This is a shot of my supplies for painting watercolor on site. I like a block or pad of 140# cold-press paper, a mechanical pencil, my travel palette, paints (I carry in a zipper bag), a brush or two, and a water carrier. A few sheets of Viva paper towels and I am ready to go. Everything fits in a tote or backpack easily. If I won't be painting long, I just take a bottle of water. If it is going to be all day, I carry a gallon of water in the trunk.
Usually I hold my pad in my lap and set the water on the ground. I do try to take a folding chair if I will be out in a field or woods. If parking nearby is available, I carry an old tv tray to set next to me. There are watercolor travel easels available. I just prefer to sit when I paint. I have tried using my french easel with the watercolors, but I felt like I was leaning over too much.
Recently I saw a watercolorist put a vinyl tablecloth on the ground under her chair. Hmmm- must have been for fire ants? I have painted places with them too. With watercolors, it is important to sit in the shade so you won't have a glare on your paper. If you can really carry a lot of supplies, you might take a beach umbrella. Just be sure that it is a solid color and not striped. The stripes will reflect on your paper and throw off your color mixing. You can usually adjust to a solid color.
For anyone who is wanting to paint plein air with watercolors or oils, for that matter, I recommend starting in your own backyard. Carrry your equipment and set up. Try different ways of sitting or standing and see what you prefer. Watch how much time it takes you to draw and paint outdoors. You will get faster as you practice. Think of your watercolors as sketches or studies and not finished pieces. Allow yourself to be looser than you would in the studio and you will enjoy your experience outdoors!
If you have any questions or comments, please let me know! I plan several excursions this spring out and about in this area.