Closing the irrigation shutoff may leave water in exposed pipe

I wouldn’t treat opening a drain as proof that the exposed pipe is empty. Gravity drainage is reassuring only if each vulnerable section can actually fall toward an accessible outlet; a sag, capped branch, closed valve, or water trapped around a valve or backflow assembly can defeat that. The useful details are the pipe route and elevation changes, where shutoffs and drains sit, and which valves must be open for each section to drain.

A blowout can help clear sections that won’t drain by gravity, but it isn’t automatically safer than relying on drain points: pressure can damage irrigation components if it exceeds their limits. I’d treat the equipment’s specific pressure limits and the backflow assembly’s requirements as constraints, not assume one setting or method fits every system. If only one section is uncertain, the safer choice may be targeted rather than blowing out the entire system.

For example, if the main exposed runs drain through accessible outlets but a short branch dips below them, I’d treat that branch as the unresolved part - not assume the whole system needs a blowout. First establish whether it has its own way to drain; if not, consider any other method only once its components’ limits and backflow requirements are known. That keeps the decision focused without assuming the branch is empty.