This should be enough for a colony though. With a recycling program it would probably last indefinitely. Much better than bringing the water from Earth.
Recyling could mean never letting it out into the atmosphere in the first place. Or maybe would, since there is no water cycle to talk of on Mars. Think closed-loop aquaponics systems etc.
I mean that water if going to be the least of your problems, unless you've mastered terraforming (we haven't) or you're okay with living under a huge dome (that we don't even have technology to build, either). Liquid water cannot exist on Mars, and liquid blood can't either, so you would die horribly if you went on Martian sole without a spacesuit. Apart from having gravity, living on Mars would be like living on the ISS, minus the food they bring to the station from Earth...