Subsurface Warming in Response to Climate Warming in Basins with Topography-Driven Groundwater Flow

Climate warming has caused increased air temperature as well as increased subsurface temperature. Many previous studies on subsurface warming simplified heat advection by neglecting the horizontal component of regional groundwater flow or even neglected heat advection accompanying groundwater flow. In this study, the simultaneous control of heat advection and conduction on subsurface warming is numerically investigated in a 2D hypothetical basin cross section. By calculating the increment of subsurface temperature, we find that heat advection could accelerate subsurface warming. In a given basin, subsurface warming in the recharge area with downward groundwater flow is more significant than that in the discharge area with upward groundwater flow. By using a 1D model with vertical groundwater flow only for comparison, we find that in any part of a basin, even if the horizontal flux is very small compared with the vertical flux, neglecting the horizontal flux would underestimate the propagation depth of climate warming. This implies that when the propagation depth of climate warming is a priori known variable, existing 1D models would overestimate downward groundwater flux or underestimate upward groundwater flux. Moreover, we find pumping would cause deeper propagation depths of climate warming by accelerating groundwater circulation, whereas basin-scale heterogeneity and anisotropy of hydraulic conductivity would cause shallower propagation depths of climate warming because of the relative dominance of horizontal flow. By demonstrating the importance of 2D groundwater flow on subsurface warming, the results provide new insight into understanding the regulation of temperature among the atmosphere, the hydrosphere and the lithosphere.