A new GRACE downscaling approach for deriving high-resolution
groundwater storage changes using ground-based scaling factors
Abstract
To compensate for the intrinsic coarse spatial resolution of groundwater
storage (GWS) anomalies (GWSA) from the Gravity Recovery and Climate
Experiment (GRACE) satellites and make better use of current dense in
situ groundwater-level data in some regions, a new statistical
downscaling method was proposed to derive high-resolution GRACE GWS
changes. A ground-based scaling factor (SFGB) method was proposed to
downscale GRACE GWS changes that were corrected using gridded scaling
factors estimated from ground-based GWS changes through forward
modeling. The proposed method was applied in the North China Plain
(NCP), where many observation wells and consistently measured specific
yield are available. Importantly, the sensitivity of the proposed method
was explored considering the uncertainties of in situ GWS changes due to
variable specific yield and/or number of observation wells. Independent
validation shows that SFGB can effectively recover GRACE GWSA at the
0.5º grid scale (r = 0.81, root mean square error = 40.51 mm/yr). The
SFGB-corrected GWSA in the NCP was -32.60{plus minus}0.99 mm/yr
(-4.6{plus minus}0.14 km3/yr) during 2004-2015, showing contrasting
GWS trends in the piedmont west (loss) and the coastal east (gains).
Uncertainties in SFGB-corrected GWSA arising from specific yield,
groundwater-level, and both can be reduced by 90%, 65%, and 84%,
respectively relative to ground-based GWSA. This study highlights the
potential value of jointly using GRACE and in situ observation data to
improve the accuracy of GRACE-derived GWSA at smaller scales. The new
downscaling method and the improved groundwater storage change estimates
would facilitate better groundwater management.