Outside the window of Helen Dahlke’s office, at the University of California at Davis, the clouds hang low, their edges seeming to brush against the building. It’s raining intensely, an unusual event in a perpetually parched state suffering from a five-year drought. “It looks like the end of the world,” says Dahlke happily.As a hydrologist and professor who studies how water flows over and through rock, soil, fields, and farms, she is something of an H2O whiz.