Introduction -- Water Rights -- Duty of Water -- Quantity of Water Appropriated -- Quality of Water Appropriated. Irrigators with prior appropriated water rights have traditionally used their total allocation of surface water to comply with "beneficial use" requirements of most Western water law. An Irrigator believes that applying all water to which he is entitled by prior appropriation constitutes beneficial use, and that such practice is necessary to maintain his water right. There is evidence, however, that overappropriating water is not essential and is, moreover, demonstrably detrimental to optimum crop yields. this paper examines state laws which address the beneficial use concept as it has applied in the past, and as it now applies to use of prior appropriated water in irrigation.