Turf type fescues are cool season grasses that normally want to be dormant in summer. You can trick them into not going dormant by consistent and frequent watering. It is possible that the fescue over the leach field is just not getting enough water to prevent dormancy. Generally, these have very little actual soil above the leach field drainage components and so the water percolates through much faster and doesn't hang around long as there is nothing there to hold on to it:-)

No turf type fescue is very drought tolerant compared to some other choices, especially the warm season turf grasses (which may not be appropriate for NoCal). I'd just suggest some specialized watering attention to the leach field areas.

The leach lines may be a bit shallow, yes. Adding top soil may help, although it might be smart to check with a professional to see if that's an OK thing to do--I have no knowledge of whether modification of a septic system is a smart thing or not and wouldn't want to advise you without expert advice. Nor would I consider "looking up an answer on the Internet" expert advice necessarily applicable to your situation. Try to find somebody local to your situation who, hopefully, knows what you're working with.

The above really didn't answer the question, per se, regarding a drought-resistant grass, so let's see if we can help you out a bit more. No, TTTF isn't drought-resistant (and doesn't actually have much of a dormancy mechanism; it's tolerant of drought, but dies rather than go truly dormant for any considerable period of time, so I imagine you also have to reseed quite regularly or suffer bare areas), but sure, you could seed there with one if you want to even out the watering regimen across the whole lawn.

The issue would be matching the growth rates and the way it looks across the whole lawn. A challenge, perhaps, but it could be done. You might consider establishment of something along the lines of a bluegrass, which some might argue may be more drought tolerant (I would argue back but it's not really worth my time), but where the dormancy mechanism (post a good establishment) would render it more likely to come back when rains resume and when cooler temperatures return.

It has the advantage of spreading easily when well-fed, so doesn't need reseeding unless something horrible happens.

That was the choice I made. Cooler, wetter summer? The lawn stays green. Dry and hot? The lawn blasts out, but comes back in September when the weather cools. If that's more tolerable on those lines, so be it.

It's a good choice for those who don't want to water--I object to throwing potable water at the lawn as a completely irresponsible personal ecological choice.

Existing areas can be made more tolerant of drought by increasing organic matter levels (slowly, over time, but it can be done). Feed organically, mulch in your fall leaves (import them if you have to). Mulch the lines and the area around them in a quarter inch of peat moss and keep doing it as it works in. Or sawdust (from untreated wood, pine pellets for horse bedding, whatever).

It takes time, but you'll note that the soil will hold more water, naturally. My "Your Lawn's Bluegrass And That Needs Water All The Time You Stupid Idiot" bluegrass lawn requires water every fifteen days or so in the hottest of summers. Yes, this is Pennsylvania. We're still talking 95 degrees.

You didn't specify your precise location, but across your range, many cultivars of Zoysia are available that will work through many of the sub-zones. It's going to depend, but through the vast majority of the heavily-populated areas, it's a good shot, so the probability is quite high. :-)

Zoysia's downside is that it looks bad during winter (think yellow-gray straw), but flourishes during hot summers. It'll drop out as soon as frost hits, and won't return until temperatures warm considerably (around here, late April, so really after our last frost date by a couple weeks--nights in the low forties at the coldest).

Its other downside is that it only flourishes in full sunlight, an absolute floor of four hours of sun per day, and seriously? Six hours minimum.

And the third downside is that it spreads like mad. It's going to take over your entire lawn eventually. Will you, nil you.

Exactly one Bermuda will tolerate the climate in Philadelphia, Pennsylvania--far harsher than the climate in maritime north CA, but I wouldn't consider it reliable even there, and I wouldn't use it too far inland. If it snows, it goes, in other words. :-) It's done very well without heating in the stadium they used it in, but Philly's climate is warmer than mine and I wouldn't dream of trying it up here in the foothills of the Poconos. Fifty miles can make a big difference.

If you can tell us your exact locale, we can help you much better.

The OP said NoCal, but then he said long, hot, dry summer. The locals in the northern part of central California like to think of themselves as living in NoCal, but they don't. If you're not north of Redding, you're geographically in the Central Valley and part of CenCal. I'm going with Sacramento as being the location, but this applies to the entire Central Valley.

Fescue is NOT appropriate for the Central Valley. Grasses that will work are UC Verde buffalograss, bermuda (any kind), St Augustine, and some zoysias. After narrowing it down to those southern type grasses, due to the long heat of summer, then you have to deal with shade tolerance. St Augustine and some of the zoysias are shade tolerant compared to buffalo or bermuda. Those are your responsible choices. Fescue, rye, and Kentucky bluegrass are irresponsible choices for the area.

You mentioned regular watering but did not mention what that means. If you are watering more than once per week in the hottest heat of summer, then you're doing it wrong and wasting your state's resources. I have Floratam St Augustine and live in the Texas Hill Country with summer temps in the high 90s for well over 100 days. Last summer I watered the back yard (south facing slope) 3 (4?) times and the front 2 times. The grass was by no means drought stressed in between watering. Rainfall counts.