Supplementary Material
GRACE Satellites Monitor Large Depletion in Water Storage
in Response to the 2011 Drought in Texas
Di Long1, Bridget R. Scanlon1, Laurent Longuevergne3, Alex-Y. Sun1, D. Nelun Fernando4, 5, and Save
Himanshu2
1. Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin,
Austin, Texas, USA
2. Center for Space Research, The University of Texas at Austin, USA
3. Geosciences Rennes, UMR CNRS 6118, Universite´ de Rennes 1, Rennes, France
4. Department of Geological Sciences, The University of Texas at Austin, USA
5. Surface Water Resources Division, Water Science Conservation, Texas Water Development Board,
Austin, Texas, USA
Material 1: Drought in the United States in 2011 and 2012
The US drought monitor in the National Integrated Drought Information System (NIDIS) shows
that ~23% of the US was subjected to severe (D3) to exceptional drought (D4) in Sep, 2011
(Figures S1 (a) and S2). Exceptional drought was concentrated mostly in Texas, Oklahoma,
southeast New Mexico, and southwest Kansas. The 2012 drought began in spring and is the
expansion of the Southern US drought in 2010-2012. About 42% of the US was subjected to
severe to exceptional drought in Sep, 2012 (Figures S1(b) and S2). In particular, the extreme and
exceptional droughts spread out from the central US. The US High Plains, including most of
Nebraska, southeast Colorado, west Kansas, northwest Oklahoma, and north of the Texas
Panhandle, suffered from exceptional drought.
Material 2: Correlations between GRACE-derived GWS and that from Water-level Monitoring
in the Literature
Reliability of GWS changes is generally evaluated by comparison with water level monitoring
from well networks. Leblanc et al. [2009] used GRACE-based TWS in combination with
simulated SMS from the Global Land Data Assimilation System (GLDAS) Noah model and
ground-based reservoir storage during the recent Millennium drought in the Murray Darling
Basin. TWS declined by 140 km3 (Aug 2002 to Dec 2006), with reductions in RESS (16 km3)
and SMS (~120 km3) occurring mostly in the early phase (2000–2002) and GWS declining by
~104 km3 over a longer time (2001–2007). Strassberg et al. [2009] and Longuevergne et al.
[2010] found good correlations (R=0.7 to 0.8) between changes in GWS from GRACE and
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ground-based monitoring data in the High Plains. GRACE-based GWS depletion of 31±3 km3 in
the Central Valley during the 2007–2009 drought (Oct 2006–Mar 2010) [Scanlon et al., 2012]
and 23.9±5.8 km3 (Apr 2006–Mar 2010) [Famiglietti et al., 2011] compared favorably with
estimates based on well data (27 km3 from Apr 2006 through Sep 2009) [Scanlon et al., 2012].
Material 3: Data Processing for GRACE CSR RL04, RL05, and GRGS RL02
Spherical harmonics solutions for CSR RL04 and RL05 were truncated at the maximum degree
and order of 60, destriped [Swenson and Wahr, 2006], and filtered using a 300 km Gaussian
filter to suppress GRACE measurement noise of high-degree and order spherical harmonics. The
spherical harmonics for GRGS RL02 were truncated at the maximum degree and order of 50.
Truncation is a type of low-pass filter and the lower degree and order applied to GRGS
constitutes a regularized solution and no further filtering was required. Bias and leakage of
GRACE signals due to truncation of the spherical harmonics and filtering associated with the
effective basin function were corrected using the method proposed by Longuevergne et al [2010]
(Figure S6). SMS from Noah in GLDAS-1 was used as a priori knowledge of the global SMS
variation to correct GRACE-based TWS.
References:
Famiglietti, J. S., M. Lo, S. L. Ho, J. Bethune, K. J. Anderson, T. H. Syed, S. C. Swenson, C. R.
de Linage, and M. Rodell (2011), Satellites measure recent rates of groundwater
depletion in California's Central Valley, Geophys. Res. Lett., 38(3), L03403, doi:
10.1029/2010GL046442.
Leblanc, M. J., P. Tregoning, G. Ramillien, S. O. Tweed, and A. Fakes (2009), Basin-scale,
integrated observations of the early 21st century multiyear drought in southeast Australia,
Water Resour Res, 45, doi: 10.1029/2008WR007333.
Longuevergne, L., B. R. Scanlon, and C. R. Wilson (2010), GRACE Hydrological estimates for
small basins: Evaluating processing approaches on the High Plains Aquifer, USA, Water
Resour Res, 46, doi:10.1029/2009WR008564.
Scanlon, B. R., L. Longuevergne, and D. Long (2012), Ground referencing GRACE satellite
estimates of groundwater storage changes in the California Central Valley, USA, Water
Resour Res, 48, doi: 10.1029/2011WR011312.
Strassberg, G., B. R. Scanlon, and D. Chambers (2009), Evaluation of groundwater storage
monitoring with the GRACE satellite: Case study of the High Plains aquifer, central
United States, Water Resour Res, 45, doi:10.1029/2008WR006892.
Swenson, S., and J. Wahr (2006), Post-processing removal of correlated errors in GRACE data,
Geophysical Research Letters, 33(8). doi: 10.1029/2005GL025285.
Figure Captions
Figure S1 (a) Drought Severity and extent from the US drought monitor in NIDIS on Sep 27,
2011, and (b) Sep 25, 2012.
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Figure S2 Area percentage of the CONUS suffering from drought ranging from D0 to D4 for the
period 2000-2012 from the US drought monitor in NIDIS.
Figure S3 Reservoirs (119), major rivers, 15 major river basins, and 8 small coastal river basins
in Texas (Data from the Texas Water Development Board).
Figure S4 (a) Clay content (%) and (b) soil depths (m) from the State Soil Geographic (STASGO)
Data Base for Texas.
Figure S5 Times series of monthly precipitation from PRISM for Texas from Jan 2003-Sep 2012.
Precipitation for Texas generally peaks in late spring (e.g., May), early summer (e.g., Jun), and
fall (e.g., Sep and Oct), and is lowest in winter (e.g., Dec, Jan, and Feb). In particular, Aug
receives the least rainfall in months of summer and early fall. The pronounced rainy seasons in
late spring, early summer, and fall are jointly impacted by polar fronts interacting with the moist
Gulf of Mexico, with the fall rainy season additionally impacted by hurricanes and tropic
depressions [TWDB, 2012], e.g., Hurricane Humberto in 2007 and Hurricane Ike in 2008.
Figure S6 Monthly reservoir storage anomaly of Texas from Jan 1978-Sep 2012 provided by the
Texas Water Development Board (TWDB)
Figure S7 CSR RL05 TWS with or without bias and leakage corrections. Grey background
represents periods with the largest TWS depletion (May-Aug 2006, Jun-Aug 2009, and Mar-Sep
2012) during droughts.
Figure S8 Time series of TWS from CSR RL05 and RL04 for Texas from Jan 2003-Sep 2012,
with uncertainties in TWS in shaded areas (red for RL04 and grey for RL05). Uncertainties in
GRACE TWS comprise: (1) uncertainties in inherent GRACE spherical harmonics solutions,
which are quantified by looking at TWS over oceans 1,000 km away from continents at the same
latitude of a study region of interest (Texas in this study). Over the oceans, variation in TWS is
assumed to be zeros; and (2) bias and leakage corrections for TWS using land surface models.
Uncertainties in TWS from LSMs are quantified by the standard deviation of TWS of all LSMs
in GLDAS-1.
Figure S9 Variability in SMS among six LSMs being tested from Jan 2003–Sep 2012 over Texas.
Note large increases in variability during wet conditions in Winter 2004/2005 and drought in
2011 exceed 2 times standard deviation.
Figure S10 Comparison of monthly forcing (a) precipitation, (b) near surface air temperature, (c)
downward longwave radiation, and (d) downward shortwave radiation between NLDAS-2 and
GLDAS-1 for Texas during the period 2003-2012. Note that all models in GLDAS-1 use the
same forcing, and this also applies to NLDAS-2.
Figure S11 Times series of monthly ETNoah and ETMosaic in NLDAS-2 for Texas from Jan 2003Sep 2012.
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Figure S12 Groundwater use (mm, converted by using the original data in acre-feet at a county
level divided by the area of county) in 2010 in Texas from the Texas Water Development Board,
with showing major aquifers in hatched areas, e.g., the High Plains (Ogallala), Gulf Coast,
Carrizo, Trinity, and Edwards-Trinity Aquifers.
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