My guess is that it’s a stub for an irrigation system that may be partially or fully installed. The purpose of the stub would be to make it easy to tap into the system if you wanted to extend the system.

Can I cut it down shorter?

Hi, Dig it down some. Maybe you will get a better clue

of what it's for.

Good Luck Woodbutcher

Sure, you can cut it down shorter, but you need to turn off the water at the meter first since you don't know if you have water at pressure in there. I'd begin with just cutting off the cap to see if there is anything unexpected inside, rather than whacking off a big chunk at first.

That's the ventilation shaft for the alien space ship that's under those rocks.

I'd proceed with caution and keep Slim Whitman's "Indian Love Call" cued up on Spotify just in case.