A Study On The Gully Formation On Loess Slopes Under An Extremely Heavy Rainstorm Event In Northern Shaanxi Province, China

Soil erosion is a very serious environmental problem,and gully erosion is the main form of soil erosion at the small watershed scale.The Loess Plateau is one of the areas with the most severe soil erosion in the world.In the Loess Plateau,the sediment yield caused by gully erosion accounts for 60-90%of the total sediment yield in the small watershed.Rainstorm is the driving force for the development of gullies.However,the research on the mechanism of gully initiation and development under heavy rain conditions is very weak.The purpose of this research is to analyze the characteristics of newly formed gully erosion caused by large-scale rainstorm conditions,reveal the mechanism of gully formation caused by rainstorm,clarify the threshold conditions for gully formation caused by rainstorm,and use rainfall and land use data to predict gully erosion caused by heavy rain.Strive to provide scientific methods and theoretical basis for the cause and prevention of gully erosion under heavy rain conditions.This research is based on the"7.26"rainstorm event in the Wuding River Basin of the Loess Plateau.Then,the low-altitude drone aerial survey technology was used to investigate the area of 64-238mm rainfall in the"7.26"rainstorm event on three transects.By comparing and analyzing the UAV orthophoto image(resolution 0.15m)and the Google image before the"7.26"rainfall event,the newly formed slope gullies caused by the rainstorm was extracted.Use remote sensing and spatial analysis to calculate the dimensional parameters of the newly formed gully on the slope.Analyze the dimensional characteristics of the newly formed slope gullies and the intensity characteristics of gully erosion at the small catchment scale caused by heavy rain.Using rainfall spatial data and land use to study rainfall thresholds for newly formed gullies on slopes caused by heavy rain.Using local geographic features and land use of the gully to study the geomorphological threshold of the newly formed slope gully caused by heavy rain.Finally,an empirical model for estimating the gully volume is established using data such as the morphological parameters,landform characteristics and land use of gullies.The main conclusions are as following:(1)The torrential rain event caused a wide range of changes in the dimensions of newly formed slope gullies and the intensity of gully erosion at the small watershed scale.This study investigated 369 small watersheds with rainfall within the range of 64-238mm,and found that 85 small watersheds added 280slope gullies,with an average of 3.3 newly formed slope gullies in each small watershed.The maximum linear density of gully erosion at the small watershed scale is 4,079.58m/km²,which is 7.8 times the average linear density.The maximum volume density of gully erosion at the small watershed scale is3,160.19m³/km²,which is 9.6 times the average volume density.(2)Rainfall has a significant impact on the intensity of gully erosion in small watersheds,and the intensity of gully erosion increases exponentially with the increase of rainfall.In the core area of the rainstorm,the average volume density is 928.39 m³/km²,which is 12.8 times that of the small watershed in the area of 106-150mm rainfall.The number of newly formed slope gullies and the intensity of gully erosion in small watersheds affected by land use.An average of 2.51 slope gullies were formed in the small grassland watershed.An average of 4.92 slope gullies were formed in the small watershed of cultivated land,and the erosion intensity was the greatest.The average linear density of gully erosion in the small watershed of cultivated land is932.29 m/km²;the average volume density of gully erosion is635.81 m³/km².Compared with small watersheds of cultivated land,the average intensity of gully erosion in small watersheds of grassland decreased by more than 60%.The type of land use affects the rainfall threshold for gully formation.This study found that the rainfall threshold of newly formed gullies in cultivated land does not exceed 106.7 mm,and the rainfall threshold of newly formed gullies in grassland does not exceed 124.2 mm.(3)The geographic threshold formed by cutting trenches will be affected by rainfall and land use.The average drainage area of the new slope gully in the rainfall core area is 0.22 ha,and the average slope is 0.51m/m,which is 66.67%and 110.87%of the 150-200mm rainfall.The geomorphic threshold equation for the formation of slope gully when the rainfall exceeds 200mm:S=0.039A^-0.547.The slopes of newly formed gullies for different land uses in areas where the rainstorm exceeds 200mm are all 0.5m/m.However,the average drainage area of grassland gullies is 0.29 ha,abandoned land gullies is 0.22 ha,and cultivated land gullies is 0.20 ha.In this study,the geomorphological threshold of gullies formed by cultivated land in areas with rainfall exceeding 200mm is:S=0.039 A^-0.547 and gullies formed by grassland is:S=0.151 A^-0.213.Calculating the critical slope of the gully formed by the catchment area of 0.01ha by the geomorphic threshold equation,the minimum slope required to form the gully in the rainstorm core area is 0.48m/m,and the minimum slope required to form the gully in the grassland is 0.40m/m.(4)When rainfall is over 150mm,the gully volume is estimated using morphological parameters such as gully length,gully perimeter,and gully area.The gully area estimates the gully volume with the best results,and the equation is:V=0.262A^1.213,R²=0.771.At the same time,under the condition of satisfying the developed landform and precipitation threshold,the non-linear regression equation of using slope and field precipitation runoff to predict the volume of newly slope gully in grassland is:V=1.653e-17S^9.395Q_g^7.048+10.206,R2=0.748(n=40),the non-linear regression equation of using slope and field precipitation runoff to predict the volume of newly slope gully in abandoned land is:V=0.03S^-0.813Q_g^0.814+5.242,R2=0.691(n=22).The use of slope and field precipitation runoff to predict the volume of newly slope gully in cultivated land is not effective.