Long-Term Hydrologic Impact of Urbanization: A Tale of Two Models

At a watershed scale, land-use change can increase runoff, flooding, and nonpoint source pollution and degrade downstream water bodies. Thus it is important to assess the potential hydrologic impacts of land-use change prior to watershed development. The L-THIA (Long-Term Hydrologic Impact Assessment) model is a tool to initially assess how land-use change affects annual average runoff and is based only on readily available data. Because L-THIA is relatively new, it is important to test it against other, well-accepted methods. The U.S. Environmental Protection Agency's SWMM (Storm Water Management Model), a well-known and widely used model, was used to perform runoff calculations for comparison with L-THIA. Applications of L-THIA and SWMM to two small watersheds in Chicago show that L-THIA predicts annual average runoff between 1.1 and 23.7% higher than SWMM. The agreement between the results is higher for larger watersheds. Both models predict a linear relationship between average annual runoff and increasing imperviousness. However, for a 10% increase in imperviousness, SWMM predicts an increase between 9.8 and 10.2% in annual average runoff, whereas L-THIA predicts an increase between 6.1 and 7.8%. Overall, L-THIA was easier and quicker to use than SWMM, because SWMM required time-consuming input data collection and formatting. Results of this and other analyses suggest that L-THIA can be an appropriate tool for initial assessment of the relative impacts of land-use change scenarios.