The levels of the other ions will vary in the same way, and you can hope that if it is a simple case of more rain diluting it then the ions will be diluted in proportion. If however it is the case of them switching to a borehole rather than a reservoir you can have a completely different profile.
Can you rely on a professional test? . . . It's valid for the day the water sample is taken. I know a very respectable member is planning on offering a testing service, and I will be using this on a monthly basis, once I know how the levels change during the year I can them measure say alkalinity and calcium, and make an educated guess as to what the rest of the ions are likely to be. . . . Unfortunately you can't really measure all of the required levels yourself, it does take some fancy equipment. (PM me about Phoenix Analytical)
If you are in a hard water area then the Salifert Calcium kit is quite handy . . .I'm always a fan of measuring what I can rather than using a formula that quite frankly does not fit any of the profiles some of the time