Greenroofs Can Save Cities Millions Of Gallons Of Water

Not to be nit-picky (but I am), just how does a green roof save millions of gallons of water?

1.) I guess they mean green roofs divert/delay water that might otherwise have to be treated as in NYC where the storm drains also run into the waste treatment systems.

2.) In the case of Baltimore, concerns are that rainwater runoff goes into the bay. Of course, one might ask what happens to contaminants that aren't washed off the streets? Don't they still eventually end up in the bay? Except perhaps in higher concentrations when they do get there? Or does sunlight (a potent force) decompose these compounds into simpler chains? Run-off into the Chesapeake Bay is considered to be the boogey-man that kills the bay. But it is a complicated problem of waste treatment plants, storm drainage, septic systems, agriculture, industry, auto emissions, coal emissions, etc.

I'm a little suspicious of the numbers quoted in the study. If the Baltimore area can divert 8 million gallons a year by using green roofs and that is said to be 10%, then what that says, is that rain water only dumps 80 million gallons a year into the bay. That seems a bit low to me.

Here are some numbers: Annual precipitation for Baltimore ~40 inches

Number of cubic inches in a gallon: 231

One square foot equals 144 square inches

Therefore each square foot of Baltimore receives= (40*144)/231 = 24.9 gallons per year

According to the U.S Census, the City of Baltimore has 80.8 square miles of land mass.

Each square mile has 27878400 sqare feet.

Therefore each square mile of Baltimore receives approximately = 24.9*27878400 = 694 million gallons of precipitation every year.

694 millions gallons per square mile multiplied by 80.8 sqaure miles results in 56.089 BILLION gallons.

So, just Baltimore proper, receives approximately 56 Billion gallons of rainfall per year. It appears to me that the numbers discussed are off by three orders of magnitude. I understand that not all rainfall actually makes it to the bay, but less than one tenth of one percent? I wonder who this student's faculty advisor is, and did he or she even question the numbers? Maybe the numbers discussed just refer to the college campus where she is studying and the article is in error.

Who knows? But just goes to show how wildly inaccurate reporting/studies can be and how many sheeple actually go through the process to validate the numbers? Even to some rough approximation?

Cheers!!