Temporal And Spatial Distribution Of Soil Water And Its Effect Mechanism On The Loess Hillslope

There are some negative problems,such as soil water imbalance and soil desiccation on the Loess hillslope covered by artificial forestland since the implement of "Grain for Green" project.These problems were mainly due to the lack of understanding of temporal and spatial distribution and effect mechanism,and not according to the characteristics of water and soil resources to plant vegetation in the Loess Plateau.In this paper,the artificial Robinia pseudoacacia forestland,natural forest land and natural grassland in the loess area of western Shanxi Province were studied.Through a large number of soil water observation experiments,dyeing tracer experiments and hydrogen-oxygen stable isotope tracing techniques,the following main results were obtained:(1)In vertical soil profile,the soil water of the three vegetation types was significantly different in the(0-100 cm soillayer),where soil water in natural grassland was significantly higher than that in natural forestland,which was significantly higher than that in artificial forestland.But the soil water in 100-400 cm soil layer in natural grassland was significantly higher than that in forestlands,while soil water in natural forestland and artificial forestland did not different significantly.The soil water of the three vegetation types decreases in the 100-400 cm soil layer with increasing distance from the bottom of the slope,and this trend in the 0-100 cm soil layer()is not obvious.This trend is not obvious in natural forestland,and in natural grassland,soil water of 0-400 cm soil layer decreases from the bottom to the top of the slope.In the severe drought year of 2015,soil water consumption throughout the growing season was dominant in the three vegetation types.In the near nonnal year of 2016,soil water consumption was dominant in artificial forestland and natural forestland,and infiltration throughout the growing season was dominant in natural grassland,while in moderate wet year of 2017,infiltration throughout the growing season was dominant in the three vegetation types.In the severe drought yearsof 2015,the most serious soil water deficit occurred in 0-100 cm soil layer in artificial and natural forestlands,while the soil water deficit in 100-200 cm soil layer in natural grassland was the most serious.(2)The factors affecting soil water in different soil layers at different time periods are varied.Using redundancy analysis,We found that,in the severe drought year of 2015 and near normal year of 2016,soil water storage of 0-200 cm soil layer is mainly affected by slope degree and slope aspect,while the deep soil water storage is affected by the vegetation type.In the moderate wet year of 2017,the vegetation type affects the surface soil water storage and the topography influence deep soil water storage.Forward selection and Monte Carlo tests of environmental factors for different time periods showed that under near-normal drought conditions,vegetation type is the dominant factor controlling soil water storage of 0-400 cm soil layer,and the slope direction and slope position dominate under severe drought conditions.(3)The preferential flow and infiltration patterns can be accurately represented by dye tracing experiments and soil water movements.Preferential infiltration amount along preferential pathway(PIV)and the contribution rate of preferential flow to total infiltration amount(contribution)can effectively assess the development degree of preferential flow.On upslope and midslope,the natural forestland has higher degree of preferential flow development compared with the artificial forestland,mainly due to the large distribution of surface roots.And in the positions,the macropore flow dominates.The contribution rate of preferential flow and spatial variability of infiltration water were higher in natural forestland than in artificial forestland.On the downslope,the lateral flow dominates due to the existence of rock fragment.The contribution of preferential flow to total amount of infiltration and the spatial variability of infiltration water in the artificial forest land and are higher than that of the natural forestland.(4)In the two moderate rainfall events,the rainfall only penetrated into the topsoil(less than 40 cm),while the infiltration depths in the 118.110 mm and 186.901 mm heavy rain events were 60 cm and 160-200 cm,respectively.In the four rainfall events,the long-term rainstorm event III produced the largest amount of infiltration.In the whole study section,soil moisture evaporation was highest in the surface layer(0-20 cm).With the delay of the time after the end of rainfall,the evaporation of forest land and grassland presents different trend.(5)In the case of long-term rainfall,the artificial forest mainly uses the soil water of 80-100 cm soil layer,the natural forest land uses the soil water of 60-100 cm soil layer,and natural grassland mainly uses the soil water of 0-40 cm soil layer.The artificial forestland and natural forestland mainly use surface soil water on the 1st day after rainfall,and the water used in the 7th day after rainfall moves to the deep soil,while natural grassland mainly uses the soil water of 0-20 cm in the surface layer.Using soil water by vegetation mainly depends on the root system.The fine roots in artificial forest and natural forest land are mainly distributed in 0-80 cm,and the fine roots in natural grassland are mainly distributed in 0-40 cm.Based on long-term field observation data,this paper analyzes and discusses the scientific problems of temporal and spatial distribution and effect mechanism on the Loess hillslope,and deepens scientific understanding of the soil water deficit,precipitation infiltration pattern and plant water consumption on the slope scale in the area.This study will provide theoretical guidance for vegetation restoration and reconstruction in the region.