Drainage Density And Its Influential Factors In The Northeastern And Eastern Margin Of The Tibetan Plateau

Drainage density is an important geometric parameter to express channel networks distribution and the degree of fluvial dissection as it reflects geology,climate,underlying surface varieties.Exploring the relationship between drainage density and other factors(topography,precipitation,vegetation etc)is a significant way to understand the drainage system and river geomorphology evolution.In this paper,we selected the Qilian Mountains in the northeastern margin of Tibetan Plateau,the West Qinling Mountains and the Yunnan Plateau in the eastern margin of Tibetan Plateau for the study area,calculated the drainage density and analyzed the influential factors based on the channel networks extracted from SRTM-DEM.The main conclusions of this article are as follows:Three small watersheds with vary precipitation,vegetation and topography conditions were selected in each study area as experimental regions and the channel networks in each watershed were extracted from DEM by using hydrology-based algorithms and morphology-based method.The comparison of distance between automatically mapped and manual digitized channel heads indicated that a dedicated contributing area threshold should be set for different watersheds while using the hydrology-based algorithms.In contrast,a constant contour curvature threshold is needed in all watersheds while using the morphology-based method.Considering the geomorphic conditions are quite different in different region on the margin of the Tibetan Plateau,the hydrology-based extracted network may depart significantly from actual streams.Accordingly,we extracted the channel networks of the study area based on the morphology-based method(contour curvature threshold= 3).We calculated the drainage density of 39 basins in the study area based on the extracted channel networks.The basin drainage density of Qilian Mountains region shows a decrease from east to west and the mean drainage density is 1.64 km/km2.The mean drainage density of West Qinling Mountains region is 2.11 km/km2 and a lower drainage density(1.94 km/km2)is observed on the northern flank of the Qinling Mountains,while a higher drainage density(2.05 km/km2)on the southern flank.The basin drainage density of Yunnan Plateau region shows a decrease from southeast to northwest with a mean drainage density of 1.59 km/km2.Based on the correlation analysis between basin drainage density and some other factors,we concluded that precipitation and vegetation are the main influential factors on basin drainage density in vary climates.In the Qilian Mountains,the vegetation is mainly grassland under the arid to semi-arid climate condition,the greater precipitation comes stronger runoff erosion so that the drainage density shows a positive correlation with the annual mean precipitation.In contrast,the Yunnan Plateau is located in a humid climate condition where the vegetation coverage is mainly forest.As the climate grows more moist,the vegetation becomes more dense and the erodibility of land surface reduces,as a result of the drainage density shows a negative correlation with the annual mean precipitation.Furthermore,5000m×5000m windows were used to sample drainage density,relief and precipitation in the study area.We classified the relief into 6 classes,that is plain<30m,tableland 30?700m,hill 70?200m,low relief mountain 200?500m,moderate relief mountain 500?1000m,high relief mountain 1000?2500m;and the precipitation was also divided into 6 classes:0?200mm,200?400 mm,400?600 mm,600?800 mm,800?1000 mm,1000?1200 mm.The average drainage density in different precipitation and relief classes was calculated.In a same relief class,the relationship between drainage density and precipitation suggests two regions of behavior:(1)For mean annual precipitation less than 400mm,drainage density is positively correlated with precipitation;(2)for precipitation exceeding 600mm annually,drainage density decreases with precipitation increases.And in a same precipitation class,the relationship between drainage density and relief also shows two regions of behavior:(1)For relief less than 1000 m(moderate relief mountains,low relief mountains,hills etc),drainage density increases with relief increases;(2)for relief exceeding 1000 m(high relief mountains),drainage density seems to show a negative trend with relief.The slope-area plot of Jinta Basin in Qilian Mountains shows that the slope profiles in low and moderate relief mountains are mainly convex,which reflect erosion by creep and overland flow.While the slope profiles in the high relief mountains are concave or straight,which is indicative of threshold-gradient dependent mass-wasting.Furthermore,the critical contributing area in low and moderate relief mountains is about 2×105 m2,which in the high relief mountains is about 7×105 m2.It is proposed that the relationship between relief and drainage density depends upon the hillslope erosion process and channel head contributing area.