During the construction and especially water filling periods of Keban and Atatürk dams in the Euphrates River Basin, there have been a series of high level political tensions among Turkey, Syria, and Iraq for the allocation of trans-boundary water resources. It has been claimed that those large investments not only contribute to those investing countries but also to those affected ones. This research measures the contributions and impacts of major investments on the basin to the riparian countries. And how spatial land use decisions on irrigated agricultural areas in rival basin countries are made by means of optimization techniques. The model application is based on with and without analyses for the selected group of dams, which are Keban, Karakaya, Atatürk, Ilisu, Tabqa, Haditha, and Mosul dams, Habbaniye Reservoir, and Urfa Tunnels. The mixed integer programming (MIP) based on the Inter Temporal Euphrates and Tigris River Basin Model (ITETRBM) is used as an optimization tool for the necessary analysis. The impacts are presented by countries and by the generated economic benefits, water withdrawals, coverage areas of irrigated agricultural land, cities feasibly supplied from the reservoirs. The results have shown that large reservoirs in Turkey provide extensive contribution to the basin countries especially during drought periods when all major reservoir investments are considered together. That can be considered as potential for coalition during drought periods instead of competition. © 2014 Elsevier Ltd.