What should a wife do if her husband requests intercourse at night and she fears remaining in a state of ritual impurity (janabah) because water is unavailable for ghusl before Fajr due to an قطع in supply, and she cannot perform ghusl from the well due to the cold weather?

It is obligatory for a wife to respond to her husband if he calls her for his need, and she should not refuse. It is stated in the Hadith: "A woman should not refuse her husband his need, even if she is on the back of a camel." The lack of water or the difficulty of obtaining it is not an excuse for refusal, for Allah has prescribed Tayammum (dry ablution) when water is not available or when one is unable to use it. It is permissible for someone who lacks water to have intercourse with his wife.