Does a watermelon or walnut have higher water activity?

A watermelon has higher water activity than a walnut. Water activity $\left(a_{w}\right)$ is a measure of the amount of free water in a food product, and it is expressed as a decimal between 0 and 1. The higher the water activity, the more free water there is in the product. Watermelons typically have a water activity of around 0.99, while walnuts have a water activity of around 0.2. This means that there is much more free water in a watermelon than in a walnut.