The first consideration is to ship water to Barcelona - by ship - from the French from the Rhone River in Marseille, France. Other considerations include receiving shipments of water from the Tarragona or even, more drastically, a desalinization of Mediterranean waters from Murcia.
10 years have passed since the Barcelona region has experienced such low levels of rain. But Barcelona isn't the only city or region which has had so little rain. Most of Spain has had the same problem in 2007.
The Catalan government continues to ask its residents to conserve water. If drought conditions continue when the summertime dry season arrives, the region will be in a desperate state.
In 2009, Catalunya hopes to start its own desalinization program for its citizens use. The desalinization plant would be located between Barcelona and Tarragona. What would this do to the water levels in the Mediterranean? Aren't special permissions required from the European Union or the neighboring countries?
So when in Barcelona, don't expect to be offered a glass of tap water in area restaurants. Besides, it's not the custom in Spain. You have to request water. And be sure to specify TAP water ("agua de grifo") - if that's what you want - or your waiter will bring you bottled water. Or just avoid the whole issue and order wine!!
Be sure to read the article entitled, "Drought and Desertification of Spain".