Impact of Impervious Surface on River Discharge in Lake Kasumigaura Basin, Japan

Impervious Surface Area (ISA) is defined as the constructed surface that prevents water from infiltrating into the soil. The ISA has emerged not only as an indicator of the degree of urbanization, but also as a major indicator of environmental quality for drainage basin management. This study focused on the relation between ISA ratio calculated by remote sensing technology and river discharge in Lake Kasumigaura Basin, Japan. ISA ratio was estimated by satellite image using Prescreened and Normalized Multiple Endmember Spectral Mixture Analysis (PNMESMA) developed for drainage basin with the estimating error smaller than 10%. Three types (vegetation, impervious surface, soil) of endmember were selected from the image, and the fraction of each endmember was calculated based on linear mixing model. River discharge dataset was collected from Kasumigaura River Office. Since the monitoring sites do not locate in the down stream, the up stream river basins of the monitoring sites were extracted by hydrological model in Geographic Information System (GIS) instead of the existing basin map. Digital Elevation Model (DEM) data with spatial resolution of 10m was collected from Geospatial Information Authority of Japan (GSI) to estimate the area of drainage. For the extracted drainage basins, the statistical relation between ISA ratio and river discharge were studied in 2000 and 2007 since the Landsat images used to estimate the fraction of ISA were in good image quality. The long-term change of river discharge was also investigated to provide the background value of this research. Results of spatial analysis suggested that the increase of the ISA raised the discharge in the rainy season, and reduced the discharge in the period of water shortage in Lake Kasumigaura Basin.