The TWI is a physically based index or indicator of the effect of local topography on runoff flow direction and accumulation. The index has applications in specialized rainfall runoff modeling and simulation of the spatial distribution of soil moisture and precision agriculture (Beven and Kirkby, 1979; Qin et al., 2011). The computation of TWI is performed using both geographic information systems (GIS) and Python, a programing software used to enhance computing capabilities. The indices help identify rainfall runoff patterns, areas of potential increased soil moisture, and ponding areas. In promoting flood risk awareness and reducing flood losses, FEMA provided funding to the ISWS to further develop the TWI for Illinois as in the UFAA report (State of Illinois, 2015). The ISWS recommended Will and DuPage Counties for TWI development because both counties have high resolution LIDAR, countywide stormwater management agencies, and capacity to implement county level planning and provide countywide mitigation plans for urban floodingissues. Furthermore, these counties also present the opportunity to demonstrate how a TWI analysis can be utilized for both urbanized (DuPage) and urbanizing (Will) counties. The TWI data provided by this project can inform activities undertaken by federal, state, and local agencies to help develop awareness of potential local flooding, assist with community’s flood risk communication, and support local efforts to reduce flooding risk within a community or watershed area. TWI information can aid in the identification of areas best suited for green infrastructure projects such as pervious pavers, rain gardens, and open space, as well as assist future development projects through the identification of areas susceptible to flooding. The products from this study serve to demonstrate the utility of developing TWI for the rest of Illinois.
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