A Research On Spatial Variance Of Ephemeral Gully At Multi-scale In The Loess Plateau

Ephemeral Gully（EG）erosion is one of the main erosion types in the Loess Plateau.Studying the spatial differentiation of EG at different scales is the key to targeted prevention and treatment of EG.In this research,the ephemeral gully was visually interpreted based on UAV images and sub-meter Google Earth remote sensing images,and the accuracy of the interpretation of the ephemeral gully was evaluated by GNSS RTK high-precision field measurements,and the factors that may affect the interpretation accuracy were analyzed.Three different scales of the Loess Plateau:small watershed,medium watershed and regional scale were used as the research objects,based on the ephemeral gully data measured by GNSS RTK,the ephemeral gully data interpreted by UAV images,and the ephemeral gully data interpreted by Google Earth remote sensing images,the multi-scale distribution of ephemeral gully and the factors affecting the distribution of ephemeral gully were analyzed.The main conclusions obtained are as follows:（1）The relative error₄)of the length of a single ephemeral gully based on UAV image interpretation is between 0.07%and 8%,and the averageis 3.04%.The relative error₄)of the length of a single ephemeral gully interpreted based on the sub-meter remote sensing images of Google Earth is between 0.07%and 12%.The methods of interpreting ephemeral gully from UAV image and Google Earth image have high accuracy and are suitable for the investigation of ephemeral gully.（2）At the Wangwugou small watershed scale,the EG density is 2.43 km/km²,the average EG length is 23.73 m,and the average critical slope length is 39.42 m;there are many EGs with a catchment area between 300 and 400 m²;The slope gradient of the slope where the ephemeral gullies located are all above 10°,and more EGs appear on the slope gradient between 20 and35°.In different slope aspects,the EG density of the shady slopes is higher.At the medium watershed scale of Chabagou,the length of EG and the critical slope length are most between20 and 40 m.The slope gradient of the slope where EG located is mostly between 15 and 30°.The catchment area above the EG is mainly concentrated between 100 and 500 m2.The land use types distributed in the ephemeral gully are mostly cropland and grassland,and the ephemeral gully is mainly distributed on the shady and semi-shady slopes.The average value of the slope gradient of the slope where the downstream EG is located is larger,and the average value of the upstream EG catchment area is larger;At present,the upstream and midstream EGs are mainly distributed in cropland,and the downstream EGs are mainly distributed in grassland.The upstream EGs are mainly distributed on sunny and semi-shady slopes,the midstream EGs are mainly distributed on shady and semi-sunny slopes,and the downstream EGs are mainly distributed on shady slopes;The topography and soil erodibility of Chabagou have great influence on the density of ephemeral gully.Although the single factor of cropland proportion did not have much influence on EG density,the interaction between cropland proportion and slope gradient,slope length and elevation had a higher effect on EG density.In the governance of EG,it is necessary to pay attention to the interaction between factors.（3）In the Loess Plateau,33.6%of the watersheds had EGs.In the surveyed units with EG,the average EG density is 3.41 km/km²,the average EG number density is about 65/km²,and the average EG length is 63.31 m.The critical slope length for EG occurrence mainly ranged from20 m to 60 m.The highest EG density was found in Wuqi and Zhidan County in the northwest of Yan’an,and Jingbian County in Yulin.The average EG density is the largest in the Loess Hilly and Gully Region,followed by the Loess sorghum gully region.At present,38.3%of EG is distributed in cropland,35.3%in grassland,and 22.8%in forest land.The elevation of the Loess Plateau generally shows a trend of high in the west and low in the east,the slope gradient in the southeast is low,and the average NDVI from 1990 to 1999 is low from the northeast to the northwest.The average annual rainfall from 1981 to 2019 and rainfall erosivity of the Loess Plateau shows a gradual increasing trend from northwest to southeast,the soil erodibility showed a trend of being larger in the north and south,and the proportion of cultivated land showed a larger trend in the north and southeast.At the regional scale of the Loess Plateau,the slope gradient and the average annual rainfall have a great influence on the formation of ephemeral gully;Soil erodibility and arable land ratio have a greater impact on the density of ephemeral gullies,rainfall erosivity and NDVI also have a higher impact on the density of ephemeral gullies.Soil erodibility and the proportion of arable land,the proportion of cropland and the slope length have a greater impact on the density of ephemeral gullies.