Background Road networks have been reported can influence local ecology and environment. The existence of road network in a watershed may increase stream density and surface runoff, therefore enhance the risk of soil erosion. As the same time, the spatial distribution of the road network showed specific regularities due to the influence of hydrological and geomorphological elements in the watershed. Studying the distribution characteristics of the road network and its relationship with topography and water system from the spatial scale is of great significance for understanding the mechanism of hydrological effect of road.

Methods In order to explore the role of road in the hydrological process of watershed, road network of small watersheds in hilly red soil region of Fujian Province was chosen as the research object. The distribution of road network in the study area was obtained from high-resolution remote sensing image. Combined with the 25 m pixel DEM data, the spatial distribution characteristics of road network and its influencing factors was analyzed through GIS spatial analysis. The linkage between road network and the hydro-geomorphologic features in the watershed were studied through GIS buffer analysis and topology analysis.

Results 1) Road networks in the study area can be classified as 5 grades, including County road, Main country road, Secondary country road, Main unpaved road and Trail. The road density is 2.3 km/km² and the structure of road network is different from that of the stream networks which can be described by bifurcation ratio. 2) The distribution of roads is driven by both the human activity and limitation of terrain conditions. Residential point is the driving factor for the extension of road network, and the road density decreases with the distance from residential buffer center. Terrain factors are the limiting factor for road network distribution. The total length of roads decreases with the increasing slope interval, but increases first and then decreases with the increasing relief amplitude interval. 3) Paved roads are mainly constructed along channels, thus may cause relatively great hydrological impact on watershed hydrology due to the short distances between streams and roads. This kind of influence mainly exist in the downstream of the watershed where most high-grade paved roads can be found. On the other hand, low-grade unpaved roads mainly affect the upstream hydrology and sediment transport by intercepting runoff and causing erosion.

Conclusions The above results revealed the spatial distribution of road networks and the road-stream relationship. It would be helpful to explain the mechanism of road networks in affecting hydrology processes in a watershed scale.