Questions about calorie deficit needed for weight loss have been discussed many times here and there plenty of sources pointing out that body fat contains about 7700 Kcal/kg. The standard answer is therefore that a deficit of 7700 Kcal is needed to lose 1 kg of body weight in the form of body fat. But this 7700 Kcal only takes into account fat tissue, which is 86% fat and 14% water, while the rest of the body contains 60% water.
So, the 7700 Kcal equals 9000 Kcal/kg *0.86, but it seems to me that since the body will keep its water percentage fixed at 60%, one should calculate the calorie deficit needed for weight loss per kg as 9000 kcal/kg * 0.4 = 3600 Kcal/kg, much lower than the usual figure.
While some sources do point out that weight loss may be more than expected due water loss, this is usually mentioned as a transient effect when people start to lose weight, not as continuous process that accompanies the loss of body fat in a steady way.