Abstract: Watershed delineation is the basis for studying the hydrological cycle with the distributed hydrological model. The process of runoff from mountains and plains is very simple; the watershed boundary delineation is based on natural dividing crest, irrigation ditches or administrative boundaries in plain areas. All of the watershed delineation methods in the mountain and plain areas are relatively effective. But in the compound mountain-plain landform areas, surface water cycle process is rather complicated, especially in areas with strong human activity, which are compound artificial-natural hydrological cycle types, the existing watershed delineation technology can not be used to define the whole basin. This paper provides a systematic method to resolve the problem. In the plain area, the DEM is preprocessed with the correction coefficient and is based on the DEM “Burn-in” elevation increment methodologies. In the mountains, the watershed boundary is delineated with D8 algorithms of the module in the SWAT model. Finally, the regional land use map was adopted to complete the whole watershed and sub-basin delineations. In the case of Hetao Irrigation District, Inner Mongolia, China, the delineated watershed boundary is composed of the mountain ridge, the boundary of irrigated lands and a natural river. This method has proved to be correct and feasible. The results show that the river network and drainage channel system are highly fitted with the real objects, the hydrologic cycle is consistent with actual water management units, and the area of watershed in the mountain area account for 58.1% of the total watershed area. It also shows that the division of the whole basin is a precondition for using the distributive hydrological model to study regional water balance. Water from the mountain sub-basins should be a large contributor to the whole basin and the sub-basins in the mountain area cannot be ignored when calculating water balance in the plain area. The watershed delineation method proposed in this paper can be used as a reference in similar topographic regions by using this model. Moreover, high-resolution DEM and detailed land use maps are an effective means to improve watershed delineation precision.

Key words: watershed delineation, mountains valley plain, DEM burn in, SWAT model