Hydrologic Impact of Urbanization on Catchment and River System Downstream from Taihu Lake

This study examined the hydrologic impacts from rapid urbanization of a river valley and associated floodplain. The catchment downstream of the Taihu Lake was selected as the study region. Remote sensing images and topographic data for the period of post-1990 were employed to estimate hydrologic indicators for analyzing evolutionary features of the river system. Time series methods were adopted to analyze the hydrological effects on the watershed and river network resulting from rapid urbanization over the past 20 years. The results demonstrated that: (1) the number, length, and drainage density of rivers decreased from 1991 to 2006. The overall number of rivers decreased by 36.3%, and the overall river length was reduced by 25.5%; (2) the relationship between river length, river number, and stream order showed a geometric series over time, indicating a self-similarity in the river network that persisted even as the network changed due to urbanization; (3) the river network fractal dimension and complexity showed a decreasing trend over time, which indicated a simplification of the river system. In 1991, 2001, and 2006, the river network fractal dimension values were 2.24, 2.12, and 1.9, respectively; (4) excluding the influence of meteorological factors, there were at different levels increases in regional and seasonal precipitation. In addition, the difference in precipitation between urban and suburban areas increased gradually. Water levels also rose when the influence of precipitation was excluded.