Water stress occurs when a plant's water demand (regulated by temperature, wind, humidity, and radiation) exceeds soil supply. The plant closes stomata to reduce losses, but this also halts photosynthesis and growth.
Meteorological indicators
Potential evapotranspiration (PET) estimates atmospheric water demand. When accumulated precipitation falls well below PET, a water deficit builds. In Spain, the dry half (south-east) accumulates severe deficits every summer, and multi-year droughts compound the problem for rainfed crops.