We always use "Go piddle" for urinate and "Hurry up!" for bowel movements. She knows the difference, too. When it's been awhile since the last "hurry up" and she comes bounding up to come back inside I'll suggest, "Don't you have to do a hurry up?" You can see the wheels turning, and if it's time she'll go back down the bank and find some convenient site to bless with her treasure.
Sometimes as a further incentive, or in praise, we sing, "Piddity piddity, hurray for liquidity!" You do get silly when you have a doodle. Luckily we have no neighbors within earshot; that would definitely have a dampening effect on our high spirits!
Leslie