The Yarmouk River basin is shared between Syria, Jordan, and Israel. Since the 1960s, Yarmouk River flows have declined more than 85% despite the signature of bilateral agreements. Syria and Jordan blame each other for the decline and have both developed their own explanatory narratives: Jordan considers that Syria violated their 1987 agreement by building more dams than what was agreed on, while Syria blames climate change. In fact, because the two countries do not share information, neither on hydrological flows nor on water management, it is increasingly difficult to distinguish between natural and anthropogenic factors affecting the flow regime. Remote sensing and multiagent simulation (MAS) are combined to carry out an independent, quantitative analysis of Jordanian and Syrian competing narratives and show that a third cause for which there is no provision in the bilateral agreements actually explains much of the changes in the flow regime: groundwater overabstraction by Syrian highland farmers.