Having leafed through that book, i came up with the following information: During dry spells metabolic water is generated by cellular biochemical conversions within the snake's tissue. According to the book, this is done by oxidizing stored fat that lies laterally along the testes. C.cerastes can also produce water by metabolizing carbohydrates and proteins, though this is to a much smaller degree. The depleted stores of fat, carbohydrates and proteins are then replenished by heavy feeding during optimum times. from "Rattlesnake: Portrait of a Predator"
IF you'd like to read the whole section on this topic, find a copy of the book and check page 89.