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APPENDIX B Techniques for Manual, Topographic.Map Measurements of Primary Drainage-Basin Characteristics Used in the Regression Equations I I I I I The drainage-basin flood-estimation method is applicable to unregulated rural stream sites in Iowa with drainage areas less than 1,060 mi2. Specific information concerning techniques for making manual measurements is outlined for the six primary drainage-basin characteristics that are used to calculate the four basin characteristics listed in the regression equations in table 2. Comparisons between manual measurements made from different scales of topographic maps are shown in table 7 for four of these six primary drainage-basin characteristics. Table 7 demonstrates that several of these primary drainage-basin characteristics are map-scale dependent. Map-scale dependency refers to a condition whereby a drainage-basin characteristic value is affected substantially by the scale of topographic map used in the measurement. The comparisons in table 7 list the percentage differences between msnusl measurements made at the same scale used for geographic- information-system (GIS) measurements (the base scale) and manual measurements made at different scales. Use of maps of scales other than the scales used to develop the equations may produce results that do not conform to the range of estimation accuracies listed for the equations in table 2. The scale of map to use for manual measurements of each primary drainage-basin characteristic is outlined in this section and in the footnotes at the end of Appendix A. I I I I I I Total Drainage Area (TDA) The stream site is located and the drainage-divide boundary upstream of the site is delineated on 1:250,OOO-scale U.S. Defense Mapping Agency (DMA) topographic maps. The drainage-divide boundary is delineated along the topographic divide that directs surface-water runoff from precipitation to the basin outlet located at the stream site. The drainage-divide boundary is an irregular line that traces the perimeter of the drainage area and is perpendicular to each elevation contour that it crosses <Office of Water Data Coordination, 1978, p. 7-9 - 7-10). In some cases it may be difficult to delineate the drainage-divide boundary on 1:250,OOO-scale topographic maps, particularly for small drainage basins or for drainage basins located in areas of low relief. In such cases it may be necessary to use larger scale topographic maps, such as 1:100,OOO-scale or 1:24,OOO-scale maps, to facilitate the delineation. Figure 4A shows the drainage-divide boundary for the Black Hawk Creek at Grundy Center streamflow-gaging station (station number 05463090; map number 73, fig. 1). Because GIS measurements of total drainage area were quantified from 1:250,000-scale topographic maps, the appropriate scale for manual measurements of total drainage area is 1:250,000. Total drainage areas for many Iowa stream sites are listed in "Drainage Areas of Iowa Streams' (Larimer, 1957). The total drainage areas listed in this publication can be used to calculate contributing drainage area (CDA) once any necessary adjustments for noncontributing drainage areas (NCDA) are accounted for. Manual measurements of total drainage area for stream sites typically are planimetered or digitized from topographic maps if drainage areas are not listed in Larimer's (1957) publication. Noncontributing Drainage Area <NCDA.) Noncontributing drainage areas usually are identified as either an area of internal drainage or as an area draining into a disappearing stream. Internal drainage areas drain into depressions, which are represented by hachured contour lines on topographic maps. Internal drainage areas may include potholes or marshes, which are common within the Des Moines Lobe landform region in north-central Iowa (Region II, fig. 2). Disappearing streams do not connect with the drainage network that reaches the basin outlet. In the karst topography of northeast Iowa, sinkholes are a common cause of disappearing streams. APPENDIX B 4tI