Résumé : Santa Juana dam is located in an arid and mountainous valley, where extensive technical and economic changes are underway. These changes are connected with the dam (the reservoir) itself as well as with the new global distribution of fruit production and exportation. Part of the transformations have obviously been unexpected, for it appears that the upstream farmers benefit of the dam more than do the downstream farmers. This paper describes the context of what appears to be an anomaly and focuses attention on security of irrigation related with security of financial investments. The main topics of hydraulic and social changes in arid Chilean mountains is treated, and the paper illustrates the problem of the profitability of the hydraulic works in the arid area, as well as the consequent problems for their beneficiaries.