Have you ever looked closely at a map of California? Fresno and Tulare counties are almost dead center.

That is significant for one very good reason. A central location means companies that truck merchandise can reach the bulk of the state's 37 million or so residents in one day. That's why Best Buy, Gap, Walmart, Sears and other heavy hitters have massive distribution centers in the region.

And those warehouses have expansive roofs. Hundreds of thousands of square feet could generate power through rooftop solar, or they could cut energy costs and water runoff by going green - literally.

Borrowing a concept popular in Europe, green roofs - complete with sod and plants - are being planted in New York, Washington D.C., Chicago, Portland and other American cities, according to this fascinating story in environment 360 by Bruce Stutz. The vegetation requires little upkeep, and helps lower power bills and water runoff.

As evidence, Stutz cites a U.S. Postal Service distribution center in Manhattan that has cut storm water runoff into the city's municipal water system by up to 75% during the summer and 40% in the winter. The green roof has also has cut the building's energy use by $30,000 per year.

Chicago has really embraced the concept. That city added 600,000 square feet of green roof last year, and has 600 projects planned. However, Illinois was also the site of an interesting structural collapse when a portion of a green roof fell in. According to this New York Times story, ice could have prevented meltwater from draining.

Portland has a program for residential and commercial properties. “It was a cost/benefit evaluation,” city storm water specialist Tom Liptan told Stutz. “The issue here was storm water. We were trying to find a way to reduce the burden on the city. If we trap it on the roofs, we don’t have to build bigger pipes to carry it or cisterns to store it for treatment.”

Climate and environment help dictate the type of plants to be used, and their effectiveness. Researchers at Colorado State University tested a variety of species and came up with recommendations. Here is a link to their findings, and links here, here and here to studies of regions with Mediterranean and desert climates.

Energy and water conservation in California are becoming bigger issues, particularly in light of stories like this. Maybe making rooftops do double duty would help.

Photo of green roof of U.S. Postal Service building