Exploring a trilateral transboundary water resource: The Yarmouk Basin. From geochemistry to numerical modelling

The Yarmouk River and its drainage basin are shared by the three riparian states of Jordan, Israel, and Syria. In this semiarid region, the relatively high rainfall makes the basin a crucial water resource for Jordan and an important one for Israel and Syria as well. Within the Lower Yarmouk Gorge, ancient thermal springs discharge, and deep boreholes produce large volumes of artesian groundwater that supply substantial parts of northwestern Jordan.
Based on geochemical and isotopic evidence, a conceptual flow model was developed indicating that these groundwaters are entirely artesian, were recharged more than 10,000 years ago, and originate from distinctly different recharge areas. Large-scale subsurface flow paths extend from the Golan Heights and the Syrian Hauran Plain, while other components are recharged in the Jordanian Ajloun Highlands.
To verify this concept, a numerical flow model was constructed based on a complex, multi-layer hydrogeological framework. The model was calibrated using scarce observation data from Syria, Jordan, and Israel and was subsequently driven by a distributed hydrological model that provided groundwater recharge time series for more than 50 years. In addition, abstraction rates from known well fields were implemented, although these values are conservatively low due to the large number of undocumented pumping activities throughout the basin.
The final transient model, for the first time, encompasses the entire subsurface catchment across national borders and provides new insights into groundwater flow dynamics and the development of extensive depression cones in response to intensive abstraction. The simulated water tables closely match observations, first confirming the conceptual model and second indicating that the resource is approaching its critical limit.
The artesian conditions are expected to diminish in the near future, and the vital contribution of groundwater discharge to the Lower Yarmouk River may cease, with serious consequences for water supply in Jordan and for transboundary water-sharing agreements among the riparian countries.