Figure 11. Distribution of root fraction in each layer for each land cover type in Figure 2(b).
  1. Conclusions
In this study, integrated land-surface–subsurface modeling was conducted in the Little Washita basin located in the southwestern Oklahoma of the U.S. based on ParFlow.CLM. The long-term effects of groundwater (GW) pumping on ground surface temperature (GST) are studied with concern on the coupling depth between ParFlow and CLM. Two groups of simulations with normal and tenfold pumping rate were performed while six scenarios of different coupling depth in each group were set. For each scenario, 1-year simulation without pumping and 10-year simulation with pumping were conducted. Thus, total 132 years per initial condition (×5 initial conditions) of simulation were completed to obtain the conclusions as follows.
  1. The subsurface can be conceptualized as a buffer on the variations of GST in the Little Washita basin. GW pumping weakens the buffer by causing hot summers and cold winters with a warming trend in average. Due to its consistence with the preliminary results obtained in our ongoing NCP study (Yang et al., 2019), the findings are probably not case dependent but can be transferred to other places with GW depletion.
  2. In the long-term pumping, the increase of GST (ΔGST) presents nonlinearly temporal trend by rapidly increasing in the beginning and gradually achieving a dynamic equilibrium. For sustainable pumping, GW flow system gradually attains a new equilibrium through self-adjustment. In this process, the water table depth (WTD) becomes stable by increasing infiltration at the land surface and decreasing discharge to streamflow, and thus the variations of GST stagnate. For unsustainable pumping, dominant mechanism for the nonlinearity of ΔGST is that WTD finally becomes lower than the critical depth range (1–10 m) with time, and thus the land surface processes, such as the variations of GST, are not sensitive to the increasing WTD anymore.
  3. The coupling depth has significant effects on the performance of the subsurface buffer. The buffer with deeper coupling depth is more effective on damping the nonlinearity and the amplitude of ΔGST. Additionally, the time-scale for GST to response the different coupling depth is largely shortened under pumping in contrast to that under natural conditions. When pumping occurs, the decrease of thermal conductivity and volumetric heat capacity have negative effects on the buffer capacity, and thus the positive effects of the coupling depth becomes more prominent than that under natural conditions.