[RichardJ]

You say it is the alternator belt slipping for sure. How do you know that? Is the alternator belt slipping on the alternator or on the water pump?

I'm not saying it's not, I just would like to know how you are making that determination.

The two grooves on the water pump pulley appear to be different diameters. Are the two grooves on the crank pulley also different diameters?
If the two grooves on the crank pulley are the same diameters, both belts will be traveling at the same speed. That means that at least one belt will be slipping on the water pump pulley at all times. One belt is on the tension side of the drive pulley, the other belt is on the take-up side of the drive pulley. That can have an effect on what you're hearing and when.

There are several things that may be in play here. Pulley diameter matchup and degrees of contact on the circumference of the pulleys.
Better pictures of all the pulley positions would be helpful.