Morphological Characteristics Of Gully On Typical Alluvial Fans And Their Hydrological Response Of Catchment In The Lhasa River Basin

In the Qinghai-Tibet Plateau,alluvial fans are valuable land resources.However,alluvial fans in this area have been suffering serious gully erosion,which led to a series of social problems such as the deterioration of ecological environment of alluvial fans and the “contradiction between human and land resources”.Studying the morphological characteristics and spatial distribution of gullies on the alluvial fans is of great significance to understand the development process and erosion mechanism of gullies deeply,and promote the protection and reasonable utilization of alluvial fans.However,due to the limitation of hypoxia and alpine climate in Tibet,it is difficult to obtain information of gullies in large scales through traditional measurements.For this reason,the shape parameters of gullies obtained from the field investigation,UAV orthophoto and Google Earth image are comparing and analyzing,the range of gullies which could be correctly extracted from these two kinds of images has been identified.Moreover,gully erosion information of 12 typical alluvial fan catchments in Lhasa River Basin has been obtained,which was further analyzed with the environmental background data such as topography,land cover,land use and human interference mode.Also the response of the gullies to hydrological factors in the upslope contributing areas was discussed.The main conclusions are as follows:1.The results of gully length extracted from UAV orthophoto and Google Earth image have high reliability in this study(Compared with the field measurement,the mean deviation extent was mainly in the range of 2.0% ~ 5.0%,0.4% ~ 14.0%,respectively),the suitable gully width range is 4.00 ~ 10.00 m(Mean deviation extent was less than 20%).Affected by the image resolution or the condition of the underlying surface of gullies,the width value extracted from UAV orthophoto is close to the measured value than Google Earth image.The gully widths extracted from the two images are in good agreement(Mean deviation extent concentrated in 2% ~ 5%),the gully widths are quite different(Mean deviation extent concentrated in 0 ~ 15%).This shows that when the measurement accuracy is high and the research range is small,it is suitable to use UAV orthophoto;when the range is large or in dangerous areas,Google Earth image is more suitable.2.The number of gullies on the 12 alluvial fans is unevenly distributed in this study,there are a large number of gullies on alluvial fans whose land use type is mainly pasture;density and dissected degree of gully were mainly in 0.37 ~ 4.20 km/km~2 and 2.00% ~ 4.10%,respectively,66.7% alluvial fans' gully volume were less than 100 000 m~3;the gullies located in the alluvial fan are mostly "slender",their length,width,area and volume are concentrated in the low value area;there was a significantly positive linear correlation between gully area and gully length,no significant correlation between gully length and width,volume.3.The number of gullies on the slope above the alluvial fan is generally greater than the alluvial fan,and its shape is more "slender".The density and dissected degree concentrated in the range of 2.00 ~ 3.67 km/km~2 and 2.61% ~ 4.84%,respectively,showing a trend of increasing first and then decreasing with the rise of altitude.The slopes with an elevation of 3800 ~ 4000 m was the most serious area of gully erosion in this study.Upper slopes at the junction with alluvial fan were the areas severely suffered with gully erosion,where should consequently be the key areas to be protected and conducted with gully control measures.4.According to the research,there was no direct correlation between catchment area,proportion of upper slope and the gully characteristics on the alluvial fan(P > 0.05).The gradient of main gully affects the gullies on alluvial fan(P < 0.05),the smaller the gradient of the main gully,the smaller gully density on the alluvial fan.Therefore,it could be considered to reduce the gradient of the main gully whereby raising the bed of main gully,hereby restraining the development of gully on alluvial fans.There was a significant positive correlation between gully density on the fans and the gully density and dissected degree on the upper slopes(P < 0.05).That may be due to gullies on the slope acting as a runoff transporting channel,thus affecting the runoff to the alluvial fan and the gully characteristics there.