Grapevines adjust how much water they release through their leaves based on the environment. Scientists developed a simple way to measure this water release in grapevine canopies. By studying vine sap flow, temperature, humidity, and sunlight absorbed by the vines, they estimated how open the vines' stomata were. The measurements showed that the water release varied throughout the day and decreased as the season progressed and the vines experienced more water stress. The most important factors affecting the water release were how much water the vines were using and how dry the air was. Understanding this process helps predict how much water grapevines need, which is crucial for managing vineyards effectively.