So I was thrilled to learn a new spattering method using an emery board. It wasn't in a watercolor book -- I found it in Kristy Kutch's book Drawing and Painting with Colored Pencil. She uses it with water-soluble colored pencils.
You can control it better because you select the area where you want the effect. You brush the area with water (which can be clear or tinted), then hold the pencil over the area and scrape it with the emery board. Flakes of water-soluble pencil color spatter over the area, and stick just to the wet part. After it's dry, you can brush off any excess flakes.
I have water-soluble colored pencils on hand, and also have both emery boards and a pad of emery sandpapers. I found the pad easier to use and it produced better flakes. I held the pencil about an inch or two above the paper, and the emery above the pencil, then rubbed them together.
This could be really good for rocks! And sand, and maybe other things.
I tried it out on tree leaves, not entirely successfully but with practice this might work for trees with small leaves.
Mostly I tested it for rocks and beaches which worked out pretty well.
I don't know how useful this will be for urban/field sketching, as it may be too awkward to work when not in the studio, but I'm going to give it a try!