Computation Method of Topogr aphic Wetness Index in Low Relief Ar ea
Received date: 2006-06-01
Revised date: 2006-09-01
Online published: 2006-11-25
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 index dramatically jumps down in other parts of wide valley area. This is unreasonable because the soil moisture should be comparatively average and high in the wide and flat valley, and the 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.
QIN Chengzhi,YANG Lin,ZHU A- Xing,LI Baolin,PEI Tao,ZHOU Chenghu . Computation Method of Topogr aphic Wetness Index in Low Relief Ar ea[J]. PROGRESS IN GEOGRAPHY, 2006 , 25(6) : 87 -93 . DOI: 10.11820/dlkxjz.2006.06.010
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