As far as I know that there are two methods 1. the “water resources vulnerability index” and 2. the “water stress indicator”. I only know the first one. For the first one the index calculates the ratio of annual water withdrawals to annual runoff (renewable freshwater supply). If the watersheds where withdrawals are less than 0.2, have low stress, 0.2-0.4 medium stress and above 0.4 high stress.