Add the watermelon juice, lemon juice, and granulated sugar to a large pot.

Heat the mixture to 190° F or 88° C, stirring often, and then remove the pot from the heat.

Carefully ladle the hot concentrate into prepared quart jars leaving 1/4-inch headspace. Remove any bubbles with a chopstick or knife, and re-adjust the liquid to fulfill headspace requirements if needed. Wipe down the rims and sides of each jar and apply two-piece canning lids until fingertip tight.

Load jars into prepared hot water bath canner and process for 10 minutes, adjusting for altitude. (See notes**)

Using a jar lifter, move the jars to a clean surface lined with a kitchen towel. Leave at least 1 inch of space between each jar.

Allow the contents of each jar to come to room temperature (12 to 24 hours). Check the seal of each jar, moving any unsealed jars to the fridge. Remove canning rings from sealed jars and store in a cool, dry place away from direct sunlight and use within 6 months.

To reconstitute this watermelon lemonade concentrate: combine with still or sparkling water (we like 1.5 parts water per 1 part concentrate).