Eight years ago we planted our new lawn to a low water mix that was also used on golf course rough. Now the bunch grasses have taken over and left a very rough surface along with 20-30% bare ground which weeds really like. What can we do to thicken up our sod?
The low water mix you planted was likely nothing but bunch grasses from the beginning. The initial planting was dense enough, when young, that it didn't appear to be comprised of bunch grasses. After "self-thinning" occurs - usually resulting from a lack of inputs (not enough water, fertilizer, etc.) - the remaining/surviving turf looks clumpy and forms a rough and uneven appearance, with empty space (where weeds will grow) between the clumps. This is basic plant ecology in action: if enough water and fertilizer isn't applied, some plants will die out and the existing plants space themselves out until there is enough in the way of resources (water, nutrients) to keep the remaining plants alive.
The lawn can be thickened up by overseeding (with a seed drill, or by broadcast seeding with is preceded by heavy core cultivation/aeration of the lawn area (to make holes for the seed to fall into; do NOT attempt to fill the holes with ANYTHING following seeding).
Once a lawn comprised of bunch type grasses has thinned out, overseeding is the only way to correct its density. After the newly seeded grass has begun growing, adequate water and fertilization is required to sustain the new grasses and the existing turf. This will not be an "instant" fix. In fact, it may take awhile (many months to a year) before the turf looks filled-in and "normal".