Sentence:  The options available for distribution of water during times of reduced supply or demand are the following: ... (b) To reduce the rate of flow in the primary canal (still maintaining continuous flow in it) and to supply full flow in rotation to secondaries and their <WSD>tertiaries</WSD>; (c) to reduce the rate of flow in both primary and secondary canals, and to supply full flow in rotation to <WSD>tertiaries</WSD>; ... 
Meaning of the word marked with <WSD> tags:  Any item considered to be of third order.