Sentence:  [paragraph 515] Now let us come to this point: you are quite clear that serious evil exists in this union of navigation and drainage? ... [paragraph 517] And you are also clear that one obstacle is the taxation of the <WSD>catchment</WSD> area, and that it needs some plan to be devised for meeting it, although this plan may have defects? 
Meaning of the word marked with <WSD> tags:  Any structure or land feature which catches and holds water; the collection of such water.