Calculating Runoff for Dummies

surface runoff types

To misquote Lewis Carroll, "Oh frabjous day!" We discovered a new Leaf Group¹ niche site where lots of the Antisocial Network's old friends ply their trade. It's called Sciencing.com, and our very first DotD candidate is repeat offender Jack Ori (sometimes known as Stephanie Silberstein), whom we found pretending to know "How to Calculate Surface Runoff."² Although Sciencing's version is supposedly copyright 2017, the Wayback machine at archive.org has a version of the post at eHow.com dating back to at least 2013. Oops...

Although the Leaf people file this under "physics," it more realistically belongs under hydrology, meteorology, geography, forestry, watershed management... Regardless, Ori is clearly out of his/her depth when it comes to calculating runoff. Despite having found an authoritative reference (the link to which Leaf didn't update when migrating the content), Jack blew it when rewording the instructions.

According to Ori, all one need do to calculate runoff is

1. Measure area length in feet...
2. Measure area width in feet...
3. Convert rainfall amount to feet...
4. Multiply the three values together to calculate total runoff in cubic feet.

Apparently for grins, Jack tosses in this bit of "trivia": "Calculate the runoff by multiplying the volume by the surface percentage. If the surface is hard (i.e. asphalt or concrete), the surface percentage is 100 percent. If the surface is grassy, the surface percentage is 60 percent." ...and then uses a "garden" 24 x 32 inches in size for his example. Ori's problems? they're multiple. Besides the abject stupidity of his example garden (more like a small flowerbed), he fails to provide even a vague notion of what that "Grassy areas..." factoid is all about. In fact, Jack extracted that number from a much longer (yet still simplified for middle-schoolers) discussion of how runoff from rainfall varies with surface characteristics. Ori never mentions permeability, slope, percolation, cover density, or any of several other factors that bear on the ratio of runoff to infiltration, although the reference material does so.

To top everything off, Jack screws up the math in his final step:

"Multiply the average rainfall by the area of the site: 5.34 square feet x 0.67 feet = 3.578 cubic feet. Grassy areas have a runoff percentage of 60 percent; 3.015 cubic feet x 0.60 = 2.147 cubic feet."

Wait: where did that 3.015 cubic feet come from? We have no idea, but 2.147 is 60% of 3.578... need we say more? In short, Ori knows nothing about this topic, yet still published a rather infantile set of instructions on "How to Calculate Surface Runoff." Returning visitors to the Antisocial Network will remember that publishing knowledge-free rubbish about science is more than enough to qualify someone as our Dumbass of the Day.

¹ Leaf Group is Demand Media Sudios (DMS, as in "You can't spell 'dumbass' without 'DMS'...") rebranded. Still the same garbage, though...
¹ The original has been sent to the rewrite team by Leaf Group, but can still be accessed using the Wayback machine at archive.org. Its URL was   ehow.com/how_6505227_calculate-surface-runoff.html

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