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I. 1 , \ 1 Noncontributing drainage areas are delineated on 1:250,Ooo-scale topographic maps. When questionable noncontributing drainage areas are encountered, hydrologic judgment is required to determine whether to delineate these areas as noncontributing. Larger scale topographic maps facilitate the delineation of questionable noncontributing areas. 1 1 1 Basin Perimeter (BP) The basin perimeter is measured along the drainage-divide boundary delineated on 1:250,OOO.scale topographic maps. Because GIS measurements of basin perimeter were quantified from 1:250,OOO-scale topographic maps, the appropriate scale for manual measurements is 1 :250,000. Basin Relief (BB) 1 I I I I , - I 1 1 I - I I Basin relief is the difference between the maximum elevation contour and the minImum interpolated elevation within the contributing drainage area delineated on 1:250,OOO-scale topographic maps. The minimum basin elevation is defined at the basin outlet as an interpolated elevation between the first elevation contour crossing the main channel upstream of the basin outlet and the first elevation contour crossing the main channel downstream of the basin outlet. Because GIS measurements of basin relief were quantified from 1:250,Ooo-scale digital elevation model (OEM) data, the appropriate scale for manual measurements is 1:250,000. Figure 4C shows the elevation contours created from OE~ data for the Black Hawk Creek at Grundy Center drainage basin. I I Number of First.Order Streams (1'08) The number of first-order streams is a count of all the stream segments defined as being a first-order drainage using Strahler's method of ordering streams (Strahler, 1952). First-order streams are defined for contributing drainage areas on 1:100,OOO-scale topographic maps. Figure 4B shows the stream ordering for the Black Hawk Creek at Grundy Center drainage basin. As shown in figure 48, a stream segment with no tributaries is defined as a first-order stream. Where two first-order streams join, they form a second-order stream; where two second-order streams join, they form a third-order stream; and so forth. Because GIS measurements of the number of first-order streams were quantified from 1: lOO.OOO-scale digital line graph data, the approprjate scale for manual measurements is 1:100,000. Comparison measurements listed in table 7 indicate that the number of first-order streams is clearly map-scale dependent and use of map scales other than 1:100,000 may produce results that do not conform to the range of estimation accuracies listed for the equations in table 2. 2.Year. 24.Hour Precipitation Intensity (1TF) The map shown in figure 5 is used to calculate 2-year, 24-hour precipitation intensities for drainage basins in Iowa and for basins that extend into southern Minnesota. This map shows polygon areas that represent averages for maximum 24-hour precipitation intensities, in inches, that are expected to be exceeded on the a\'erage once every 2 years. These polygons were created from the precipitation contours depicted on 2-year, 24-hour precipitation intensity maps for Iowa (Waite, 1988, p. 31) and the United States (Hershfield, 1961, p. 95). The polygon areas for southern Minnesota were interpolated from the precipitation contours depicted on the United States map. The polygons shown in figure 5 represent the average value, in inches, of rainfall between the precipitation contours and are not intended to represent interpolated values between the contours. Figure 5 was used to compute a weighted average of the 2-year, 24-hour precipitation intensity for each drainage basin processed by the GIS procedure. A manual measurement. of 2-year, 24-hour precipitation intensity can be made by delineating the approximate location of the drainage-divide boundary for a stream site in figure 5. The approximate percentage of the total drainage area for the APPENDIX B Sl