Abstract: Gully is the essential geographical and hydrological feature of loess plateau. Runoff accumulation threshold is the key to extracting loess gully by hydrological analysis. Present method for determining the accumulation threshold is highly subjective. To address the uncertainty problem of the accumulation threshold, we present a method of determining the runoff accumulation threshold based on the receiver operating characteristic (ROC) curve. We select the 30 m resolution DEM of Nianzhuanggou catchment in Baota District of Yan’an City as the basic data. Firstly, we collect 69 starting points of actual gullies and analyze the convergence characteristics of the starting points, and determine the interval of confluence gathering. Secondly, we extracted 500 first-order gully points as sample points and obtain the optimal threshold of runoff accumulation by analyzing the diagnostic value of runoff accumulation to the gully based on ROC curve. Research results demonstrate that the runoff accumulation is of high diagnostic value for gully. An accumulation threshold could well determine whether the sample point is a gully; the root mean square error of the gully extracted under the optimal accumulation threshold determined by the ROC curve analysis method is less than 2.5 pixels, and the result is ideal.

Key words: loess channel, gully extraction, hydrological analysis, runoff accumulation threshold, ROC curve