index = renewable_freshwater_m3_per_yr / population
| > 1700 m³ | No stress |
| 1000 – 1700 m³ | Water stress |
| 500 – 1000 m³ | Water scarcity |
| < 500 m³ | Absolute scarcity |
The Falkenmark Water Stress Indicator (Malin Falkenmark, 1989) divides a region's total renewable freshwater — precipitation-fed rivers, lakes and recharged aquifers, not a one-off store — by its population. The same freshwater total spread across more people pushes the index down even if the water supply itself hasn't shrunk; a drought that cuts the supply pushes it down even if population is flat.