Threshold and scaling effect of impervious surface impact on stream water quality in city with river networks

Rapid expanding of urban impervious surface in cities with river networks has generated a notable impact on the regional environ-hydrological process. Discussion on the water quality response of impervious surface is of great importance for the regional land use regulation and river management. Considering the special hydro-geomorphic characteristics of city with river network, Shanghai was selected as the study area. Based on the 48 water environmental monitoring cross-sections, a serial of buffers in different spatial scale from 100 m to 1500 m were established surrounding each cross-section, which would be used as hydro-logical units to detect the scaling effect. It demonstrated that there was a clear non-linear relationship between TIA (Total Impervious Area) percentage and water quality index, and the threshold effect was clearly verified for these indices, with a common threshold (TIA=60%). However, there was a simple linear relationship and not any threshold effect between TIA percentage and DO(Dissolved Oxygen). The sudden change of different water quality index was different when TIA exceeded the threshold, especially the pollution of NH ₃-N and TN were the most. At the same time, the results were also proved that the threshold effect was independent of the scaling.