In a medium saucepan over medium-high heat, bring the rhubarb and water to a boil. Lower the heat to medium-low, cover, and cook the rhubarb until it is tender, about 10 minutes. Stir to break up and release all of its juices.

Set a fine mesh strainer lined with a double layer of cheesecloth over a large bowl. Pour the rhubarb mixture through the strainer to remove the pulp. To get the most rhubarb juice out, pull up the corners of the cheesecloth and gently press on it with a wooden spoon to squeeze out the remaining juice. Measure the strained juice and add enough water to equal 3 cups.

Return the liquid to the saucepan over medium-high heat and stir in the sugar. Bring to a boil and cook until the sugar dissolves, about 8 minutes. Remove from the heat, stir in the lemon juice, and let cool. Strain again for the clearest ruby syrup, then pour into a container and refrigerate. Keeps well in the refrigerator for up to 3 weeks.