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The drainage-basin flood-estimation method developed in this study is similar to the regional flood-estimation method developed by Lara (1987) because both methods estimate flood discharges on the basis of morphologic relations. While the standard errors of estimate appear to be higher for the drainage-basin equations than for Lara's equations (Lara, 1987, p. 28), a direct comparison cannot be made because of the different methodologies used to develop the equations. Lara's method is based on the physiography of broad geographic landform regions defined for the State, whereas the drainage-basin method presented in this report is based on specific measurements of basin morphology. The drainage-basin equations are independent of hydrologic regionalization. The application of regional equations often requires that subjective judgments be made concerning basin anomalies and the weighting of regional discharge estimates. This subjectivity may introduce additional unmeasured error to the estimation accuracy of the regional discharge estimates. The drainage-basin regression equations presented in this ro~port provide a flood-estimation method that minimizes the subjectivity in its application to the ability of the user to measure the characteristics. Example of Equation Use-- Example 1 Examole I.--An application of the drainage- basin flood-estimation method can be illustrated by using the equation (listed in table 2) to estimate the 100-year peak discharge for the discontinued Black Hawk Creek at Grundy Center crest-stage gaging station (station number 05463090; map number 73, fig. 1), located in Grundy County, at a bridge crossing on State Highway 14, at thE' north edge of Grundy Center, in the NWl/4, sec. 7, T. 87 N., R. 16 W. Differences between manually computed values (table 1) and values computed using the GIS procedure (tables 1 and 9) are due to differences in applying the techniques. Step 1. The characteristics used in the drainage-basin equation (table 2) are contributing drainage area (CDA), relative relief (RR), drainage frequency (D.fi'), and 2-year, 24-hour precipitation intensity (TTF). The primary drainage-basin characteristics used in this equation are total drainage area (TDA), noncontributing drainage area (NCDA), basin relief (BR), basin perimeter (BP), number of first-order streams (FOS), and 2-year, 24-hour precipitation intensity (TTF). These primary drainage-basin characteristic measurements and the scale of maps to use for each manual measurement are described in Appendix A and Appendix B. Step 2. The topographic maps used to delineate the drainage-divide boundary for this gaging station are the DMA 1:250,000-scale Waterloo topographic map and the USGS 1:100,000-scale Grundy County map. Figure 4A shows the drainage-divide boundary that was delineated for this gaging station on the 1:250,000-scale map. Contributing drainage area (CDA) is calculated from the primary drainage-basin characteristics total drainage area (TDA) and noncontributing drainage area (NCDA). The total drainage area published for this gaging station in the annual streamflow reports of the U.S. Geological Survey is 56.9 mi2 (table 9). Inspection of the 1:100,000-scale map does not show any noncontributing drainage areas within the drainage-divide boundary of this basin. The contributing drainage area (CDA) is calculated as CDA = TDA-NCDA, (10) = 56.9-0, ::: 56.9 mi 2 . Step 3. Relative relief (RR) is calculated from the primary drainage-basin characteristics basin relief (BR) and basin perimeter (BP). The difference between the highest elevation contour and the lowest interpolated elevation in the basin measured from the 1:250,000-scale topographic map gives a basin relief of 181 ft (table 1). Figure 4C shows an approximate represent.ation of the topography for this drainage basin. The drainage-divide boundary delineated on the 1:250,OOO-scale topographic map (fig. 4A) is used to measure the basin perimeter, which is 33.5 mi (table 1). Relative relief (RR) is calculated as 18 ESTiMATING DESIGN-FLOOD DISCHARGES FOR STREAMS IN IOWA ~