Preheat oven to 350°F (180°C).
In a medium bowl, stir together ⅓ cup plus 1½ tablespoons (102.5 grams) water, orange zest and juice, and castor sugar. Add rhubarb, tossing to combine. Pour onto a rimmed baking sheet, and cover with foil.
Bake for 8 minutes. Using a sharp paring knife, test rhubarb until it cuts easily but still holds its shape. If not ready, cover and bake for 3 to 4 minutes more, and test again. Let cool completely before using.