broken ball valve lever handle replacement

I can still turn the valve using vice grips, but i want to replace the handle. My question is this... do I need to shut off the water main to the house in order to change the handle? Can I take the half inch nut

Sure you can. Your yard needs watering anyhow, and taking out the nut and the stem will allow you to do an excellent job.

The other possibility is that there is a valve somewhere, in addition to the one that broke, that will turn off the water to the sprinkler system. In that case, turning that off will have the same effect as turning off water to the whole house: your grass will still need watering.

Yes, you can remove the nut that holds the handle on to the end of the stem without turning the water off. The only question is what condition the end of the stem is in and if you can find a new valve that has the same handle. If it does not come off easily, spray it with some penetrating lube, let it soak awhile. If you don't have that, use some WD40 or similar.

I can't see exactly what you have but yes, you can take off the screw or nut holding the handle and take the broken piece off. Reverse to assemble. I've seen some that even come with a new nut.