Analyses And Stochastic Modelling Of Soil Moisture Dynamics At Different Spatial Scales On The Loess Plateau

The Loess Plateau,located in the northwest of China,is a typical ecological fragile region in China and even in the world.In order to restore the ecology,a series of ecological engineering measures have been implemented on the Loess Plateau.However,due to the limited and uneven rainfall and the deep groundwater,the contradiction between vegetation restoration and limited water resources is increasingly prominent,and soil moisture becomes the key limiting factor for ecological restoration in this region.Therefore,the study of soil water dynamics reveals the soil water cycle mechanism in the artificial forest ecosystem,and provides some recomendations to manage the limited water resources practices and keep the development of vegetation restoration sustainability.In this study,we investigated the soil water characteristics in farmland,grassland,shrub land and forest land on a typical north-south transect of the Loess Plateau and monitored the soil moisture dynamics in three typical hillslopes?Changwu,Ansai and Shenmu,which locates in the middle and the both ends of the transect,respectively?for two years.The objectives of this study were:1)to investigate spatial distribution characteristics of soil water content in the transect;2)to analysis the dynamic of the slope soil moisture;3)to explore the main controlling factors of soil moisture in different spatial scales;4)to simulate the probability density characteristics of soil moisture,and;5)to discusse the optimum vegetation types and coverage using different kind of statiscal method such as classical statistics,geostatistics,Partial Least Squares Regression analysis,and model such as stochastic simulation model.The main results can be drawn as follows:?1?Soil moisture contents?SMC?under four land use types decreased gradually from the southern to the northern of Loess Plateau with increasing latitude.The change trend was consistent with the distribution of the mean annual precipitation,potential evapotranspiration,and soil clay texture across the Loess Plateau.By comparing the soil moisture in horizontal and vertical directions,soil moisture was significantly different under different land use types.The general trend for the soil moisture was:farmland>grassland>shrub land>woodland.The main reason are the differences in root distribution and evapotranspiration under different land use types.?2?According to the mean variation coefficient?CV?along the soil profile at three locations,vertical changes in the soil profile at 0–500 cm were divided into four layers:a fast-changing layer?0–40 cm?,an active layer?40–100 cm?,a sub-active layer?100–200 cm?,and a relatively stable layer?200–500 cm?,respectively.The SMC had good spatial structure characteristics except for the 0-40 cm and 100-200 cm soil layers in the Ansai slope,which can be well simulated by the theoretical semi-variance function model.The surface SMC of the fitting model were spherical model in general,while the deep SMC were gaussian model.There were significant differences for the correlations between SMC in the three slopes and the influencing factors of topography,soil texture and vegetation.There was a significant correlation between the SMC in Changwu and the bulk density,altitude and leaf area index?P>0.01?.The SMCs in Ansai and Shenmu were only significantly correlated with the maximum leaf area and soil texture?P>0.05?,respectively.?3?PLSR models simulated SMC accurately in diverse soil layers at different scales.Upper and lower layer SMCs were the two most important factors that influenced diverse soil layers at three scales.The 7-day antecedent precipitation and 7-day antecedent potential evapotranspiration contributed significantly to SMC only at the 0–40 cm soil layer.The variable importance in project?VIP?of soil properties,especially sand and silt content,which influenced SMC strongly,increased significantly after increasing the measured scale.Mean annual precipitation and potential evapotranspiration also influenced SMC at the regional scale significantly.Overall,this study indicated that dominant controls of SMC varied among three spatial scales on the Loess Plateau,and VIP was a function of spatial scale and soil depth.?4?Soil water data were collected over a 3-year period with a frequency of bi-weekly at plots in the Chinese Loess Plateau under the grass?Bothriochloa ischaemum L.,BOI?,shrub,Sea-buckthorn?Hippophae rhamnoides L.,SEB?,and tree,Chinese pine?Pinus tabulaeformis Carr,CHP?.The data was compared with values simulated using the Laio stochastic model.The results showed that the mean relative SMC differed in the order:BOI>CHP>SEB.Based on the shape of the PDFs and their statistical moments,it was concluded that the Laio stochastic model accurately simulated the soil water PDFs under all three vegetation types in the semi-humid area of the Loess Plateau.Under the same climate,topography and soil conditions,the soil water PDF was found to be sensitive to a critical water content at which plants begin closing stomata?s_e*?and the mean maximum daily evapotranspiration rate(E_max).The soil water PDFs for three vegetation types were simulated with four LAI scenarios,BOI and CHP were determined the sustainable vegetation types as compared with SEB by considering soil water availability and soil erosion control.?5?The soil moisture PDF was simulated by Laio model on a typical north-south transect of the Loess Plateau,and the results showed that:Both climatic factors and soil texture factors affect the PDF of SMC significantly in the Laio model,while the climatic factors mainly affect the proportions of SMC in a same distribution interval,and soil texture factors affect the distribution interval of SMC.Using the line of the characteristic values of the PDF curves of six sample points to evaluate the soil water condition of the transect,most of the SMC values are distributed in the simulated range,only some woodlands showed low soil moisture content,which indicated that these places were not suitable for planting forest,and some place showed high SMC due to local topographic factors.The soil moisture PDFs were simulated and been compared under different vegetation coverage conditions around the transect.The results showed that the SMC was low and had little response to vegetation coverage change in the northern part of the transect which proved it was not suitable for forest vegetation growth;in the middle of the transect,vegetation coverage should be effectively reduced?about 25%-50%?to maintain normal SMC;at the southern part of the transect,the soil moisture is sufficient,but it is not suitable to plant water-intensive vegetation and high-density forest.