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 landuse 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.