Bring chicken broth and water to a simmer in a medium saucepan. Add asparagus and simmer, uncovered, until just tender, about 4 minutes. Transfer asparagus to an ice bath to stop cooking and then drain. Keep broth at a bare simmer, covered.
Cook onion in 2 tablespoons butter with 1/4 teaspoon sea salt in a 4-quart heavy saucepan over medium heat, stirring occasionally, until softened, about 5 minutes.
Add rice and cook, stirring constantly, 1 minute. Add wine and simmer, stirring constantly, until absorbed.
Stir in 1/2 cup broth mixture and simmer, stirring frequently, until absorbed. Continue adding broth, 1/2 cup at a time, stirring frequently and letting each addition be absorbed before adding the next, until rice is creamy and tender, about 18 minutes.
Stir in asparagus, zest, remaining 2 tablespoons butter, parmesan, parsley, and pepper to taste.