The author is a lady. Go figure.

With an underground tank outdoors, and a swimming pool up top, the pool could be a good solar collector and heat dump to assist the cistern. The main factor in the consideration is Delta T and surface area, both underground and up top. Passively gaining and losing heat is pretty much free, minus transportation energy cost. Creativity and innovation are well rewarded. The control strategy has a critical role in the system operation as well. Trial and error will yield oodles of answers once a plan is devised.

Another good article..
http://www.cchrc.org/docs/reports/thermal_storage.pdf

So I have a hypothetical question.. Say I wanted to freeze my cistern into a large block of ice of the winter. How much insulation woul it take to make it last about 2 months?