In a large pot, combine the diced rhubarb with water. Bring to a gentle simmer over medium heat and cook for about 15 minutes, stirring occasionally, until the rhubarb is soft and has released its juice. Strain the mixture to extract the rhubarb juice.

In a clean pot, pour the strained rhubarb juice and add the strained lemon juice along with granulated sugar. Heat gently over medium heat, stirring until the sugar completely dissolves, about 5-10 minutes.

While the concentrate heats, prepare your canning jars in a boiling water bath canner. Once the concentrate reaches the desired temperature, ladle it into the jars, leaving about 1/4 inch headspace. Seal the jars and process in a boiling water bath for 15 minutes.

After processing, remove the jars and let them sit undisturbed for 12-24 hours. Check the seals and store the jars in a cool, dark pantry.