The sentence implicitly has 'water' in the position below:
_____is only enough [water] to supply the needs of the house.
You're talking about a quantity of water, and hence you must specify which water you are quantifying - the spring water, or the water from the spring.
The sentence really means:
The [amount of] spring water is only enough [water] to supply the needs of the house.
You could say "Spring water is good for drinking", but not "Spring water is enough for drinking". In the first, you are referring to any spring water, or all spring water.
In the second, you must be talking about some specific spring water, because you are able to quantify it.
Student or Learner