Korean farmers use levees to separate and irrigate their rice fields. The word underlined in the sentence above is best defined as .

Levees are ridges that surround and keep water in irrigated areas

Explanation:

Levees are ridges that help us keep the water out from where we don't want it to be. In this case, these ridges are there to surround and keep the water inside the irrigated areas so that the rice can have enough water to grow and later be harvested by us for many uses. They are very popular in Korea but not seen only there.