Freezing-thawing process and soil moisture migration within the black soil plow layer in seasonally frozen ground regions

The freezing-thawing process,ground temperature and moisture migration within the black soil plowlayer were analyzed with the measured data of soil temperature and moisture within the black soil plowlayer at the Water Conservancy Comprehensive Experimental Research Center of Heilongjiang Province on the outskirts of Harbin,Heilongjiang Province of China from November 2011 to April 2012,where a typical middle-deep seasonally frozen soil experimental observation field has been set up. In this paper,the freezing-thawing process of middle-deep seasonally frozen plowlayer of black soil on shady slopes and sunny slopes are analyzed,including the characteristics of the freezing and thawing process at different depths of black soil plowlayer. At different depths of the black soil plowlayer,soil moisture migration and soil moisture change with precipitation at different depths are also discussed. The results showthat in middle-deep seasonally frozen ground regions,black soil under the plowlayer usually has a freezing-thawing period of 164 days,approximately 5. 5 months,which can be divided into five stages. Stage I lasts 18 days,when the plowlayer of surface black soil begins daily frozen state by day; at this stage the plowlayer of black soil thawing by day and freezing at night,with a freezing-thawing cycle in every day; on the sunny slopes of plowlayer of black soil,before 08: 00 and after20: 00 the air temperatures is lower than soil temperature,and the air temperature is higher than the soil temperature at 14: 00 at the depths of 1 cm and 5 cm; on the shady slopes of the plowlayer of black soil,ground temperature at 08: 00,14: 00 is lower than air temperature,and at 20: 00 the air temperature is the same as ground temperature at the depth of 1 cm basically. Stage II lasts 72 days,black topsoil is in a frozen state by day,with a frozen deep going down; at the same depth of soil temperature on shady slopes is lower than that on sunny slopes. Stage III lasts 22 days,which is a soil stably frozen period,when all the frozen layer of soil presents completely frozen state; at 08: 00 and 14: 00 air temperature substantially exceeds soil temperature on shady slopes. Stage IV lasts 52 days,when black topsoil thaws by day and freezes at night,in an alternating state between day and night,soil freezing-thawing cycle gradually develops downwards; on sunny slopes soil thawing is deeper and earlier than that on shady slopes; on shady slopes topsoil experiences 47 freeze-thawcycles,and on sunny slopes it is 39. Stage V is a stably thawing period,when alternate freezing and thawing does not present in the thawing process until the frozen layer entirely thawed. In the seasonally frozen ground regions of black soil plowlayer,soil temperature continuous curve functional forms of expression are in line with cyclical fluctuations in ground temperature equations. After the whole period of freezing and thawing,soil moisture content on shady slopes and sunny slopes of black soil plowlayer is greater than that before freezing. The black soil plowlayer soil moisture on shady slope is significantly lower than that on sunny slope. On the sunny slopes,within the black soil plowlayer the maximum depth of soil moisture is at the depth of 5 cm in the whole period of soil freezing and thawing cycles. On the shady slopes,soil moisture change within black soil plowlayer is not obvious. But on shady slope,within black soil plowlayer topsoil moisture changes less at the depth of 1 cm,5 cm and 10 cm,which significantly dependents on precipitation in the thawing period. Again,soil temperature change will affect the soil moisture content variation in seasonally frozen ground regions with topsoil black soil freezing and thawing cycles.