Model-based assessment of interbasin groundwater flow in data scarce areas: the Gallocanta Lake endorheic watershed (Spain)

Abstract Aquifer systems, because of the presence of frequently complex geological structures, may extend beyond watersheds limits. Interbasin groundwater flow is often identified among watersheds. Because geological systems are complex ones, modelling tools are needed for its estimation. In this paper, we quantify the outflows from the endhoreic Gallocanta watershed (Spain) by means of a MODFLOW numerical model in order to assess the robustness of the boundaries of the Gallocanta Groundwater Body. Our results show the Gallocanta watershed is hydrogeologically connected with the adjoining Piedra-Ortiz and Jiloca watersheds (discharging annually in these basins about 4 and 1 Mm 3 respectively). Furthermore, we hypothesized the presence of geological features altering the groundwater flow. Additional simulations were run to analyse the changes in the water budget in the cases of: i) groundwater pumping no longer allowed by the authorities, and ii) a potential drought scenario. In the first case, the results forecast an increase in discharge to the Piedra-Ortiz and Jiloca watersheds, while in the second case a diminution of the outflows to the two neighboring basins is foreseen. We then propose a larger and unique groundwater body, spanning from the Caminreal Springs on the east and the Piedra-Ortiz basin on the west, including a moving groundwater divide internal to the Gallocanta watershed. Monitoring the baseflow of the Piedra-Ortiz river and of the Caminreal Springs will allow to get information on the evolution of the groundwater resource availability in the Gallocanta watershed. Our results stress the importance of conjunctively using data and traditional geologic knowledge (i.e. surface geology maps) along with numerical modelling analyses. This holds especially true in areas, such those of hard-rock aquifers, where scarce hydrogeologic data are available, to test conceptual models, to derive and to infer information on water budgets and on the presence of relevant structural features driving the groundwater flow. This approach may lead to informed decision-making on groundwater body boundaries definition for the application of relevant groundwater management regulations.