To save water, I typically let my lawn go a bit dormant in summer, but under extreme conditions (i.e., 2 weeks without any rain), I want to give it an inch or so once a week.  

Is there some combination of custom settings re: field capacity, etc. that would let me make this happen (I'm thinking of some way to let RM know that the soil can tolerate 2 weeks of no rain just fine, but then needs to start getting water)?.  

What I do NOT want is to be watering a bit each week when the grass doesn't absolutely need it.

Thanks for any ideas!