Chinese characters written with water

Chinese characters written on the pavement with brush and water. I have seen this a few times in Beijing, once in the Houhai park (see picture) and once on Wangfujing opposite of the Kaogu bookstore, near the Fine Arts Museum. The scenario is this: an old man dips a huge brush into water and writes large Chinese characters on the ground, which then slowly fade away. So the first characters are gone before he has finished writing the last ones.

Now where does this art of hydro-calligraphy come from? Or what is the significance of it? To be honest, it does grab you as you see beautifully written characters fade away under the rays of the sun. Something about temporality. And you can never tell when they are really gone, in one moment you still seem to see them but then maybe you don't anymore. There must be a second in time when they are actually gone but you still register them as being there.