Assessing Impervious Area Ratios of Grid-based Land Use Classifications on the Example of an Urban Watershed

When applying a distributed hydrological model in urban watersheds, grid-based land use classification data with 10 m resolution are typically used in Japan. Land use classifications into 17 categories are made without taking into account their impermeable properties. Thus, for such a grid-based urban hydrological model, the estimation of the Impervious Area Ratio (IAR) of each land use classification is a crucial factor for accurate runoff analysis in urban watersheds. However, so far IAR of each classification is estimated very roughly and applied in the corresponding hydrologic models almost empirically. Thus in order to assess the IAR accurately, we created a set of vector-based “urban landscape GIS delineation” data for a typical urban watershed in Tokyo using detailed land use recognition, which exactly delineated the pervious and impervious features into 20 land use types in the watershed. These vector data are used to improve the impervious area depiction of grid-based land use classifications. By superimposing the vector-based delineation map on the grid-based map, the IAR of each grid-based land use classification was estimated with very little uncertainty, after calculating the IARs of all grid cells in the entire urban watershed. As a result, we were able to calculate the frequency distribution of IAR for each land use classification as well as the spatial distribution of IARs for the urban watershed.