Water is Required but be Cautious: It can't be stated enough that the pitaya cactus will tolerate a period of dryness more than it will tolerate a period of excessive wetness! However, when the pitaya cactus is healthy and growing and producing lots of fruit (which are very rich in water content), the pitaya cactus requires a considerable amount of water! This may seem a contradiction, but remember that the pitaya cactus is a cactus after all - and that the fruits are very high in water content.
Too much water can cause rot and increase the chance of both fungi and bacteria attacking the pitaya cactus and kill the pitaya cactus directly - err on the side of caution.