Sheathing warping in wet conditions

Hello, I'm finishing up a shed build and am having trouble with moisture. Until recently, I've covered it with tarps during the rain. Before hanging the sheathing, I painted the bottom edges and back side of each sheet of plywood to seal out water. After hanging it and finishing the painting, I started leaving the tarps off. The problem is this one corner has started to warp after a recent rain. It's elevated 10-12 inches, and my roof has a 12 inch eave, so I'm surprised that it's getting wet at all. My proposed solutions are:

1. Digging out around the base a little more so there's more elevation.
2. Putting in some gravel to hopefully prevent water from pooling and creating a splash.
3. Adding 1x4 trim along the bottom and caulking.

Gutters aren't an option, unless someone knows how to install them on rafters that have a square tail cut rather than a plumb cut.

If you have advice about which is the best option, or have any other suggestions, please let me know.

Thanks!