Many presidential messages on domestic policy are somewhat like final-exam projects of student architects: They are elaborate, finely crafted proposals that show the draftsman's sense of design and his theoretical knowledge of engineering, economics and consumer tastes. They don't show whether his plans will be accepted in the market-place and whether, if it carried out, they will standup.

President Carter's message on water policy is of that sort. It has a fine emphasis on water conservation, better planning of federal projects, and more attention to the environmental import of dams, flood-control systems and the like. The message also confirms that, through the months of tussing with the western states, Mr Carter has come to appreciate much more the vital role in that dry region and westerners' hypersensitivity to any suggestion of federal inteference with the established state and private water rights and prerogatives.

Instead of trying to bludgeon the states into accepting his new principles, Mr Carter wants to bring them gradually along. The clearest sign of this is that the president, who tried to stop those18 water projects so abruptly 17 months ago, is now about to recommend a number of new starts. He has also proposed giving higher priority to some projects now on the drawing boards if the states involved volunteer to assume small shares of the cost. His water-conservation program, too, stops short of the tough demands, especially involving agricultural irrigation practices, that environmental groups have urged. The conservation theme is not necessarily insubstantial, however. The test will be what requirements the administration really includes in various programs of federal aid. It's not yet clear, for instance, whether the administration will go so far as to require water meters and water-saving plumbing in all federally assisted new housing in Arizona, Denver, metropolitan Washinton and other water-short areas.

Besides saving water, Mr. Carter wants to save federal money by requiring the beneficiaries of future projects to pay more of the cost. These proposals are intended in the long run, not only to reduce the federal share of individual projects, but also to temper the states' enthusiasm for more and more enormous public works. Again, though, politics has moderated Mr. Carter's approach. Some administration officials used to suggest recalculating the costs of long-pending projects at current discount rates. That would eliminate a number of marginal proposals - but that step was not recommended this week.

All in all, Mr. Carter has tried hard to produce a reasonable and politically acceptable program. Will it have much effect? That depends on Mr. Carter's willingness to be very firm with Congress - and there's going to be an early test fo that. The House Appropriations Committee has just reported a public-works bill that, as usual, is much more generous than the administration's request. This time the panel has resurrected eight of the 10 projects that Mr. Carter did persuade Congress to shelve last year. Overall, the bill would add $221 million to the $3-billion budget request for dams and other water-resource projects, and would provide construction funds for 41 new projects - all unbudgeted. If Mr. Carter is serious about enforcing the sound standards he has just set out, he will have to tell Congress something that was not in Tuesday's message: that if the lawmakers send him such legislation, he will veto it.