Cacti don’t need a lot of water, but sufficient rainfall at least a few times a year is of course desirable. When soil is soaked by recent rainfall, cacti binge on the soil water. Water is absorbed faster than the cacti can utilize it and the excess water is stored in succulent tissue. This stored water ensures the plant remains healthy through periods of drought but generally is utilized within a few seasons of drought. This is different from some desert shrubs such as creosote that can go three years without any rainfall. Some cacti are better than others at surviving dry periods. For example, the eastern prickly pear which grows in the central to eastern United States needs quite a bit of rainfall every year to survive, 25 and upwards to 40 inches annually. Beaver Tail Prickly Pear and the Teddy Bear Cholla of very hot and dry deserts of the southwestern U.S. on the other hand require only a few inches of rain annually and can go up to a few years without water. These cacti are the extremes though in North America, most species occupy the slightly wetter deserts were say six to 12 inches of rain fall annually.
In general, cacti also require rain to fall during certain times of the year to ensure success. Precipitation that falls as snow cannot be used. Freezing temperatures first of all kill most cacti and cold moisture is not absorbed well. The Great Basin Desert commonly having snowfall and dry summers, therefore has few cacti. Cool weather precipitation above freezing can be absorbed and utilized though. Being warm to hot weather plants, summer precipitation is also well utilized. As mentioned previously, cacti typically do not endure extended seasons of drought though. The Mojave Desert of southern California, northwestern Arizona, and southern Nevada has winter rainfall but long hot dry summers and therefore supports few cacti. The Sonoran Desert of Southern Arizona and northwest Mexico however, has both a winter and summer rainy season and therefore supports an abundance of cacti. Of course cacti can survive many types of desert conditions but they do generally require evenly dispersed precipitation above freezing to proliferate.
Once water is absorbed into the tissue of a cactus, the plant holds onto it like gold. Contrary to Hollywood myth, cacti don’t contain a swimming pool of water swishing around inside. Water absorbed by roots is carried to internal succulent tissues where the cells are filled with and store the water. Cells swelling with water causes the entire cactus to swell as mentioned before. Once water is in the cells through it is not easily removed by anything other than the cactus itself. To remove the water one would literally have to burst billions of microscopic cells. So extract a glass of water from a cactus would take a lot of work. It would be much easier simply to eat the cactus and obtain the moisture that way. Unfortunately, cacti contain oxalic acid, a chemical that can cause kidney stones and tastes bad, and for that reason is a major deterrent to any creature that may want to consume a cactus.