Question: How much of the Netherlands is affected by the problem regulated by the Hoogheemraadschap van Rijnland? 
Sentence (from "Hoogheemraadschap van Rijnland"): The Hoogheemraadschap van Rijnland is the oldest water board of the Netherlands, having received its first commission to protect the land from flooding back in 1248 from William II of Holland.
Relevant evidence: Yes