In a saucepan, bring 4 cups of water to a boil. Remove from heat and add 5 black tea bags. Steep for about 5 minutes.
In a separate saucepan, bring 1 ½ cups water to a boil. Remove from heat and stir in 15 Stevia bags.
Allow liquids from both saucepans to cool.
In a food processor, pulse 2 cups of chopped watermelon. Strain to remove watermelon flesh and any rogue seeds.
Combine all liquids in a pitcher, add water to fill pitcher (4 - 8 more cups) and stir in 1 teaspoon baking soda to help maintain red color. Chill for several hours before serving.
