Combine 1/4 cup of water, rhubarb and sugar in a large saucepan over medium heat. Bring to a boil, stirring to dissolve the sugar. Reduce to a simmer and cook until rhubarb is tender, about 10 minutes. Remove from heat and let cool.
Combine rhubarb, lemon wedges, and water in a blender (you may need to do this in batches depending on the size of your blender. Blend on high until smooth and pureed. Strain.
Add more water (1 cup at a time) if it seems too thick. Chill completely and serve over ice.