Sentence:  If this first catch-water is to be employed at any time as a <WSD>flooder</WSD>, the upper edge of its bank must be carefully trimmed to the level; but I believe, on the whole, it will be more advisable to make a separate <WSD>flooder</WSD> immediately below, which may be cut at the exact level by means of the plumb rule, to the depth of one, and width of two feet; with an outlet from the catch-water at every five or ten Irish chains. 
Meaning of the word marked with <WSD> tags:  A channel or device for carrying and controlling water used in flood irrigation.