Add potatoes, carrots, onions, water, chicken base and salt to a 6 qt soup pot and cook over medium heat until tender. Add 1 c milk to pot. Blend sour cream and flour in a small bowl until incorporated, add rest of milk. Put this into pot along with ham. Cook over low heat 30 to 45 minutes, stirring occasionally until soup thickens. Add Velveeta and parsley flake and cook for an additional 10 minutes. Serve hot.