The effect of urbanization on the hydrologic regime of the Big Darby Creek Watershed, Ohio

The Big Darby Creek Watershed located near the Columbus, Ohio, metropolitan area, is widely recognized for its excellent quality of environmental health and high biological diversity in a Midwest agricultural landscape. The watershed has been affected by an increasing amount of runoff due to urban growth, causing occasional flooding, nonpoint pollution problems, and reduced biological diversity. This study focused on how urban growth in a once agricultural watershed impacted the surface hydrology. To accomplish this, an improved classification algorithm---the singular value decomposition (SVD) method with key vector analysis---for mapping land use/land cover (LULC) was developed. The developed algorithm resulted in an improvement of the LULC classification accuracy by: (1) using a hierarchical classification approach, (2) separately classifying urban and rural land using an object recognition technique, and (3) integrating the GIS layers (road, streamline, park area shapefile, and golf course) in the postclassification stage. The process produced high accuracy LULC maps for the years of 1974, 1984, 1994, and 2005 with an overall accuracy of >89%. Also, an urban change matrix was introduced to better interpret urban change scenarios in this study. The urban change matrix showed that major urbanization in the Big Darby Creek Watershed occurred between 1994 and 2005.; To investigate the hydrologic impact of the land use change in the Big Darby Creek Watershed, the improved LULC maps were used. The HEC-GeoHMS/HEC-HMS tools were used to develop a hydrologic model for the watershed in a distributed modeling scheme. The SCS Curve Number method was used for precipitation-runoff calculation, and the ModClark method was applied for the overland flow computation. The Muskingum-Cunge routing method was used for the river routing calculation. A storm event (April 15-30, 2005 period) was used to calibrate the HEC-HMS model at the Darbyville gauging station. The hydrologic simulation results showed that there is a significant increase (15%) in the peak hydrograph for Hellbranch Run, which is the most urbanized subwatershed in Big Darby Creek. When compared to the population forecast and recent reports on the Darby Creek development plan, the impact of the urban development will have a significant effect on the hydrology of the watershed. A careful hydrologic design and analysis will be required for considering any future urban development in the Big Darby Creek Watershed. The sensitivity analysis of the LULC maps showed that the resulting hydrograph from the simulated LULC error would be minor for small urbanizing areas.; In conclusion, an improved method for mapping LULC and its change in an urbanizing watershed was developed to investigate the impact of surface hydrology. The research resulted in a framework that can be used to study urbanizing watersheds. This framework can be used for future modeling efforts to understand the hydrological impact of LULC change in a watershed on a large scale.