Cut the rhubarb into 1-in. (2.5 cm) slices. Combine the syrup, rhubarb, and water in a stainless-steel saucepan. Bring to a simmer and cook until the rhubarb is tender, about 10 minutes. Let the mixture cool, then strain through a fine strainer. Do not press down on the rhubarb solids, but let the rhubarb stand in the strainer about 30 minutes to let all the flavored syrup drain off. This will keep the syrup clear. Measure the syrup and add enough cold water to bring the volume to 20 oz (625 mL). Freeze the syrup. Reserve the rhubarb for another use (for example, add sugar to taste to make a simple rhubarb compote).