soil depth increases slightly.The ADA of soil moisture at 4,20,60,100,160 and 210 cm soil depth are 0.97,0.22,0.03,0.01,0.01 and 0.03%,respectively.(2) According to the variation characteristic in soil moisture at different time scales and vertical section,the variation in soil moisture throughout the year can be divided into three phases: accumulation phase(March to August),attenuation phase(August to December) and the relatively stable phase(December to March),respectively.(3) During the period 2001 to 2005,the soil moisture at 4,20,60,100 and 160 cm soil depth in August increases with a linear trend.Conversely,the soil moisture at 210 cm soil depth in August decreases with a linear trend.During the wet season,soil moisture has an obvious response to precipitation;however,during the dry season,soil moisture affected by soil temperature significantly.