Another aspect of the art of wine making is knowing when to top off oak wine barrels during the winemaking process. Winemakers need to routinely do this during barrel aging.

Here's why…

1. Evaporation Through the Barrel

While the oak staves in the barrel do expand when saturated with the wine, these barrels are not completely airtight. So, they actually allow a small amount of oxygen to enter the barrel and interact with the wine.
Conversely, oak barrels also allow evaporation of the wine through the wood over time, especially in dry or warm conditions.
This evaporation creates air gaps, called “headspace” inside the barrel.
The amount of wine that evaporates from the barrel is often referred to as the "Angel’s Share" because it was imagined that angels must be taking their share of the wine from the heavens above.

2. Risk of Oxidation

The headspace is filled with air (that includes oxygen), and too much oxygen exposure can lead to oxidation, which can dull the wine’s flavors and aromas and, in the extreme, spoil the wine and give it a nutty-like flavor (much like Port wine flavor)

3. Knowing When to “Topping Off” the Barrels

The “art” of knowing when to top off a wine barrel is a mix of routine checks, sensory observation, and understanding the wine’s environment. Experienced winemakers can even predict when barrels need topping based on subtle cues like how the barrels plug (known as a bung plug) fits or the cellar conditions.
Topping off simply means refilling the barrel to its full capacity with the same wine (typically from a reserve tank or another barrel).
This minimizes the oxygen in the headspace and helps preserve wine quality.

4. Frequency of Topping Off

Winemakers typically top off barrels every few weeks to every couple of months, depending on cellar conditions (humidity, temperature), barrel size, and wine style.

So in short, topping off an oak wine barrel is a key part of maintaining the wine’s freshness during barrel aging. Yet another part of the art of wine making. Cheers!