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UJ . ,~ '"' _..... J '..I 23 Relations of Mean Annual Flood The composite frequency curves, derived in the preceding section, define dimensionless ratios to the mean annual flood for floods of other recurrence intervals. In order to define the flood-frequency curve in terms of discharge for a specific site, the magnitude of the mean annual flood is required. The magnitude of the mean annual flood is obtained by relating it to some measur- able characteristics of the drainage basins. Excluding climatic factors, the physical characteristics of drainage basins that primarily influence the mean annual flood are size of the basin, shape of basin, alignment with the prevailing direction of storm travel, land and stream slopes, depth and porosity of soil mantle, flood water storage, vegetal cover, and land use. Some of these factors are not subject to numerical evaluation and cannot be used directly in a correlation. However, the mean flow of a stream is a hydrologic measure that must integrate all the factors affecting runoff and consequently it is indicative of flood potential. The mean flow of a stream is the average or arithmetic mean discharge passing a specific point during a certain time interval. As such it provides a numerical integration of the numerous factors that influence streamflow and it is susceptible of correlative analysis. Mean flow, adjusted to the period 1921-56, has been determined for the areas above gaging stations located in and adjacent to Kansas and for the intervening subareas between gaging stations (Furness, 1960). Mean flow was correlated geographically so that the varia- tions portrayed by a map may be used to define mean flow reliably in rnost areas of the State. The map has been reproduced as figure 9 and shows mean flow in units of cubic feet per second per square mile. The mean annual flood was graphically correlated with drainage area, stream slope, shape of basin factors, mean precipitation, and mean flow. Of these variables, only drainage area and mean flow were significant in defining a relation with the mean annual flood. Some factors or combination of factors that influence floods were neither reflected in the size of basin nor in mean flow but their effect was found to be fairly constant over areas having some- what similar geologic features. Accordingly, the State was subdivided into five hydrologic areas, shown in figure 7, and described separately in the following subsections. The geographic boundaries of the areas generally conform wi th topographic, physiographic, and geologic variations and follow stream divides where the preceding characteristics are not seriously violated. Satisfactory relations of mean annual flood to drainage area and mean flow were defined by graphical multiple-correlation techniques for each area that included main- stem stations as well as tributary stations. The regression constants thus de- termined provide the best emperical relation statistically. However, they may not represent the underlying relation of mean annual flood to drainage area and mean flow because the effects of many other factors are included in the relations. The variation in the regression constants from area to area is attributed to the variation in these other factors.