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<p>Climate change induced alteration of recharge is expected to have diverse effects on groundwater levels, which could also modify the fragmentation and hierarchy of groundwater flow systems, including their dimensions and relative positions.</p><p>This study put emphasis on how flow system hierarchy may change due to recharge reduction in complex, vertically superimposed groundwater flow systems with different fluid driving forces through an example of the Duna-Tisza Interfluve in Hungary. Two main groundwater flow domain was identified by previous authors in this area with a separate source of water. Recharge to the upper, unconfined, gravitational regime is inferred to occur from infiltrating precipitation, while the underlying confined, overpressured flow domain is maintained by pore volume reduction due to tectonic compression of the basement (Tóth and Almási 2001, Almási 2003, Mádl-Szőnyi and Tóth 2009). The exposure of these groundwater flow systems, one is driven by gravity and other one is by overpressure, is completely different to the effects of changes in hydrologic parameters. Local scale gravity-driven flow systems are identified to be the most vulnerable to atmospheric processes (Kurylyk et al., 2014), while overpressured upward flow is driven by tectonic compression, and thus independent of climatic variation.</p><p>Two-dimensional transient numerical simulations were performed to gain insight into the response of this complex flow system to the predicted climate change of the region. Special emphasis is placed on i) how relative rate and influence of the different driving forces may change due to the predicted recharge reduction, ii) how the fragmentation of the flow field may alter, iii) how the penetration depth of upper, gravity-driven flow field may adjust to these changes and iv) how groundwater-related shallow surface water bodies will be affected by these changes.</p><p>Understanding the effects of changed hydrologic conditions on such complex flow patterns and recharge-discharge relationships as well as on interactions with surface water bodies can help to set-up three-dimensional site-specific models. These models provide a base to better mitigate and prepare for the consequences of predicted future changes.</p><p>The research is supported by the ÚNKP-20-4 New National Excellence Program of the Ministry for Innovation and Technology from the source of the National Research, Development and Innovation Fund, as well as by the József and Erzsébet Tóth Endowed Hydrogeology Chair. This work is part of a project that has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 810980.</p>

<p>The depth of the Earth’s critical zone can be questionable especially in thousands meter deep sedimentary basins. Therefore, extension of the critical zone’s usually studied 10s of meters depth considering groundwater flow systems has critical importance. Growing demand for groundwater resources (water, geothermal energy), economic services of the groundwater flow related surface and subsurface processes and phenomena (e.g., groundwater dependent ecosystems, surface salinization), and the potential role of groundwater in the adaptation to and mitigation of the effects of human activities and climate change represent the significance and functions of groundwater flow systems in the critical zone.</p> <p>Regarding the complexity of these flow systems, the primary goal could be the determination of their relative significance in the shallower parts of the critical zone. To this end, the present study proposes a methodology based on the hydrodynamic analysis of measured data to separate flow systems with different driving forces (topography, vertical compaction) and pore pressure regimes (normal or  close to hydrostatic, overpressured, underpressured). These characteristics define the renewability of groundwater resources, the near-surface conditions (e.g., distribution of nutrients, salts and heat, type of vegetation and soils, slope stability, etc.), and the exposure of flow systems to the effects of global and climate change.</p> <p>As a case study, groundwater flow systems of the Great Hungarian Plain (Pannonian Basin, Hungary) were evaluated and characterized by analyzing about 5,800 measured hydraulic data (pre-production static water levels and static formation pressures) in hydraulic head vs. elevation and pressure vs. elevation profiles, tomographic maps, and hydraulic cross sections in combination with the geologic build-up and some surface phenomena (distribution of saline soils and vegetation). As a result, spatial extension and distinct functions in the critical zone were defined for three flow regimes, namely i) the near-surface topography-driven groundwater flow systems, ii) an underlying overpressured regime, and iii) the transition zone of i) and ii). For instance, outstanding significance of the upward flows of saline water from the transition zone was revealed in the generation of saline soils and vegetation.</p> <p>The research was funded by the National Multidisciplinary Laboratory for Climate Change, RRF-2.3.1-21-2022-00014 project.</p>