From Arthur Barbour in the June 1985 issue of The Artist’s Magazine:
To make a graded watercolor wash, turn the paper so that the part you want darker is at the top. then dampen the paper with a sponge and clean water. Start with a loaded brush of color and stroke rapidly across the top of the paper, moving down with even strokes across the width of the paper. When the brush is nearly depleted, recharge it with paint and start again at the top, stroking across and down until the desired depth of value is reached.