Seeding into existing lawn

Back story:
My mother's lawn requires way too much water and fertilizer. I live in California so the summer is hot and we go a good 4-5 months without any significant rainfall. Before I started mowing it for her this spring it was getting watered by automatic sprinklers 20 minutes a day 6 days a week. I'm still tinkering with the sprinkler timer but I plan to reduce it to 3 days a week this summer and hopefully less than that by next year.

The result of so frequent watering is very shallow roots. I dug a hole in the lawn (no pictures) and the soil was pretty dry and lifeless. the roots didn't go down in the soil more than about one or two inches. There were no earthworms or any other detectable soil life. The soil is a clay loam according to my own squeeze test. Because it is not very compacted I think it really just needs a lot more organic matter and getting the grass to root deeper would really help.

The changes I have made so far are reduced watering frequency, mowing high (4"), and leaving the clippings on the lawn instead of bagging them. When I fertilize I'm going to use organic fertilizer instead whatever they were using before.

Question:
I'm not sure what species of grass her lawn is but it is not very drought tolerant. If I were to select a more suitable grass for my climate and overseed the lawn with it in the fall, perhaps using the seedball method discussed elsewhere on this forum and in Fukuoka's books, would it grow and out compete the old grass because of the reduced watering schedule? Or would the seeds just sit on top of the thatch of the old turf and do nothing?

Renting a slit seeder is a good idea, and I would use tall turf type fescues for low water and fertility requirements... do it in the fall and you won't have the weed problems you would with spring or summer seeding...

S

I would not put in new seed.

You are already on the right path.  Get those roots to go deep. 

How long did it take to dig the hole?  What was the soil like?  Sandy?  Lots of clay? 

Paul, what kind of grass are we encouraging to go deeper? Rye? Blue? Bermuda?

How will they do with reduced ferts? Water? Will they drive down with reduced inputs or just curl up? Are they even capable? (Deep rooted Bermuda?)

The right grass will make a lot of difference. One that needs far less water, or far less foods. Or both. TTTF fits all bills...

Slit seeding also aerates the soil in those top two inches, allowing for increased root mass and incorporation of humus. Top dressing with compost before hand can be even more beneficial, and the best herbicide for any lawn is more grass seed. And in fall, you give it all a bangin' headstart on weed competition...

I stand by my earlier pronouncement... if reducing water and ferts is the goal, better be starting with the right stuff...

S