installed but unused irrigation systems

I'd suggest a middle ground of sorts. By carefully explaining that the system has been shut down (completely I assume) for X years and that shutdown was a strategic decision rather than simple convenience due to abundant rainfall, I think you're halfway there. But to really convince the reviewer that you are committed to zero irrigation water use going forward, you might consider some physical interruption to the system that would render its use more difficult than opening a valve. Perhaps there is a main line that could be cut or removed while the remainder of the system stays in place? That would ensure that there was no reactivation of the system by a new facility manager or landscape contractor without some kind of formal approval, and mitigate costs by leaving 99% of the system undisturbed.