Firstly, you should not be using only water in your cooling system. If so, not only will it evaporate in your system but you will incur internal rusting over time and destroy your engine.
Secondly, how do you know you're losing fluid only when on the highway? Where have you observed the leak? Are you sure the hoses are properly tightened and the radiator cap isn't leaking?
For your own sake, please make an effort to write clearly and spell correctly. Misspelling "water" in the title can lead some people to think you may be incompetent, which doesn't encourage positive attention.