Figure 7. Two of the soil water characteristic curves used in
this study (a), and mean infiltration at the land surface for 10 years
of pumping (b).
However, in G1, the increase of WTD is less than 0.5 m in average after
10 years of pumping (Figure 4a), and thus the WTD in most of the
modeling area is still in the critical depth range. Normally, in G1, the
continuous increase of WTD with pumping would remain in effect on the
variations of GST. Therefore, the attainment of dynamic equilibrium in
GST variations (Figure 5a) seems incompatible with the above
explanations for G2. In fact, this ‘paradox’ can be explained from the
perspective of GW flow system as follows. The GW flow system becomes
nonequilibrium with an obvious increase of WTD in the beginning of
pumping (Figure 4a). With pumping, the increase of WTD promotes the
infiltration at the land surface (or recharge at the water table)
(Figure 7b) and decreases the discharge to streams (Condon and Maxwell,
2019), and thus a new dynamic equilibrium of the flow system is
gradually achieved. Such a self-adjustment of the GW flow system has
also been reported in previous studies (Cao et al., 2013; Condon and
Maxwell, 2019). Hence, the GST increases with WTD in the beginning while
it becomes dynamic stabilized when a new equilibrium of the GW flow
system is achieved. Therefore, at a sustainable pumping rate such as
that in G1, the dynamic equilibrium of GST can be achieved due to the
self-adjustment of the GW flow system even though the WTD is still in
the critical depth range. For an unsustainable pumping rate such as that
in G2 and NCP, the combined effects of the self-adjustment of the GW
flow system and the critical depth range theory should be responsible
for the nonlinear variations of GST. In addition, the slight
nonlinearity in Figure 4b indicates, in G2, the effect of the critical
depth range is dominant while that of the self-adjustment of the flow
system can be almost neglected.
- Effects of soil properties on the subsurface buffer
The discussion in section 3.1 indicates the buffer capacity of the
subsurface on variations of GST (or the land surface heat fluxes), which
is consistent to our hypothesis and other studies from a more general
perspective (Condon and Maxwell, 2014b; Cuthbert et al., 2019; Smerdon
and Stieglitz, 2006). For example, Condon and Maxwell (2014b)
conceptualized the GW system as a buffer on the hydrological variations.
Smerdon and Stieglitz (2006) described the damping effect of the
subsurface buffer on the propagation of temperature signals from the
land surface to the deep subsurface. The same results obtained from the
Little Washita basin of the U.S. and the NCP of China (Yang et al.,
2019), which are two totally different study areas, indicate the
generalizability of our conclusions that deserve more attention in the
areas with over-exploitation of GW worldwide. It is noted that the
change of the buffer capacity with pumping is mainly due to the change
of thermal properties in the subsurface. The thermal conductivity and
volumetric heat capacity of the subsurface decrease with the decreasing
soil moisture induced by pumping (Figure 8). Figure 8 was plotted based
on the source code of ParFlow.CLM. Hence, when a hot or cold signal
(e.g., the increase or decrease of the ground heat flux) is input at the
land surface, the decreased thermal conductivity cannot propagate the
signal to deep subsurface in time while the decreased volumetric heat
capacity cannot effectively damp the signal due to the limited
storage/release ability of heat. Therefore, the coupling depth (BBCP),
which determines the heat capacity of the subsurface in terms of volume,
should also be responsible for the buffer capacity of the subsurface.