[Kartographer]

That's I wonder if some of these earthbag/cisterns tied together wouldn't work.

http://earthbagplans.wordpress.com/2009/06/24/earthbag-cistern/

[Bean Counter]

Great concept, but if I were going to do that much digging I’d want a lot more than just 2000 gallons. From the description that looks like the biggest size you could build from earthbags....