When looking at the equation, it appears that the units on the right do not match those on the left.
How could cubic feet per minute equal inches per hour?
Actually the units do match because, very conveniently, l-inch of rain falling on 1-acre of ground equals 1.008
cfs.
This is so close to unity that no correction factor is needed in the formula.
For simplicity the Rational Method is based on an idealized rainstorm; one that really never occurs in nature. In
this rainstorm, the rainfall is steady and uniform over the entire watershed. In its attempts to represent real
rainstorms, these idealized rainstorms have higher rates of rainfall for storms that are short in duration. (This was
explained in Lesson 1 using the hydrographs.) For such idealized storms, the maximum discharge can take place
only when the entire basin contributes simultaneously to that runoff. This is because it takes time for water to
flow from where it falls, over the surface, to the outlet of the watershed (see figure 2-1).
Figure 2-1. Flow across a regular drainage basin
2-3
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