Ground Cover
I had previously painted the piece of plywood that serves as the diorama's base with a first available paint, which was flat black. I did that just to protect the wood from any moisture. Before installing the newly-constructed lumber storage shed, I painted it with a more appropriate base color.

I then glued the lumber storage shed to the diorama board using 5-minute epoxy. As this photo shows, I am in the middle of covering the board with some Arizona Rock & Mineral Co. brown sand. This was done by covering sections with matte medium and then liberally sprinkling the sand. Letting it dry overnight guaranteed maximum adhesion. The next day I brushed all the loose sand off and recovered it to be re-used again. The darker "blobs" in the photo is the wet sand still drying. The lower-left section hasn't been covered yet, while the middle and upper-right section is already dry.

Note: the edges of the overlapped sections can build up matte medium, which is visible in the final result. I'm going to have to strategically hide that in the future.