This document defines several key terms related to irrigation: - Base period refers to the time from initial watering to final watering of a crop before harvest. - Crop period is the time from planting to harvest. - Duty is the area irrigated by a unit discharge over the base period and relates water volume to crop area. - Delta is the total water depth required by a crop over its lifetime in the field. - Gross command area is the total area bounded by drainage boundaries that can be irrigated by a canal system. - Culturable command area is the area within the gross command area suitable for crop growth, excluding non-arable land.