The Effect Of Rainfall On Soil Moisture Of Initial Planting Alfalfa Land Under Dried Soil Layers Condition In The Loess Hilly Region

In semi-arid Loess Plateau,a large area of artificial forest and grassland has had a problem of dried soil layer for a long time.Dried soil layer is caused by imbalance of soil moisture.Alfalfas were replanted in dried soil.A 0-600 cm large-scale soil column experiment was done in the first year to observe the soil moisture.Soil moisture infiltration process,dynamic change of soil moisture and soil moisture circulation process of replanted alfalfa field under condition of dried soil were discussed.Soil moisture characteristics and soil desiccation change of replanted alfalfa field of artificial forest and grassland in loess hilly region with serious vegetation desiccation were evaluated.The following conclusion is reached:(1)In the context of soil desiccation,the response of soil moisture of the initially planted(1a)alfalfa field at 0-600 cm to rainfall showed lagging with the depth.In case of light rain(<10 mm),soil moisture content within depth of 0～10 cm showed a sensitive fluctuation;in case of moderate rain(<25 mm),soil moisture content within depth of 0～30 cm fluctuated;in case of heavy rain(>25 mm),soil moisture content within depth of 0～50 cm fluctuated.Moisture content at the upper layer increased significantly and gradually decreased with the increase of soil depth.(2)Under different types of rainfall,the heavier the rain was,the deeper the infiltration and migration were.Migration depth was also affected by continuous rainfall.Interaction of multiple rainfalls had a sustainable influence on moisture migration process,and the later rainfall would drive early rainwater stored to migrate downwards.There were only a few times that rainwater infiltrated to 10 cm in initially planted alfalfa field.The effective rates of number of rainfalls and rainfall capacity with valid infiltration depth up to 10 cm were 39.7%and 86.9%respectively.As soil depth increased,the response of soil moisture change to rainfall showed significant vertical migration hysteresis.(3)Surface moisture changed daily due to continuous rainfall.Soil moisture at 0～100 cm infiltrated continuously,soil moisture content at 100～300cm increased slowly,and soil moisture at 300～600 cm showed no significant fluctuation.From May to September,average effective soil moisture storage at 0～100 cm was higher than effective soil moisture storage at 100～300 cm.Soil moisture at 100～200 cm was the same as the soil moisture at 200～300 cm in October.Soil moisture shortage at 0～600 cm showed a trend of progressive increase as depth increased,and soil moisture shortage at 0～600 cm decreased first and then increased in October.From May to September,no dried soil layers at 0～600 cm increased month by month.From May to October,the seriously dried layers decreased.(4)In case of light rain in alfalfa field,soil moisture storage and effective soil moisture storage changed slightly;in case of moderate rain,soil moisture storage and effective soil moisture storage at 0～50 cm increased;in case of heavy rain,soil moisture storage and effective soil moisture storage at 0～60 cm increased,and the average values of soil moisture storage and effective soil moisture storage changed slightly.(5)In the context of soil desiccation,surface moisture circulation of initially planted alfalfa field responded intensely to rainfall,so there were a large number of surface soil moisture circulation.As soil depth increased,the influence of soil moisture on rainfall weakened,so the number of moisture circulation reduced.During experiment,there were 7 moisture circulations at 0～30 cm,5 moisture circulations at 0～100 cm,2 moisture circulations at 100～200 cm and 1 moisture circulation at 200～300 cm.Soil moisture below 300 cm was stable and did not circulate.The response of section soil moisture circulation in initially planted alfalfa field to rainfall showed a certain hysteresis with the depth,which meant the hysteresis was more distinct as depth increased.In typical rainwater circulation,circulation of light rain occurred at shallow layers,while circulation of moderate rain occurred at deeper layers than light rain,and circulation of heavy rain occurred at deeper layers than moderate rain.