Temporal And Spatial Characteristics Of Soil Moisture On Typical Slope Of Purple Soil

Soil moisture is an important carrier of surface energy flow and material migration.Its formation and redistribution process are closely related to the generation of slope runoff,soil moisture is the main influencing factor of the slope soil erosion process.The soil layer is a key factor in the distribution of rainfall and moisture.The properties of the soil directly affect the temporal and spatial distribution pattern of soil moisture at the slope scale.The weathering of purple parent rock is strong,the surface material is loose,and the gravel content is high.Under the action of rainfall,water and soil loss is very easy to occur.The water and soil loss caused by hydraulic erosion is the main problem faced by the purple soil slope.Therefore,studying the temporal and spatial variability characteristics of purple soil moisture on the slope scale provides an important scientific basis for the effective control of soil erosion and the efficient use of water resources.This paper takes two typical purple soil slopes(orchard slopes and cultivated slopes)in the Wangjiaqiao small watershed of the Three Gorges Reservoir area as the research object.Long time series monitoring of soil water content by MP-406 III,field investigation of vegetation type,land use and topographic environment,combined with indoor soil property determination,analyze the spatial and temporal variability of soil moisture on slopes in both time and space dimensions,and clarify the main influencing factors of spatial and temporal variability of soil moisture at the slope scale;the specific effects of different main controlling factors on soil moisture change were identified;reveal the temporal stability characteristics of soil moisture on slopes,in order to provide a theoretical basis for soil erosion control and land use allocation planning in this area.The main findings are as follows:(1)There is a big difference in soil water content during dry and wet periods.Under drought conditions,the soil water content is low but increases with the increase of soil layers;during wet periods,the water content of each soil layer is higher,but there is little difference between soil layers;The water content increased with the increase of soil layer in the range of 0～30 cm during the whole period.The main controlling factors of soil moisture on slopes are mainly divided into two categories.One is the terrain factor based on altitude,which is always negatively correlated with soil moisture;the other is the soil properties based on gravel content,silt content and sand content.Among them,the gravel content is positively correlated with soil moisture during the wet period,but negatively correlated during the dry period.The silt content is positively correlated with the soil moisture during the entire monitoring period,while the sand content is the opposite.(2)The soil water holding capacity and soil water content of different land uses are all expressed as cultivated land>grassland>orchard>woodland.The degree of impact of land use on soil moisture is related to the previous rainfall.When there is no significant rainfall in the current period,the difference in soil water content between different land uses is obvious;when there is rainfall in the current period,the soil is in a moist state,and another rainfall will reduce the impact of land use on soil water content.Land use has different effects on soil water content during dry and wet periods.Water content of 30-50 cm varies greatly during dry periods.The differences in soil water content between the four soil layers(0-10,10-20,20-30,30-50 cm)for different land uses during wet periods were all significant,and the significance increased with the increase in soil layer.(3)The soil water content of different slope positions responds differently to different rain patterns.Compared with before the rain,the difference of the soil water content of different slope positions under light and moderate rain conditions is reduced,but it is mainly reflected in the soil layer within 30 cm.After light rain and moderate rain,the soil water content in the middle of the orchard slope increased significantly,and the response to rainfall was the greatest;except for the middle of the slope,the response of the other slope positions to light rain and moderate rain was small.The heavy rain significantly increased the water content of the soil profile,and made the overall soil water content of the slope surface saturated.The difference in soil water content of different slope positions was reduced,and the soil water content in the middle of the orchard slope and the bottom of the cultivated slope increased significantly.(4)In terms of soil depth,soil moisture in the shallow layer(0-20 cm)is easily affected by environmental factors such as precipitation,evapotranspiration and root uptake,and the spatial distribution structure is unstable;the deeper the soil layer is,the fluctuation range and mean value of the relative difference standard deviation(SDRD)of soil moisture content decreases with increasing soil layer,so the temporal stability of soil moisture is better.The time stability theory can be used to accurately predict the average value of the soil water content on the purple soil slope.When the average relative difference(MRD)value is close to zero and the corresponding SDRD is also small as the representative measurement point,its prediction effect The best,but there are differences due to the different slope structure;after establishing the relationship between representative measurement points and time stability,the average soil water content and space of the slope can be accurately predicted by measuring the soil water content of certain sample points distribution characteristics.