Heuristic or intuitive rules based on experience may not be efficient when applied to the management of water supply systems that contain both surface and subsurface storage. In particular, rules that assign subsurface storage the role of a backup to surface storage do not recognize the different capabilities of surface and subsurface storage. We demonstrate how to incorporate the different capabilities of surface and subsurface storage in appropriately cautious long-term control of conjunctive-use systems. We also demonstrate how these results may be used to evaluate the benefit of adding groundwater supplies to an existing surface water supply system. ¿ 1998 American Geophysical Union