Bring broth and water to a simmer in a medium saucepan. Add asparagus and simmer, uncovered, until just tender, about 4 minutes. Transfer asparagus with a slotted spoon to an ice bath to stop cooking, then drain. Keep broth barely simmering, covered.
Cook onion in 2 tbsp butter with 1/4 tsp salt in a 4 qt heavy saucepan over medium heat, stirring occasionally, until softened, about 5 minutes.
Add rice and cook, stirring constant;y, 1 minute. Add wine and simmer, stirring constantly until absorbed.
Stir in 1/2 cup broth mixture and briskly 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 but still al dente (it should be the consistency of thick soup), about 18 minutes. There will be some left-over broth mixture.
Sir in shrimp, zest and remaining2 tbsp butter, parmesan, parsley and pepper to taste. (thin risotto with some of remaining broth if necessary).