I have a polygon layer that represents sub-catchments and a line layer that represents a stream network. There is exactly one feature in the line layer for each polygon. I have used v.rast.stats to calculate the average slope for each line feature.

Now I would like to transfer this attribute to the polygon layer. In other words, the polygon layer should receive the information about the average flow gradient in each feature. How can this be done?

Here is a screenshot:

The red lines are the line layer, the white lines are the boundaries of the polygons and in the background is the slope grid.