In a saucepan, combine chopped rhubarb, water, and sugar. Bring to a boil over medium heat, then reduce heat and simmer for about 10 minutes until rhubarb is soft.

Remove from heat and let cool slightly. Blend the mixture until smooth, then stir in lemon juice and lemon zest. Chill in the refrigerator for at least 2 hours.

Pour the chilled mixture into an ice cream maker and churn according to the manufacturer's instructions until it reaches a soft-serve consistency.

Transfer the sorbet to a freezer-safe container and freeze for at least 4 hours or until firm.