This paper develops a spatial conjunctive use model of an irrigation project in which the regulatory agency determines investments in the centralized distribution system and farmers decide the level of on-farm technology in the field. Irrigation return flows are assumed to recharge the groundwater aquifer. It is shown that there is specialization in production with upstream farmers using surface water and downstream farmers pumping from the aquifer. An empirical model suggests that the proportion of return flows has a significant effect on the level of investments in water distribution as well as in the field. For example, if return flows are relatively high, it may be optimal to allow for significant water losses from the canal and the fields. It suggests that the project pricing and technology adoption policies may need to be tempered by consideration of the basinwide impacts of water diversions. © 1998 Elsevier Science B.V. All rights reserved.