I rake it out in the spring when it is still mostly one big floating ice block to get all the leaves an gunk out from the previous year. Then refill it with well water. Most of the time there is very little left in the tank. I Let rain refill it over spring/summer, (try to get it on a downhill slope or low spot). Then when I'm making my final rounds on the farm in Aug I will top them off if needed.
Been thinking of rigging something up to attach to a tree to catch rain an funel down to the tub with a piece of hose, but still working on that.