The duty of water is the relationship between the volume of water and the area of the crop it matures. This volume of water is generally expressed as, "a unit discharge flowing for a time equal to the base period of the crop, called Base of a duty". The duty varies with the place of its measurement, because of the continuous conveyance losses as the water flows. If the variation in duty of water from the head of a main canal (M) to that in the field (F) then duty of water at M is always less than duty of water at F.