An longtime leak outside that based on a photo was probably a worn or broken part in the backflow preventer. Suggested the tenant contact a local plumber but... Called Armstrong Plumbing, which dispatched a tech within a couple of hours. The leak was behind a bush and once found, the tech concurred that the culprit was the backflow preventer. He turned on the sprinkler system to observe the leak, took apart the backflow preventer unit, examined it, swapped around some parts, reassembled and reattached it to the irrigation line. Re-testing resulted in no leaks. The tech answered all my questions thoroughly. It was a pleasure to work with the friendly and informative tech and office staff of this family-owned business.