Changes characteristics of the soil moisture in semi-arid areas

Analysised the change characteristic of soil moisture in semi-arid area by using the soil moisture data 5～80 cm soil layer observed every half hour in SACOL station from May 2006 to December 2009.The results show that the seasonal changes in soil moisture in 5～40 cm soil layers are obvious,which manifest characteristic of three high and three low trend;in the 80 cm soil layer the seasonal changes are not obvious.The fluctuation of soil moisture in shallow layer is larger than that in deep layer,the average daily range of the soil moisture content of each layer 5～80 cm is 1.08%,0.57%,0.25%,0.11%and 0.04%respectively.The variance of the 10 cm layer is 5.71%,while the 80 cm layer's is only 2.69%.Soil moisture changes with the depth of the soil layer, season and weather background.With the deepening of soil layer,the soil moisture is increasing in winter, however the situation is opposite in other seasons.The time lag of soil moisture between the upper and lower soil layer is increasing when the soil layer deepens,the shortest is 3.5 hours and the longest is 35 hours;but the correlation of soil moisture between the upper and lower soil layer decreases.