The inversion produces an estimate of the load
that resembles the uplift pattern but is smoother
because of the constraints imposed. In March
2014, when most vertical displacements are
farthest from their long-term averages, our results show crustal unloading over the entire
WUSA, with a maximum in the central Sierra
Nevada equivalent to 50 cm of water (Fig. 3).
There appears to be a small amount of real non-tectonic loading in Montana, whereas the apparent loading just south of the U.S.-Mexico border
is probably caused by postseismic effects from
the moment magnitude 7.2 El Mayor–Cucapah
earthquake in 2010 (26). The arid regions of
eastern California, Oregon, Washington, and western Nevada show little loading. Estimated loads
near the northern and southern boundaries of the
grid, as well as in Arizona, Utah, and Montana, are
poorly constrained by the GPS data.
We interpret the widespread negative loading,
with its central California maximum, to represent
changes in terrestrial water storage due to the
current WUSA drought. The implied drying relative to the long-term mean appears to be most
acute in coastal and mountainous areas and
subdued in highly arid regions. This is expected,
because the change in precipitation in a drought
is proportional to the climatological mean value,
so that arid regions lose less water than do wet
regions. The area-integrated water deficit over
the WUSA in March 2014 is 240 gigatons, a
value that is insensitive to the degree of smoothing
used in the inversion (fig. S6). For perspective,
this deficit is equivalent to a uniform 10-cm
layer of water over the entire WUSA and is the
magnitude of the current annual mass loss from
the Greenland Ice Sheet.
The temporal and spatial water storage variations implied by the observed displacements are
consistent with contemporaneous observations
of precipitation and streamflow, all of which underscore the extent and severity of the current
drought in the WUSA. The departure of annual
precipitation from the long-term average (Fig. 4)
highlights the changes over the past few years: a
wet 2011; a variable 2012; a dry 2013 for the
western Rockies, the Great Basin, and parts of
California; and severe drought in 2014 along the
Pacific coast, with dry conditions extending inland to the Rocky Mountains. Precipitation patterns in 2011 and 2014, in particular, match the
pattern of vertical displacements for, respectively,
excess (wet) and deficit (dry) loading conditions.
Streamflow data from the U.S. Geological Survey
(USGS) stream gauge network (fig. S9) exhibit
wet/dry patterns similar to the precipitation data,
although it is the years 2013 and 2014 that most
closely match the vertical displacements. These
data sets are consistent with each other and also
complementary, each highlighting a different
component of the hydrological system.
It has been suggested that long-term and seasonal variations in mass loading due to the hydrological cycle may affect seismicity rates along the
San Andreas fault (27). We computed the Coulomb
stress change on the San Andreas fault from the
load shown in Fig. 3 (12, 28) and found that the
past 2 years of unloading have increased Coulomb
stresses by 100 to 200 Pa, approximately the same
amount as a week of tectonic strain accumulation
(29). Therefore, stress changes from the drought
unloading seem unlikely to affect seismicity.
Other methods that can directly monitor
changes in total water mass change are sensitive
to different spatial scales. Gravimeters allow very
sensitive detection of mass changes, but because
their sensitivity falls off as r–2 (where r is the distance to the mass), they are best at measuring very
local changes (30). Conversely, perturbations to
satellite orbits can be used to detect changing
mass distributions over the whole Earth but are
insensitive to load variations with wavelengths
much less than the orbital height. The nominal
resolution for the GRACE satellite is 400 to 500 km
(31), which is consistent with results from GRACE
studies of drought-induced mass changes (1, 2).
For vertical displacements from GPS, the loading
Green function varies as r–1: Load-induced sig-
nals reflect both local and regional changes. Com-
bining these different measurement types offers
the greatest promise for monitoring terrestrial
In the WUSA, interannual changes in crustal
loading are driven by changes in cool-season precipitation, which cause variations in surface water, snowpack, soil moisture, and groundwater.
We have demonstrated that GPS can be used to
recover loading changes due to both wet (e.g.,
2011) and dry (e.g., post-2013) climate patterns,
which suggests a new role for GPS networks such
as that of the PBO. Although precipitation and
surface water levels are currently well sampled,
other components of terrestrial water storage are
monitored only at a limited number of locations,
including a growing number of GPS stations for
which GPS reflectometry measurements of snow
depth and soil moisture are available (32–34).
Our analysis shows that the existing network of
continuous GPS stations in the WUSA measures
vertical crustal motion at sufficient precision and
sampling density to allow the estimation of interannual changes in water loads, providing a new
view of the ongoing drought in much of the WUSA.
Furthermore, the exceptional stability of the GPS
monumentation (35) means that this network is
also capable of monitoring the long-term effects of
regional climate change. Surface displacement observations from GPS, in the WUSA and globally,
have the potential to dramatically expand the
capabilities of the current hydrological observing
network, and continued operation of these instruments will provide considerable value in
understanding current and future hydrological
changes, with obvious social and economic benefits.
REFERENCES AND NOTES
1. F. Frappart et al., Environ. Res. Lett. 7, 044010 (2012).
2. A. C. Thomas, J. T. Reager, J. S. Famiglietti, M. Rodell,
Geophys. Res. Lett. 41, 1537–
3. W. E. Farrell, Rev. Geophys. 10, 761–797 (1972).
4. K. J. Ouellette, C. de Linage, J. S. Famiglietti, Water Resour. Res.
49, 2508–2518 (2013).
5. P. Elósegui, J. L. Davis, J. X. Mitrovica, R. A. Bennett,
B. P. Wernicke, Geophys. Res. Lett. 30, 1111 (2003).
sciencemag.org 26 SEPTEMBER 2014 • VOL 345 ISSUE 6204 1589
Fig. 4. Maps of annual precipitation anomalies. Deviation of annual precipitation from the 2003-to-2013 mean at meteorological stations in the National
Oceanic and Atmospheric Administration’s Global Historical Climatology Network, for 2011 to 2014. The pattern of precipitation—in particular, the surplus in
California in 2011 and the deficit in 2014—mirrors the pattern of uplift seen in the GPS data.