Clapp Spring is also listed as a permanent watering hole, yet he texted a friend from the Spring saying he was out of water, so it sounds to me like the Spring dried up, and he was counting on refilling his water (if indeed he brought any) from the spring.
http://www.desertusa.com/mag07/mar07/tubeagate.html"Reflecting the amount of rainfall during the preceding several years, the main pool at the spring varies from a few inches to five feet in diameter and from three to nine inches deep. It is located in the heart of the oasis, at the base of a palm. Except during and a few years after a serious drought, there may be several additional seeps and smaller pools of water in shallow holes dug by wild burros."
A few inches in diameter and a few inches deep. I would not depend on that as my water source.