I guess that would put the Arduinos at the nodes (sensors). These would need to be in watertight containers and you certainly would need locally regulated power since you could not just send 5volts over a long distance down a bit of CAT5 (or whatever). An ATTiny 84 would be admirably suited to this task since it has sufficient I/O and is dirt cheap.
Can you design this? Do you need help? Just let me know. I have a number of Adruinos running various lights in my house.
I am now even more worried about lightning strikes, etc. with buried cables, arduinos, power regulation and the sensors all in a watertight enclosure, some buried and some not buried. This is an interesting challenge indeed.