Blair - we did a simul-edit 
I added a few words to clarify my thoughts.
If you have a gravity feed water source, that seems ecologically effective. Maybe you could arrange the valve to shut in the event of failure (but I'm no gravity fed watering expert )
I suppose reasonable questions to think through are:
- how long will watering take each day, and how fast is the flow?
- what happens if it fails with water on?
- what happens if it fails with water off?
- how long might it be broken before anyone notices and can fix it?
In Houston, they used to water overnight to reduce evaporation, and a stuck valve could water-log gardens. There were storm drains, so it was never a disaster, but plants would suffer. If there is no reliable run-off, a stuck valve might be expensive.
A friend uses a slow drip feed, so a monitoring system would need to be active for the duration of watering, quite a long time.
If you go away on holiday for a week, would failure be a disaster? What could you do to avoid failure or reduce its impacts.
I am not trying to put you off. I strongly believe smarter use of water is critical for our species survival. I'm just trying to stimulate your creative juices into designing a simple, robust solution.
HTH
GB-)