中文摘要：

地形湿度指数（topographic wetness index）可定量模拟流域内土壤水分的干湿状况．在流域的土壤及分布式水文模型等研究中具有重要的意义。但现有的地形湿度指数计算方法在应用于地形平缓地区时会得到明显不合理的结果。即在河谷地区内。地形湿度指数仅在狭窄的汇水线上数值较高，而在汇水线以外的位置则阶跃式地变为异常低的地形湿度指数值。本文针对此问题对地形湿度指数的计算方法提出改进：以多流向算法MFD-fg计算汇水面积．相应地以最大下坡计算地形湿度指数。再基于一个正态分布函数对河谷平原地区内的地形湿度指数进行插值处理。应用结果表明．所得地形湿度指数的空间分布不但能合理地反映平缓地区坡面上的水分分布状况，并且在河谷地区内地形湿度指数值也都比较高。其空间分布呈平滑过渡，因而整个研究区域的水分分布状况得到了比较合理的反映．

英文摘要：

Topographic wetness index, which is designed for modeling the status （＂dry＂ or ＂wet＂） of the soil moisture quantitatively, is an important index for both predictive soil mapping and distributed hydrological modeling in a catchment. Current methods for calculating topographic wetness index have evident problems when applied in low relief area. Outside the positions of narrow accumulation line with high topographic wetness index, the topographic wetness value of topographic wetness index should be high. This problem is caused by both the flow accumulation algorithm and the slope gradient used during computing the topographic wetness index. A new method for computing topographic wetness index is proposed in this paper to address this problem. Firstly, flow accumulation is calculated by a multiple flow direction algorithm （MFD-fg）. Topographic wetness index is then computed by the flow accumulation and maximum downslope. The maximum downslope used in the computation of topographic wetness index is matched with the idea of both MFD-fg and topographic wetness index. Furthermore, a post-processing method is also proposed to compute the topographic wetness index in valley area. The topographic wetness index in the valley area is interpolated by a Gaussian function based on the value of the topographic wetness index on the nearest position on extracted flow accumulation line. The application in a small watershed shows that the method proposed in this paper can get a comparatively reasonable distribution of topographic wetness index for not only the hillslope but also the wide valley area. The value of topographic wetness index in valley area is averagely high and with a smooth transition, which reflects the natural status of the soil moisture in application area. In the future research, the method proposed in this paper will be evaluated by both artificial surfaces and the real applications.