Diagnosing the subsurface buffer on ground-surface temperature under long-term groundwater pumping: effects of the bottom boundary condition placement

The terrestrial subsurface acts as a buffer in regulating land-surface water and energy processes. In integrated modeling with land-surface models, the regulating capacity of the subsurface is largely determined by the bottom boundary condition placement (BBCP). However, the sensitivity of land-surface processes to BBCP in Earth system modeling and integrated hydrologic modeling under conditions of human disturbance has not been investigated. In the research reported here, numerical experiments by integrated hydrologic modeling in the Little Washita Basin (Oklahoma, USA) were conducted to study the behavior of the subsurface buffer in regulating ground-surface temperature (GST) under long-term groundwater pumping and the role of BBCP in modeling such processes. Results reveal that long-term groundwater pumping weakens the subsurface buffer and leads to a warming trend. Under either sustainable or unsustainable pumping scenarios, the increase in GST exhibits temporal nonlinearity by rapidly increasing in the beginning and gradually achieving a dynamic equilibrium. It is also found that the subsurface buffer with deeper BBCP is more effective on damping the nonlinearity and the amplitude of the increase in GST. In addition, variations in GST are more sensitive to BBCP under pumping than in the natural state due to decreased thermal properties. In summary, this study diagnoses the subsurface buffer on variations of GST with the role of BBCP in integrated modeling under long-term groundwater pumping. These results are expected to have important implications on integrated modeling with land-surface models in areas of groundwater depletion worldwide.