As an important geomorphological divide in the hilly Loess Plateau, valley boundary plays a pivotal role in the construction of distributed models for studying the processes and mechanism of soil erosion, sediment transfer and runoffs within a watershed. In the past, this boundary manually interpreted from large scale maps and aerial photographs. In this paper, the authors first present a simple and practical definition for topographic structures unique to the highly dissected topography in the hilly Loess Plateau in DEMs; then define the valley boundary cells in a DEM with the following features: 1) they are most likely located on a concave slope, 2) these cells have the sharpest transition in slope, 3) those cells on the downward side of the valley boundary should have a slope gradient larger than 20° ,4) as the boundary of the entire valley in a basin valley boundary must be continuous; then devise an algorithm to automatically delineate valley boundary from DEMs and implement it in a computer. This algorithm was applied to detect drainage networks and the valley boundary in the Wangjiagou basin in the hilly loess area of Lishi County, Shanxi Province, China from a 10 m DEM. The extracted results are highly comparable to those manually interpreted from aerial photographs.
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