Investigation And Evaluation Of Soil Erosion In Dapo Small Watershed By The “7·20” Extraordinary Rainstorm Disaster In Zhengzhou

From July 18 to 23,2021,Henan Province and its surrounding areas encountered a rare super extremely heavy rainstorm in history.At 4-5 p.m.on July 20,the local one hour rainfall reached the highest record in the history of the country.According to the data from Henan Meteorological Station,74 meteorological stations recorded more than 50 mm rainfall,53 more than 100 mm rainfall,and 17 more than 250 mm extremely heavy rainstorm.The study on the impact of the extremely heavy rainstorm on water and soil conservation projects and measures is typical.At present,there are few investigation and evaluation results on the water and soil loss disasters caused by the super standard rainstorm and the benefits of water and soil conservation measures at home and abroad.Therefore,field investigation on the water and soil loss disasters caused by the rainstorm in a typical small watershed can better understand the water and soil loss caused by the rainstorm and the ability of water and soil conservation to prevent and mitigate disasters,To provide reference for preventing floods and soil erosion caused by extreme weather.Xinmi Dapo small watershed is located in the central area of the strong rainstorm of the "7.20" extremely heavy rainstorm in Zhengzhou,and the survey is highly representative.Due to epidemic control,the research team conducted a field survey of the research area(Xinmi Dapo Small Watershed)in early September 2021.The survey targets rainfall characteristics,runoff characteristics,gully erosion,road erosion,gravity erosion,and terrace damage.According to the relevant conditions of erosion gullies obtained from the field survey,the spatial distribution characteristics of erosion gullies,road erosion,gravity erosion and terrace damage in the study area are proposed by comparing with the local high-precision aerial images before the rainstorm;The soil erosion modulus of small watershed caused by this rainstorm was calculated by combining CSLE model with investigation;By investigating the conservation of various water and soil conservation measures,the damage of water and soil conservation measures under the extremely heavy rainstorm was understood,and the benefits of water and soil conservation in the basin were evaluated.The main conclusions are as follows:(1)The "7.20" rainfall in Dapo small watershed is a extremely heavy rainstorm.The maximum rainfall observed by the single rainfall stations around it from July 18 to 22 is681.5mm.The average rainfall of the eight rainfall stations around Dapo small watershed is616.6mm,the average hourly maximum rainfall intensity is 50.6mm/h,the hourly maximum rainfall intensity is 73.0mm/h,and the average rainfall erosivity is 7550 MJ · mm/(hm2· h).(2)The water and soil loss in Dapo small watershed was serious in the rainstorm.The extremely heavy rainstorm resulted in 31 erosion ditches,120 m long road rills,210 m long road shallow ditches and 210 m long road cut ditches in Dapo small watershed;39 locations where gravity erosion occurred;The damaged length of terraced fields and ridges is 435.26 m.Erosion gullies mainly occur within the slope range of 10°-45°;The surface erosion of roads in the large slope and small watershed mainly occurs in the non hardened production road area,and there is no significant difference in the surface erosion intensity of production roads in different slope grades within the watershed;Gravity erosion mainly occurs within the slope range of 20°-50°;Terrace measures played an important role in erosion resistance during the extremely heavy rainstorm,so the overall erosion modulus of terraces in the small watershed is relatively low(the average surface erosion modulus is 846.99t/km2).(3)The extremely heavy rainstorm caused 2208.77 t gully erosion in Dapo small watershed,and its soil erosion modulus is 2726.877t/km2;The erosion amount of road rills is1.84 t,and the soil erosion modulus is 2.27t/km2;The erosion amount of shallow gullies on roads is 12.38 t,and the soil erosion modulus is 15.28t/km2;Gravity erosion amount is 6110 t,and soil erosion modulus is 7540t/km2.(4)The average surface erosion soil erosion modulus of the large slope and small watershed is 1477.44t/km calculated through the CSLE model,Superimposed gully erosion soil erosion modulus,the overall soil erosion modulus of Dapo small watershed caused by this extremely heavy rainstorm is 4945.14t/km2.The location of high soil erosion modulus in the large slope and small watershed is mainly distributed in the western part of the gully with large slope and the area of human disturbance in the middle and upper part of the gully.(5)Based on the field investigation of soil and water conservation measures such as terraced fields,forest and grass in the post disaster study area,the CSLE model was used to calculate the regional soil erosion modulus with and without soil and water conservation measures.The comparative analysis found that under this level of rainstorm intensity,compared with the sloping farmland without terraced fields,terraced fields,forest and grass measures have significant soil and water conservation benefits,that is,compared with the sloping farmland,terraced fields can reduce the soil erosion modulus of cultivated land by89.75%,Forest and grass biological measures can reduce soil erosion modulus by 91.90%.