A magnolia tree in north-central Texas is receiving sufficient water, but many of its leaves are falling off,

This tree periodically looses leaves, however if it is showing that it is not "happy" (by putting out new buds of growth), several things could be going on:

1. Was it transplanted within the last several years? It may now only be in shock, but this is rare -- usually happens earlier to trees, but may take five years for a tall sahuaro cactus...

2. It may have a fungal infection in the root system, or a parasite that is plugging its xylem and phloem....similar to bark beetles in conifers.

3. Too much water, or too little, or not penetrating the hard pan of the soil

4. pH of the soil has changed

5. ground water may may have risen, and now may be drowning the roots

6. someone may have put a plant/weed killer under the canopy of the tree, and the roots have absolved it, in effect killing the tree some time (maybe even years) later.

7. Check one of the leave that has not yet fallen.... it may have a leaf miner...

That would be about all of your choices.

If this is a SOUTHERN Magnolia, then it is perfectly normal to lose lots of leaves at this time of the year. And how do you know it is getting plenty of water? Is this a large tree? If it is brand new it may be going into transplant shock.

Maybe it's getting too much water.

heat. ?