To me, the central horizon and centered barrel placement are just fine, graphically speaking. There is a slight V shaped triangle formed by the horizon, tree and fence, with the barrel at its base. The branches fill up the upper half nicely. The vertical (portrait) orientation makes this triangle work better, too.
As a storytelling tool, I agree with Luke, showing more of the emptinees of the landscape might help.
If that tree is the only one in the immediate area, a wider, horizontally oriented shot with the barrel dead center (ignoring all rule of thirds and golden ratios and other such "pleasing balance rules") can be very effective in showing the tree and barrel's isolation.
Alternatively, for a different take on the vertical orientation, walking some way along the fence and allowing that to be your leading line to the tree and barrel can imply the distance to be walked, and that the barrel is a distant target. Having the barrel at the top of the frame allows for a longer fence and longer implied walk (you could accentuate the texture of the terrain that one has to walkxon in the foreground). Having the barrel / horizon mid frame in a vertical shot enhances the "big sky" effect.
If you're gonna have the barrel be smaller in the frame, waiting for a sunny day to make the blue pop more might work.
I just realized that the AGUA letters would probably not be legible if you were to stand alongside the fence. If you walk straight back from your current vantage point, would there be a road parallel to the fence? If you can walk a decent way beyond the road, to show the road and fence as a line across the barren landscape with the tree and barrel as a punctuation, that might be nice as well.