Runoff Estimation 

Comparative SCS CN and L-THIA based Estimation

Land use maps, hydrologic soil group maps, event rainfall data, and long-term daily precipitation data were prepared to run L-THIA GIS for estimating runoff volume. Land use maps for the study area for 1920, 1943, and 1990 were edited so they contained the land use classifications required for L-THIA GIS. The land use classes considered by L-THIA are: Agricultural, Commercial, Forest, Grass/Pasture, High Density (HD) Residential, Low Density (LD) Residential, and Industrial. The hydrologic soil group map was reclassified into 4 groups – A, B, C, D (Refer to Table 2). The land use management practice was assumed good for the entire study area. The land use maps of the IRLB and KSC areas for 1920, 1942, and 1990 and the hydrologic soil group maps were used to create CN maps of the study areas for each year using L-THIA GIS. 

Rainfall events of 1-year, 5-year, 10-year, 50-year, and 100-year return periods for 24 hours were selected for analysis in addition to the long-term L-THIA analysis. Rainfall depths for these events were obtained from US Weather Bureau precipitation data (US Department of Commerce, Weather Bureau, 1961). Long-term daily precipitation data was obtained from "Summary of the Day" CD-ROMs (NCDS, 1996). Precipitation data from the Melbourne, Florida, weather station was used for the entire study area.

The L-THIA GIS system was used to compute runoff depth maps for the group of events as well as the long-term average annual runoff. From the runoff maps, the L-THIA GIS system was used to estimate runoff volumes. 

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