You probably know your Social Security number, your driver’s license number and perhaps the latest wrinkle in mattress marketing, your sleep number. But do you know your drought number? The latter represents the amount of water you are allowed to use per day. If you don’t know it, you probably should. Not knowing could cost you money. As California’s severe drought moves into a fourth year, state and local water agencies are working on something called “allocation-based rate structures,” a kind of precursor to water rationing that’s all the rage in Sacramento and in some areas such as Santa Cruz,...