Walker Art Center's Cinema of Urgency series, a selection of global-issue films from PBS' long-running P.O.V. documentary series, begins Friday with Irena Salina's "Flow: For Love of Water". Exploring the planet's coming shortage of drinking water and corporate plans to privatize the delivery of clean water, the film couples outrage and alarm in the perfect explosive activist package. The villains in the story are the usual suspects (one of them starts with a "C" and ends with "oca Cola"). But the heroes are unexpected and inspiring -- especially the ragtag team of African plumbers who work secretly through the night to reconnect a shantytown's water pipes.