Dryback
The share of its water content a substrate loses between irrigations, usually given as a percentage of volumetric water content. It is the central measurement in crop steering.
A block that peaks at 60 percent volumetric water content and falls to 45 percent before the next irrigation has taken a 15 point dryback. Growers track it with in-substrate sensors and treat both its size and its timing as steering levers.
Larger and faster drybacks push a plant generative, favoring flower development. Smaller, gentler drybacks with earlier irrigation keep it vegetative. Overnight dryback in particular is a standard generative tool in flower.
Because dryback is driven by transpiration, anything that changes transpiration changes dryback. CO2 enrichment reduces transpiration, so drybacks slow, and a steering program calibrated before enrichment will quietly drift once the gas is turned on.