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<br />Table 1. Metropolitan areas included in nationwide urban flood-frequency study-Continued <br /> <br />State <br /> <br />Metropolitan area <br /> <br />Number <br />of <br />sites <br /> <br />City <br />skew <br /> <br />Source of equivalent rural discharge <br />(see references) <br /> <br />Texas <br />Texas <br />Washington <br />Washington <br /> <br />Houston <br />San Antonio <br />Portland-Vancouver <br />Seattle-Tacoma <br /> <br />21 <br />5 <br />3 <br />6 <br /> <br />-.3 <br />-.6 <br />.1 <br />.0 <br /> <br />Liscum and Massey (1980) <br />Schroeder and Massey (1977) <br />CUmntans and others (1975) <br />Do. <br /> <br />total, an average, a percentage, or an index for the total <br />drainage basin. A few of the variables were computed <br />for "thirds" of the basin in an attempt to define some <br />variables further and to provide locations of basin devel- <br />opment. For this study, some basins were divided into <br />upper, middle, and lower thirds on a drainage map with <br />the drainage divide delineated. Each third contains <br />approximately one-third of the contributing drainage <br />area and drains the upper, middle, or lower reaches of <br />the basin. Because travel time or flow time was consid- <br />ered in drawing the lines separating the basin thirds <br />distances along main streams and tributaries were <br />marked to help locate the boundaries of the thirds. This <br />drawing of the boundaries means not that all thirds of <br />the basin bave equal travel distances but that within <br />each third the travel distances of two or more streams <br />are about equal. Since precise definition of the lines <br />dividing the basin into thirds was not considered neces- <br />sary for the variables that utilize this concept, the lines <br />can generally be drawn on the drainage map by eye, <br />without precise measurements. Figure 2 shows sche- <br />matics of three typical basin shapes and their division <br />into thirds. Complex basin shapes and drainage patterns <br />are sometimes encountered; they require more judgment <br />in subdividing. <br /> <br />Topographic and Climatic Variables <br /> <br />The physical and climatic conditions existing in <br />each basin are described by a selected set of topographic <br />and climatic variables. Parameters of physical charac- <br />teristics include drainage-area size, channel length, valley <br />length, stream slope, storage, Soil Conservation Service <br />(SCS) soil classification, SCS soil-cover-complex curve <br />number, and SCS index of potential infiltration. Each <br />basin is divided into thirds, as previously described, and <br />dominant soil classifications are given for the upper <br />third, middle third, and lower third of the basin. The <br />percentage of the total basin covered by each soil type is <br />included. The channel and drainage system efficiency is <br />described by a coefficient estimated according to proce- <br />dures dermed by Espey and Winslow (1974). Bankfull <br />discharge at each gaging station is included, and each <br /> <br />basin that has significant in-channe1 storage is identified. <br />In-channel storage, distinguished from basin storage, is <br />dermed as temporary storage created by detention ponds <br />or ponding at roadway embankments. Climatic variables <br />include mean annual precipitation, rainfall intensity of <br />the 2-hour-duration 2-year recurrence interval, and <br />rainfall intensity of the 2-hour-duration lOO-year recur- <br />rence interval. <br /> <br />Land-Use Variables <br /> <br />Land use within each drainage basin is described <br />with two sets of land-use variables. Each set is derived <br />from an independent source, and although similar results <br />were obtained for most stations, there are some stations <br />for which the two data sources yielded quite dissimilar <br />results. No attempt was made to resolve the differences <br />nor to indicate which was more nearly correct. Land use <br />was not significant in the final results of this study. <br />The first set of land-use data was obtained from <br />I :250,000 land-use maps compiled by the Geological <br />Survey from recent high-altitude photography. Because <br />maps are not available for all cities, these data are not <br />available for some basins. Classifications of land use <br />follow the standard system for remote sensing described <br />by Anderson and others (1976) and include percentages <br />of the basin occupied by residential areas, commercial <br />areas, industrial areas, transportation facilities, mixed <br />urban areas, cropland, forests, lakes and reservoirs, <br />wetlands, rangelands, and a few other miscellaneous <br />types of land use. Dates of the maps used are given in <br />the data base. <br />The second set of land-use data was compiled from <br />recent maps and field surveys by the Soil Conservation <br />Service. Again, because of a lack of suitable maps for <br />some cities, these data were not determined for some <br />stations. Categories of land use follow the SCS classifi- <br />cation system and include residential areas (percentages <br />of the basin having lot sizes of :la, :la, Y., Y., and I acre are <br />provided), paved areas, streets, industrial areas, com- <br />mercial areas, forests, meadows, pasture and rangelands, <br />cultivated lands, and open spaces. <br /> <br />Data Base 5 <br />