Even the best fire department will be less than fully effective if it has a less-than-adequate water supply. Similarly, even a superior water supply will be less than fully effective if the fire department lacks the equipment, personnel, or operational considerations to use the water. Your preliminary Fire Suppression Rating Schedule (FSRS) score is subject to modification by a divergence factor, which recognizes any disparity in the relative level of effectiveness of your fire department and water supply.
The divergence factor mathematically reduces your preliminary scores if the fire department and water supply scores are incompatible with each other.