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Table 1. Comparisons of manual measurements and geographic-information-system-procedure measurements of selected drainage-basin characteristics at selected streamflow-gaging stations--Continued Measure- Selected drainage-basin characteristics Station ment number technique TDA1 BP BR FOS TI'F 06807780 MAN 42.7 47.4 268 18 3.05 GIS 42.8 48.8 280 19 3.05 % DIFF +0.2 +3.0 +4.5 +5.6 0 06903400 MAN 182 79.0 224 80 3.25 GIS 184 79.6 256 80 3.25 % DIFF +1.1 +0.8 +14.3 0 0 WILCOXON SIGNED-RANKS TEST STATISTIC2 -1.726 -1.334 -1.843 -0.365 NO TES-r3 p-VALUE STATISTIC 0.0844 0.1823 0.0653 0.7150 1 Manual TDA measurements are streamflow-gaging-station drainage areas published by the U.S. Geological Survey in annual streamflow reports. Noncontributing drainage areas (NCDA) are not listed because none were identified for these drainage basins. 2 Using a 95-percent level of significance, the T-value statistic = 2.2010 (Iman and Conover, 1983, p.438). 3 All values for % DIFF = O. Comparison measurements for total drainage area (TDA) indicate that the GIS procedure was within about 1 percent of the drainage areas published by the USGS in annual streamflow reports for 8 of the 12 selected gaging stations. This comparison indicates that delineations of drainage areas used in the GIS procedure, made from 1:250,000-scale topographic maps, were generally valid. The Wilcoxon signed-ranks test was applied to four of the five drainage-basin characteristics listed in table 1 using STATIT procedure SGNRNK (Statware, Inc., 1990, p. 3-25 - 3-26). Results (table 1) indicate that GIS procedure measurements of total drainage area, basin perimeter (BP), basin relief (BR), and number of first-order streams (FOB) were not significantly different from manual topographic-map measurements at the 95-percent level of significance. The greater , variation in measurement comparisons of basin relief are believed to be due to limitations in the 1:250,000-scale OEM data. Results of the comparison tests (table 1) indicate that GIS procedure measurements are generally valid for the primary drainage-basin characteristics used in the regression equations presented in the following section. Basin slope (BS) is another drainage-basin characteristic that was quantified using OEM data. It is hypothesized that basin slope may have a significant effect on surface-water runoff. Basin slope was indicated as being a significant characteristic in a few of the initial multiple-regression analyses. Comparison measurements indicated that the GIS procedure greatly underestimated basin slope. Measure- ment differences for basin slope were between minus 9 and 66 percent, with an average ESTIMATING DESIGN-FLOOD DISCHARGES USING DRAINAGE-BASIN CHARACTERISTICS Hi