Influence of groundwater recharge projections on climate-driven subsurface warming: insights from numerical modeling

Abstract. Groundwater warming due to rising surface temperatures has been documented in both urban and natural settings. However, the potential for long-term changes in the magnitude and seasonality of groundwater recharge to modulate this warming trend has not yet been systematically investigated. In this study, we integrate a stochastic weather generator, distributed hydrologic modeling, and regional thermo-hydraulic groundwater modeling into a unified workflow and apply it to the area of Brandenburg (northeastern Germany). We conduct numerical simulations to assess changes in the subsurface thermal field between present day and 2100, evaluating two climate change scenarios, and incorporate a spectrum of ensemble-based and discrete recharge projections. Our results demonstrate that, while surface temperature rise is the primary driver of the projected groundwater warming of up to 2.5 °C, groundwater flow is responsible for its regional variability in magnitude and affected depths. Higher hydraulic gradients on topographic highs and increased thickness of the permeable Quaternary unit may allow the warming signal to propagate below 200 m depth, whereas groundwater discharge in the river valleys tends to limit it to <200 m. By the late century, the difference in groundwater temperatures between recharge-reduction and recharge-increase scenarios can reach 0.4 °C. Under the high-emissions pathway, a 20 % recharge reduction, from a mean of 75 to 60 mm a-1, causes a 2–5 m water level decline, reducing the area of unconfined aquifer subjected to seasonal temperature fluctuations. Model experiments show that even a hypothetical increase in winter recharge does not suffice to counteract the groundwater warming induced by rising surface temperatures. Changes in advection rates are not expected to affect the net climate-driven accumulated heat in the subsurface due to counterbalancing of heat gains and losses between recharge and discharge areas. Nevertheless, long-term reconfiguration of the potentiometric surface may further impact both the annual and long-term thermal state of key aquifers targeted for water supply and shallow geothermal energy utilization.