@DavidBrown They use flood irrigation on their exhibits, similar circumstances can be seen in the photos of the wildebeest, Spotted Hyena, Thomson's Gazelle, and more. There's a bit of discussion on it on this photo: Thomson’s Gazelle Exhibit - ZooChat
There must be 50 signs up at this zoo about flood irrigation, with even the lion and tiger enclosures almost entirely underwater. I'm not sure how long the flooding lasts, but many animals at the zoo were stuck on small dry patches of land during my visit. Back in 2011, loads of primates had their entire outdoor cages as mini lakes and it kept them up on the branches.
Water also evaporates very quickly in the Southwest; if it rains at night there's often no sign of it by midday. The fact that @snowleopard encountered standing water on 2 out of 2 random visits implies they're doing it pretty often, though.
@Coelacanth18 yes, they do it in different yards/habitats throughout the week every day, so one will likely always encounter standing water in different habitats. The amount of yards, cages, and habitats they have is staggering, so even seeing it in 10 would be only a fraction of the facility.
