Ogden Pleissner (1905-1983) was an American watercolorist who painted Europe after World War II and became best known for his sporting artwork.
His plein-air watercolor setup was perfect for working large. The wooden tripod easel can expand to hold very large boards, and the angle can be tilted to allow washes to move. The palette has its own tripod to keep it right next to the painting, and it can be tilted as well. A big porcelain bowl holds the rinse water. The umbrella has its own stand, not attached to the easel.
One of Pleissner's plein-air watercolors: Old Mill, Winchendon, Massachusetts, c. 1960, 16 x 26 inches, courtesy Adelson Galleries.
The second photo is from the Archives of American Art, which has Pleissner's papers.
The Pleissner Gallery at the Shelburne Museum in Burlington, VT has many originals and studio effects.