That would/could also work. Though I am not sure much evaporation happens from these basins. These serve the same purpose as a slurry storage tank but earthen in nature almost always lined with clay to stop seepage. See here: https://wiki.openstreetmap.org/wiki/Tag:building%3Dslurry_tank
I'm just a new mapper trying to figure out how to tag these types of land uses, though I have been around cattle my whole life.