Who steals river gauges?
The unit doesn't appear to be quiet energy consuming as it works in one of the two modes: periodical monitoring (30 sec every 15 mins) and on-demand monitoring (react to ph calls). Ley! you're right. That's 12V Gel-Cell Battery.
BTW, "If vandalism is a concern, the system should perhaps be mounted on top of a tree or a high pole." A tree is the way to go!