Are you are confusing dissolved solids (which in your case is freaking high!) with alkalinity (the measurement of pH or acidity)? Plus tap water pH is usually variable throughout the year.

Water pH is not material. There is very little buffering capacity in water. And while most soil buffering is not great, it is much stronger than water. So regardless of water pH the soil pH remains unchanged by the water added. Ideal soil pH is 6.5+/-0.3.

Nope, 200 - 560 ppm is the actual reported Alkalinity from the official water quality report (link below). I don't know much about this subject, but I read somewhere that 100+ ppm is considered high alkalinity, and this is MUCH higher ... hence my concern about how it might affect the health and viability of garden plants.

Here is a link that might be useful: Water quality report

Or is it hardness (carbonate), which is measured in ppm?

We have crappy water here, and things grow fine. I think the Davis area has some issues not with water but with some areas that have a soil problem. Can't remember what it was though. Hopefully someone from Davis will chime in soon...ahhhh...looking at the water report, looks like boron is a problem in some areas. I think it was boron you have to watch out for.

Whatever it is, mulching and keeping the garden mulched with good quality compost makes everything better. Also saving rain water and drenching soil with it washes a lot of the bad stuff irrigation water leaves behind below the root systems of most plants. You learn to deal with what you have...

Here is a link that might be useful: davis wiki on Boron