In zone 10.... the promised land for citrus. But surrounded by concrete...unhappy zone. 1 - concrete can raise the pH of the soil (making it basic due to chemistry of the concrete). 2 - the concrete limits the feeder root area which are predominantly surface - think inches. So #1, can inhibit uptake of nutrients in the limited area of #2. Feeder roots generically speaking spread to the foliage coverage area.

The lighter and yellow tinged, lack of growth, unhealthy aspect could easily be a combination of the two. I'd recommend taking soil samples and sending them to your extension service where they will tell you the ph and mineral and sometimes organic % content.

I wouldn't write off any of the trees, once you get a handle on the soil and nutrition.

Lemons (and any citrus that are fruiting) are heavy feeders. So again, the principles above apply.

In the interim, if you're only watering, for sure, given the lack of available nutrition for the roots (blocked in by concrete and no natural fertilizer - due to bio-decay of other plant material/compost), they are very likely in need of serious fertilizer (and possibly, acidifier, I would do two prong, vinegar for short term lower pH and longer term with sulfur).

Thank you for the advice! Yes, many of my neighbors have very healthy citrus trees (and we have a bountiful orange tree in our front yard), so my lemon trees are in a very sorry state by comparison :)

I'll definitely look into soil analysis / making the soil for acidic if necessary -- I bought a cheap soil pH analyzer and it said the soil was fairly alkaline (~7.5 pH), but I'm not sure how much I can trust it.

As a rough rule-of-thumb, does a fairly thorough watering once per week sound like enough water, too much water, or about right?

It's probably the concrete raising the pH and limited uptake of limited of plant food. Citrus want acidity, so below 7, all the way down to 6. The cheap meters aren't great. And you want and overall soil assessment, so you can buy pH strips, get de-ionized water (not tap or bottled), then take a couple of tablespoons of soil, pour in enough water to make moist, not wet and dripping. Then squeeze water out and apply test strip. That said, if it's measuring alkaline, it's definitely alkaline. They're not accurate on scale - but they are accurate on being acid or basic.

As for watering, depends on your soil. Being in ground, generally is very forgiving. I don't know you're soil - is it sandy or clay? A few inches down in the soil, is it moist? I put 1" of water per week for my in ground fruit trees (none citrus in VA) and if I was zone 10, I would feel that would work for my citrus (soil retention dependent... if it was really sandy, it would require more frequent watering at that amount). I'm sure there are some CA citrus growers here who can perhaps give more salient advice on the soil retention and watering.