An irrigation system is far more complicated than a thermostat however there are already strong contenders in this space. I use a Cyber-Rain[1] controller which adjusts based on humidity, temperature and forecast along with details provided during setup such as sprinkler types, soil types, grade and more. When your system has a flow sensor the controller will even shut down the system in the event of a broken head and send an email instead of letting hundreds of gallons of water run down the street.

[1]http://cyber-rain.com