A modified topographic index that incorporates the hydraulic and physical properties of soil

Aiming at quantifying the impacts of soil properties on rainfall–runoff processes, the soil saturated hydraulic conductivity ( K s ) and the soil erodibility factor ( K ) were selected and incorporated into the classical topographic index ln(α/tanβ) ( TI ) to construct a modified topographic index ( TI′ ). Stream network extractions and performance evaluations of topography-based hydrological models based on TI and TI′ were carried out in three watersheds with different climate conditions. The investigations indicated that: (1) the changes of stream networks caused by the incorporation of K s ·K could correctly present the phenomenon that the points would show greater potential to be saturated to become contributing areas if their underlying soils possess higher hydraulic conductivities and stronger erodibility; and (2) the performances of the topography-based hydrological models TOPMODEL and TOPX were improved when simulating the daily rainfall–runoff processes with the input of ln(α/(tanβ·Ks·K) ( TI3 ). TI3 was suitable for rainfall–runoff simulation in arid and semi-arid, humid and semi-humid, and humid regions. The performance improvements increased as the spatial heterogeneity of Ks·K enlarged. Based on these investigations, TI3 was recommended for the modified form of TI′ .