If the water body can contain the rainfall event with no discharge to the sanitary sewer, then the runoff coefficient is essentially zero. If, on the other hand, the rain that falls directly into the water body raises the water level sufficiently to overflow into the sanitary sewer, the runoff coefficient is 1.0. For example, pools will often have a bit of freeboard before the water level rises enough to overflow and this will allow you to capture the rainfall, and even make up for some evaporation that would otherwise be made up using potable water.