Dear me, I've been watching this thread with interest (disbelief)
As an old civil engineer who had to do earthwork calcs, do a search for "prismoidal calculations" first hit on google
A 1000 gallons is only a lot when taken as a proportion of the total volume.
How much does a water truck carry. Depends on the truck. Do they not have the capacity shown on the truck in the states?
1000 US gallons = 3.79 cu.metres so it is a little less than 4 tons in weight. say, quarter of a small tanker or 1/10 of a big tanker.