Drought: water balance, not rainfall

The standard

The measured water balance judges drought from the ratio of available soil water to evaporation and vine demand, not from rainfall alone.

Criteria

• What matters is the plant-available soil water, not the absolute rainfall
• The soil's storage capacity changes the effect of the same drought
• Duration and timing decide between benefit and harm
• Mild water deprivation concentrates, sustained deficit interrupts ripening

Distinction

Drought ≠ water stress: drought is the external water deficit, water stress the vine's measurable response to it. Drought ≠ meteorological drought: the latter is a defined regional event, the former also a local, short-lived state of a single plot. Only the measured water balance separates fertile scarcity from damaging deficit.