Figure 2. The elevation (a), land cover (b), and soil (c) and
aquifer (d) properties in the modeling area.
- Discretization and coupling depth
The subsurface is divided into 1
×
1 km2 cells in the horizontal direction with terrain
following grids to accommodate the topography. Layering is customized in
this study. The subsurface of 100 m thickness is discretized into 15
layers with 6 top layers of 0.1, 0.3, 0.6, 1.0, 3.0 and 5.0 m and 9
bottom layers of 10 m per layer. Thus, the number of grid cells are
25215 (41 × 41 × 15) in total. The top 4 layers (2 m) are set to be
soils (Figure 2c) while the deep subsurface (98 m) has the hydraulic
properties of the aquifers (Figure 2d). Six scenarios with different
coupling depths (2, 10, 20, 30, 60, and 90 m) are tested to study the
feedbacks between land surface and subsurface processes, especially with
the long-term GW pumping. The coupling depth corresponds to the number
of coupling layers of 4, 6, 7, 8, 11, and 14, respectively. Hence, the
scenarios are noted as L4, L6, L7, L8, L11, and L14 respectively, where
L represents the layer.
- Boundary conditions and simulations
The boundary conditions are all no flow boundaries except for the
overland flow boundary at the land surface. Meteorological forcings from
10/01/1970 to 09/30/1971 in this study are obtained from the 1.25-degree
gridded Japanese 55-year Reanalysis (JRA-55) (Harada et al., 2016;
Kobayashi et al., 2015). The forcings are spatially uniform by using the
data near the center of the modeling area (Kollet and Maxwell, 2008).
The original JRA-55 data of 3-hour resolution are linearly interpolated
into hourly resolution. The 1-year forcings are repeatedly used in the
following 10-year pumping simulations.
After spin-up (refer to section 2.2.4), one more year is simulated to
represent the natural state without pumping while 10 years of simulation
with pumping is conducted to explore its long-term effects on GST. GW
pumping is assumed only in the croplands shown in Figure 2b. Multi-year
averaged annual pumping rate of about 80 million cubic meters is adopted
from Condon and Maxwell (2014a). One more group of pumping simulations
with tenfold pumping rate are also conducted to understand the
mechanisms of the subsurface buffer. Those two groups of simulation are
noted as G1 and G2 respectively. Hourly time step is used in all
simulations.
- Initial conditions
Initial conditions are critical and specifically considered in this
study due to the thickened coupling depth. In Stevens et al. (2007), a
spin-up of 500 years was conducted for 500 years of simulation, and the
heat propagating downward did not reach the 1000 m BBCP. In this study,
for L4, which is the common set in previous studies (e.g., Maxwell and
Condon, 2016), 4 years of spin-up are enough to achieve the dynamic
equilibrium of the model. Nevertheless, considering the 10 years of
pumping, longer spin-ups of 10, 30, 50, and 100 years (≥ 10 years
pumping) were performed additionally to ensure the reliability of the
simulations. Therefore, the following discussion in section 3 is based
on 4 years of spin-up while the results with longer spin-up are provided
in Supporting Information (Figures S1 and S2).
- Model evaluation
The model was evaluated in Condon and Maxwell (2014a), which
demonstrated its capability to capture the interactions of water and
energy processes between land surface and subsurface. Also as pointed by
many previous studies based on such integrated models (e.g., Condon and
Maxwell, 2014b, Maxwell and Condon, 2016), the aim here is to diagnose
the subsurface buffer based on a realistic platform but not to fit
history or predict future by a calibrated model. Hence, no special
calibration and verification of the model were conducted in this study.
- Results and discussion
- Performance of the subsurface buffer
- WTD and GST variations under long-term GW pumping
If not otherwise specified, the discussion in this section is based on
scenario L4 in G1. Annual average WTD in the year without pumping is
2.32 m in average and 25.08 m at the maximum (Figure 3a). The difference
between annual average WTD in each pumping year and that in the year
without pumping was calculated (ΔWTD) (Figure 3). Obvious increase of
WTD occurs not only in the croplands with pumping but also at the
uplands without pumping (Figure 3), which is consistent with Ferguson
and Maxwell (2011) and Condon and Maxwell (2014a). The mean ΔWTD in the
modeling area was plotted in Figure 4. The mean ΔWTD after 10 years of
pumping is about 0.5 m for G1 and 10 m for G2. Hence, after 10 years of
pumping, for G1, most WTD in the modeling area is still in the critical
depth range (1–10 m), in which the land surface heat fluxes are
sensitive to the variations of WTD (Condon and Maxwell, 2019; Ferguson
and Maxwell, 2011). In contrast, for G2, most WTD has become lower than
the critical depth range. In addition, the rate of increase of WTD is
decreasing with time in G1 while it is almost linear in G2. The mean,
maximum, and minimum change of GST (ΔGST) are shown in Figure 5. GW
pumping leads to increased GST in summer (March to September) and
decreased GST in winter (October to February) (Figures 5a and 5c). The
mean ΔGST indicates the warming trend in average (Figures 5a and 5c),
though the decrease of GST is over its increase at extreme values at
some moments (Figure 5b).
The
mean ΔGST is in -0.5 K–0.5 K in G1, and in -1.5 K–1.5 K in G2. In
addition, the variations of GST occur most obviously in the pumping area
(Figure 6). These verify the hypothesis proposed in section 1 (i.e., the
first objective in this study) that the subsurface may be conceptualized
as a buffer on variations of GST in the Little Washita basin. GW pumping
can weaken this buffer by causing hotter summer and colder winter with a
warming trend in average.