Figure 11. Distribution of root fraction in each layer for each
land cover type in Figure 2(b).
- 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.
- 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.
- 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.
- 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.