The state’s years-long drought may have also had a psychological effect on residents, who lately have been praying for rain, said Amir AghaKouchak, a civil and environmental engineering professor at University of California, Irvine. “Fire, when you see it, you immediately feel the danger,” he said. “But rain is different, especially in California, where we consider it a good thing.” Floods, then, can blindside people, he said. And it doesn’t take much water — sometimes just a quarter of an inch in a matter of minutes — to transform a benign hill into a mudslide, AghaKouchak said.