The small-scale tank irrigation system is an effective option for sustainable agricultural water development. The management strategy for water release in such a system is considered in the framework of variational calculus. A minimum principle is deduced to characterize an optimal strategy minimizing a performance index that evaluates water content deviation in the command area and the time when the irrigation tank dries up. The optimal strategies are demonstrated in two practical example systems. (c) 2004 Elsevier Ltd. All rights reserved.