Check the status of a cistern when SIP zone is active

[tfcb]

Hi,

My sprinkler system uses both cistern and city water. I'd like to use the cistern water until empty and then switch to city water. 

If the system is running I need to check once per minute to see if the cistern still has water (I already have a way to test this) and turn on the city water if not. I'd also like to ensure that the city water value is closed and cistern pump off if the system isn't running.

What are your thoughts on the best way to implement?

Thanks,

Tim