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Text S1
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Supplementary Methods and Results
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1. Data
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The latest Gravity Recovery and Climate Experiment (GRACE) terrestrial water
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storage (TWS) land products (RL05) provided by the Center for Space Research
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(CSR), GeoForschungsZentrum Potsdam (GFZ), and Jet Propulsion Laboratory (JPL)
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were used (data available at
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ftp://podaac-ftp.jpl.nasa.gov/allData/tellus/L3/land_mass/RL05/). The monthly TWS
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was scaled by using the scaling factors provided with the data in order to restore much
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of the energy removed by the by de-striping, filtering, and truncation processes
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[Landerer and Swenson, 2012].
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The hotspot of groundwater depletion is the North China Plain (NCP) [Cao et
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al., 2013]. The plain area with extensive groundwater exploitation is the region where
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the Ministry of Water Resources of China [MWR, 2013] provides the estimates of
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shallow aquifer storage change according to the monitoring well network observations
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(Fig. S3). The groundwater depletion is unevenly distributed in the area with the
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highest water table decline in the piedmont region (i.e. the west part of the plain area)
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[Cao et al., 2013]. We divided the area with decreasing GRACE TWS into two
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regions. The east region (Table S1) covers the plain area with extensive groundwater
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exploitation, and the west region (Table S2) covers the mountainous areas. We
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delineated the two regions following the administrative boundary. The east region
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covers an area of 1.4×105 km2 and the west region has an area of 2.3×105 km2. The
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east region, which covers the region with the highest water table decline [Cao et al.,
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2013), is larger than the plain area covered by the monitoring well network. The
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shallow aquifer depletion in the east region (in water thickness) was calculated as the
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water volume change of the shallow aquifer divided by the area of the east region.
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The coal transport data used in this study area were obtained from the National
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Bureau of Statistics of China [NBS, 2012]. The data were provided at the
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administrative zones. For the administrative zones with part of the area within the
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study region, the net coal mass change was assumed to be evenly distributed in the
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administrative zone and only the portion in the study region was accounted for.
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The east region (Hebei province and Tianjin city), a part of the Haihe River
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basin, received water supply mostly for domestic use from the Yellow River (Fig. S3;
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Table S3) [MWR, 2013]. The inter-basin water diversion (in water thickness) was
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calculated as the water diversion amount divided by the area of the east region.
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2. Land-Surface Hydrologic Model
We used the Variable Infiltration Capacity (VIC) model [Liang et al., 1995] to
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estimate the natural surface-water storage change. The VIC model was applied at
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0.25° spatial resolution. The meteorological observations (including precipitation,
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daily maximum and minimum temperature, and surface wind speed) were obtained
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from China Meteorological Administration (CMA). The VIC forcings were gridded
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from the station observations to the model spatial resolution for the period of
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1952-2011 (Maurer et al., 2002). The gridded precipitation data were scaled to match
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the long-term average of the precipitation climatology produced with a more intensive
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station network [Xie et al., 2007] in the historical period of 1962-2001. The model
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was initialized by a long model spin-up period starting in 1952 using the gridded
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forcing. A routing model [Lohmann et al., 1998] was used to simulate streamflow. The
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VIC model and the routing scheme together represent a parameterization that has
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about the necessary degree of sophistication for the representation of the natural
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surface-water budget [Tang et al., 2010].
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The VIC model performance was optimized using the naturalized streamflow at
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2 hydrologic stations, Luanxian and Guaitai, in the study area (Fig. S3). The modeled
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streamflow matched well with the naturalized streamflow in the period of 1958-1977
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(Fig. S4). The Nash-Sutcliffe efficiency of the model is 0.84 and 0.77 for Luanxian
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and Guaitai stations, respectively. The correlation coefficient is high (>0.97),
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indicating that the model can capture the monthly variation of the naturalized
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streamflow well. In this study, we used the standard version of the VIC model in
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which the effects of dams and water diversion are not accounted for in the scheme.
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The modeled evaporation and runoff, together with the precipitation, were used to
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calculate the modeled natural surface-water storage change [Tang et al., 2010].
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3. The GRACE TWS Data without Scaling
Figure S5 shows the estimates of the contributions (natural surface-water,
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reservoir storage, coal transport, inter-basin water diversion when applicable, and
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groundwater) of GRACE TWS from 2003 to 2011 if the GRACE TWS data without
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scaling were used. Comparing to the beginning of 2003, the estimated groundwater
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decline at the end of 2011 (17-23 mm) is smaller than the coal mass loss (28.5 mm) in
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the west region. The trend of the estimated groundwater storage in the west region is
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insignificant, with a value between -3.68 (VIC estimated) to 3.30 (Noah estimated)
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mm yr-1. In the east region, the estimated mass loss signal of groundwater depletion
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(115-140 mm) was largely offset by the mass gains in equivalent water caused by
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natural surface-water (14-39 mm), reservoir (26 mm), inter-basin water diversion (46
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mm), and coal transport (16.6 mm). The trend of original GRACE TWS is
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insignificant with a value of -5.05 mm yr-1 while the estimated rate of groundwater
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depletion is significant with a value between -13.73 (VIC estimated) to -8.10 (Noah
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estimated) mm yr-1. These estimates are close to those using the scaled GRACE TWS.
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