Combine rhubarb, sugar, and lemon peel in a large saucepan; bring to a boil. Reduce heat to medium-low; simmer until sugar has dissolved and rhubarb releases its juice and starts to break up, about 10 minutes. Off heat, stir in lemon juice.

Pour rhubarb mixture through a fine-mesh sieve, pressing out as much liquid as possible.

Fill a glass with ice; add 1 cup water and 1/3 cup syrup. Stir to combine.