Drought has tightened its grip on the Western U.S., as dry conditions tick on into their second decade and strain a river that supplies 40 million people. Experts agree that things are bad and getting worse. But how exactly do you measure a drought, and how can you tell where it’s going?

Brad Udall is an expert on the subject, studying water and climate at Colorado State University’s Colorado Water Center. Lately, his forecasts for the basin haven’t been particularly uplifting.