Temporal And Spatial Distribution Characteristics Of Soil Moisture And Soil Moisture Simulation In Deep Profile Of Critical Zone In Chinese Loess Plateau

The Loess Critical Zone(LCZ),as a typical environmental sensitive and ecologically fragile zone,ecological construction and vegetation restoration are particularly important in this area.Deep-going research of the spatial distribution,dynamic changes and feasible deep soil moisture prediction methods is of great significance for soil moisture research and vegetation ecological construction in LCZ.In this paper,two 18 m deep profiles under different land use types were selected in the Wangdonggou watershed,Changwu,in the semi-humid climate zone,and four21 m profiles with different land use types were selected in the Liudaogou watershed,Shenmu,in the semi-arid climate zone.Field sampling,long-term in situ observation,indoor analysis and simulation were then implemented.The main conclusions are as follows:(1)0~2 m is the relatively active layer of soil moisture in the LCZ.Within 0~2 m,the soil moisture increased first and then decreased with the depth increasing.In the same profile,the soil moisture’s coefficient of variation(CV)within 0~2 m was the largest;below 2 m,the soil moisture distribution varied at different depths with no obvious regularity in different soil profiles.Within the same soil profile,the spatial distribution of soil moisture at different depths,which have significant time stability,varies little with time.In the two watersheds,the mean soil moisture and minimum water content in soil profiles with deep-rooted plants were smaller than those with shallow root plants.In different climatic zones,the CV of soil water content within the same range vary slightly.(2)Temporal and spatial dynamics of soil moisture are affected by climatic factors such as vegetation,precipitation and soil particle composition in 0~2 m,and are affected by vegetation and soil particle composition below 2 m.Specifically,climatic factors such as precipitation have significant effects on shallow soil moisture dynamics and distribution,but have little effect on deep layers.Roots water absorption increase soil water dynamics in shallow layers.The phenomenon is significant in profile of shallow-rooted plants in semi-arid climatic areas but non-significant in profile of deep-rooted plants in semi-humid climatic and semi-arid climatic areas.In the deeper layers,roots and soil particle composition affect soil moisture dynamics.The effect of soil particle composition on shallow soil moisture is significant in semi-arid climate regions but non-significant in semi-humid climate regions.It’s impact on shallow soil moisture dynamics is significant in the semi-arid climate region but non-significant in profile of deep-rooted plants in semi-humid and semi-arid climatic zone.The composition of soil particles has significant effect on the distribution and dynamic changes of soil moisture in the deep layer.When there exist deep roots,the two factors will act together on the spatial and temporal distribution and dynamic changes of soil moisture.(3)The temporal stability characteristics of soil moisture can be used to quickly predict the mean soil moisture in a certain area.The representative depth of wheat and apple in Wangdonggou watershed are 9 m and 13.6 m.The representative depth of stipa bungeanaand,alfalfa,soybean and Caragana korshinskii in Liudaogou watershed are 4.4 m,2.6 m,6.8 m and 17.6 m,respectively;The temporal stability of soil moisture in 0~2 m is weaker than that below 2 m.(4)During the calibration period and the validation period,the relative error(RE),the root mean square error(RMSE)and the determination coefficient(R~2)of simulation in the Wangdonggou and Liudaogou watershed are all acceptable.The Hydrus-1D model can be used to simulate the spatial distribution and dynamic changes of soil moisture in deep sections at long-term scales.(5)In terms of time,the effect of the simulation in initial stage is better than the later,and the simulation accuracy decreases with the simulation period increasing;in terms of space,the deep layer(2~21 m)soil moisture simulation effect is better than the shallow layer(0~2 m).The simulation accuracy of soil layers with less soil types change is higher than that of the soil layer with much change.Root distribution and growth,climatic factors such as precipitation,soil texture are the main external factors affecting the accuracy of simulation.The fineness of soil stratification when built a model is one of the internal factors affecting the accuracy of simulation.When comes to long term simulation in deep profile,the measured root and soil moisture data should be used regularly to correct the model.