Oops. I forgot to comment ...
Any idea what hardware I would use? I'm assuming some kind of servo/solenoid something? (No idea where to look to find that kind of thing)
Or, is there hardware (diy) to turn a 120v outlet on/off?
Do you need the Arduino to turn the water on/off, or can that be your host/server?
An area of concern is the amount of power needed for an occasional bit of sensing is quite low, so the whole thing could run off a battery for a long time, or even be solar powered with a small panel. Further, it isn't super critical. If it dies due to a flat battery, and you miss a few hours data, it is annoying, but probably not mission critical.
EDIT: When you need to control an external device like a pump, your power need may go well beyond a simple self contained sensor system. I've noticed a few threads describing solenoids 'latching' on or off, but that may overly limit your options. Think about powering irrigation from something else, with comparable power requirements, or use the irrigation systems power to run the sensor.
Another concern is when you want to control external systems, like a water pump or irrigation system, failure may have a large impact, like flooding your crops, which may be much more hassle than losing some data.
EDIT: Control the irrigation system with something with comparable levels of availability, or use something which makes it is easy to detect failure, and react appropriately.
Just something to think about.
HTH
GB-)