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Most people love blue skies and clearly prefer being drenched with sunshine rather than rain.  The sunny regions of the country are booming as people move to where they know the calendar will be filled with blue-sky days.

But think a bit more about what a blue sky means. In other words, think about the absence of clouds.

If you know where your water comes from, you know towns and cities draw their water from rivers, lakes or groundwater. These sources are replenished by rain and snow, and we all know that rain and snow come from…clouds. 

So clouds are like water-delivery trucks rolling into town. What does this say about places that are rarely visited by clouds?

To better understand the relationships between clouds and water supply, I did a very quick analysis of some readily available numbers.

• First, I looked at SustainLane’s ranking of the sustainability of major U.S. cities’ water supply systems. These rankings provide an indication of how reliable or, on the other hand, how stressed or vulnerable a city’s source of water is.
• Second, for each city I looked at the average number of cloudy days per year, from the National Weather Service.

The result shows that the most water-sustainable cities are the most cloudy and the least water-sustainable cities are those with the most sun. (See the graph below; the most water-sustainable cities have low numbers and are on the left side of the graph.) This correlation will not shock anyone who remembers the hydrological cycle from high school.

Water chart

The 10 most water-sustainable cities have, on average, 160 cloudy days and 93 days of full sun. The 10 least water-sustainable cities are nearly the mirror image: 90 cloudy days and 168 sunny days per year.  (For those who like statistics, let’s just say the differences in these averages are highly significant.).

People are clearly choosing abundant blue skies over abundant freshwater.

While the bottom five cities for water sustainability (the very sunny Las Vegas, Phoenix, Mesa, Tucson, and L.A.) grew by an average of 37% from 1990 to 2000, according to Census data, the five most water-sustainable cities actually lost population, with an average loss of 3%.

In fact, of the five most water-sustainable (and cloudy) cities, only Chicago grew. The other four — Cleveland, Milwaukee, Detroit and New Orleans — all lost population.  The 2010 census will show that these trends have continued.

There are two reasons for concern here.

First, rapidly growing populations threaten to outpace water supplies in many cities. Even in the relatively wet southeast, recent droughts have revealed that booming populations are straining water supplies in places like North Carolina and Atlanta.

These trends will be worsened by climate change, which is predicted to increase drought risk throughout the United States. And the regions most at risk for climate-change-induced drought encompass the cities that today have both the lowest water sustainability and rapidly growing populations.

In short, Americans are collectively moving from the places that are best equipped to deal with climate change to those that are least equipped.

The second reason for concern is that supplying water to regions that lack abundant local supplies requires intense manipulation and alteration of rivers. Massive dams store water for diversion into long canals — artificial rivers for cities that lack their own. The result is blocked rivers, decimated populations of salmon and other native fish, and parched deltas and estuaries (where rivers meet the sea).

For example, dams on the Colorado River provide the water to support the rapid growth of cities from San Diego to Las Vegas to Phoenix. But as a result, the Colorado River rarely flows to the sea and its delta has now been reduced to 5% of its original size.

The Colorado Delta was once one of the world’s great desert estuaries. Legendary conservationist Aldo Leopold fell in love with the place, enraptured by its labyrinth of channels and ponds fringed by lush vegetation, teeming with birds and haunted by the nocturnal screams of jaguars.

The first time I flew into Phoenix I couldn’t help but think: “That’s where the Delta went.” With unintentional irony, the city had recreated an orderly proxy of the unruly Colorado Delta landscape. The delta was once a mosaic of water and jungle, an island of green in a sea of desert. Phoenix, too, is an island of green in a sea of desert, with its own mosaic of swimming pools and lush yards and golf courses (see photo below).

I don’t mean to pick on Phoenix here. I do want to raise awareness of the implications of our choices about where to live.

If you live in a place where it’s rarely cloudy (e.g., Las Vegas), or where it never rains during the warm half of the year (e.g., California), understand that getting water to your tap requires a massive reworking of natural systems and that water is particularly precious in such places.

And if you live where you wish you saw more of the sun, remember those clouds are water-delivery trucks lining up to keep your city sustainably watered and more resilient and prepared for climate change.

Googlemaps images

SEE THE PROBLEM?: On left, the parched Colorado River Delta in Mexico, showing the maze of twisting channels that once were filled with water and surrounded by lush forests and wetlands; on right, Phoenix, AZ, with lawns, golf courses and pools sustained by Colorado River water. (Photo: Google Earth)

— Text by Jeff OppermanCool Green Science Blog

Americans choose sun over water: Why that's bad news
Which do you prefer: abundant blue skies or abundant blue (fresh) water? It may be hard to have both, especially as climate change continues, says Conservancy s