Hydrological information extracted from digital elevation models (DEMs)is the basis of distributed hydrological models.Algorithms determining water flow direction over terrain surface are basis for extracting hydrological information from the DEM.Flow direction and accumulation distributions have a direct effect on catchment area.Single flow direction algorithm is widely used because of its easiness to implement and to track upstream catchment areas.However,the single flow direction (SFD)algorithms tend to produce unnatural parallel flow paths in gentle slope areas.Therefore,multiple flow direction (MFD)algorithms have been proposed to calculate water flow directions.However,MFD algorithms create overlap upstream boundaries of watersheds.Considering applicability of SFD and MFD algorithms,a hybrid approach is proposed to calculate a more reasonable distribution of water under different terrain conditions.First,a template-based terrain classification algorithm is applied to the given DEM which is categorized into five classes:valleys,ridges,passes,gentle slope area and steep slope area with a slope threshold value.A single flow direction algorithm is applied to steep slope,valley and ridge area.A multiple flow direction algorithm is used to determine water flow directions in the gentle slope and passes area.In this paper,two small watersheds in the Linfen Basin of the Loess Plateau and the Sichuan Basin in the Yangtze River Basin were selected as study areas.SRTM DEMs of 30 m and 90 m are used in experiments.The results of hybrid flow direction algorithm are compared with the results of typical SFD and MFD algorithms.The divergent effect is apparently suppressed compared to multi-flow direction algorithms.The occurrence of unnatural parallel drainage lines is decreased compared to the results of single flow direction algorithms.And improvements of hybrid flow direction on DEMs of 30 m is better than 90 m datasets.%从数字高程模型提取的汇水网络和汇水区等信息是分布式水文模型及应用分析的基础参数,基于地表汇水模拟的算法是提取该类信息的主要方法,其中,水流方向的确定对提取结果有着直接的影响.单流向算法因其易于实现、易于确定上游汇水区等特性,得到了广泛应用,然而单流向算法在坡度平缓区域会产生不自然的平行径流,能模拟地表水流分散径流特点的多流向算法可以在一定程度上避免此问题,但多流向算法使得不同区域的汇水单元可能存在交叉.本文结合两类流向算法各自的优点和适用性,设计实现了一种混合流向算法,以期在不同的地形条件下模拟得到更加合理的水流分配.首先,使用基于模板的形态检测方法,在给定阈值的基础上,对数字地形进行了分类,D EM被划分为山谷、山脊、鞍部、缓坡和陡坡5类.对陡坡、山谷和山脊区域运用单流向算法;对缓坡和鞍部区域采用多流向算法确定径流方向并进行水量分配.本文选取了黄土地貌和中低山丘陵的两个流域作为研究区,利用并采用了30 m和90 m两个分辨率的DEM.本文研究将混合流向算法与现有其他算法的结果进行比较.相比于多流向算法,该算法结果中的分散效应受到明显的抑制,相比于单流向算法,非自然的平行径流也大幅减少.同时,混合流向算法在较大分辨率D EM上(30 m)改进效果更加明显.
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