Development of two indices for determining hydrologic implications of land use changes in urban areas

Simple metrics are needed by decision makers to quantify hydrologic impacts of land uses as a result of urbanization. Two indices to evaluate hydrologic implications of land use changes were developed and demonstrated in case studies. The first index, pre-development versus post-development index (PPH), compares pre-development to post-development hydrologic conditions for time of concentration, peak runoff for a 2-year 24-hour storm and a 10-year 24-hour storm, and average annual runoff volume. The second, extent of maximum index (E-H), compares post-development with potential minimum and maximum hydrologic conditions of a site. The indices were applied in three case studies of residential subdivisions in Lafayette, Indiana. These case studies illustrate how to interpret the resulting index values. The Orchards subdivision, which was designed with conservation principles, scored the best for both indices. Although the two indices can be used separately, it is advised to use them together.