Road distribution and controlling factors in watershed of the Loess Plateau
黄土高原典型小流域道路特征及影响因素

An agricultural watershed generally consists of farmland and road networks,of which unpaved roads are a significant land use type in rural watersheds.Road surfaces are quite different from farmland soil in physical properties,compared with farmlands,road surfaces may limit infiltration and increase the rate of sediment yield in watershed.Also,road networks can influence and change hydrologic and geomorphic processes greatly in a watersheds.Hence,roads distribution patterns should be taken into consideration in soil loss prediction models as crucial factors. In order to explore the spatial distribution characteristics of roads in small watersheds of the Loess Plateau,a field survey was conducted in the small watershed of Zhifanggou located on central Loess Plateau to get roads network data with the aid of GPS in the study.On the basis of field data and DEM of Zhifanggou watershed,GIS spatial analysis and statistical methods were used to analyze road network in view of their quantity,structure and morphology respectively as well as controlling factors.The results showed that roads on Loess Plateau could be classified into 4 types based on their functions and scales.Unpaved roads of classs 2,3 and 4 are representatives in small watersheds.Total road lengths are linearly related with road classes.Roads network was similar to the stream networks in construction and could be described by Horton laws.Generally,road construction was controlled by landforms and human activities,roads controlled by landforms were distributed along main road and total length changed as exponential function of distance from the main road.Roads that are relatively freely-distributed are mainly in zones of the same elevation with residential zones.Linear relationship exists between the slope gradient sine values of road controlled by landform and that of land surface.Based on which the mean angle between road and contour can be calculated as 29.3.The microcosmic features of roads were different on surfaces of different slopes.Both the curve span and the angle between road and contour are increased with the decrease of slope gradients.Zigzag rarely happens and roads nearly upright with contours when slope is less than 25.This study will be helpful in a good understanding of road network characteristics in a watershed and can provide powerful supports to the establishment and application of process-based models which estimate the effects derived from road networks on runoff and soil loss in watersheds.