The Luxembourg National Museum of Natural History. 2) Department of Physics and CIRES, University of Colorado. 3) U.S. Geological Survey and GFDL/NOAA, ...
測 地 学 会 誌,第47巻,第1号
Journal of the Geodetic Society of Japan Vol. 47, No. 1, (2001), pp. 249-254
(2001),249-254頁
Gravity
Changes
Due
to Continental
Water
Storage
Tonie M. van Dam') , John M. Wahr2), P. Chris D. Mi11y3)and Olivier Francis4) 1) European Center for Geodynamics and Seismology and The Luxembourg National Museum of Natural History 2) Department of Physics and CIRES, University of Colorado 3) U.S. Geological Survey and GFDL/NOAA, Princeton, New Jersey, U.S.A. 4) Institut Superieur de Technologie and European Center for Geodynamics and Seismology (Received September 30, 2000; Revised January 4, 2001; Accepted January 4, 2001) Abstract Five years of global continental water storage variations are used to predict the effects of long-wavelength, long-period variability in water storage on gravity obser vations. At the sites of existing superconducting gravimeters, the modeled gravity changes have root-mean-square (RMS) values of as much as 7 Gals, with ranges of up to 22 uGals. Variations much larger than these values can be found over large regions the globe. We find that the gravity effects are predominantly annual in char acter. We also find that the modeled responses to water loading exhibit long-period variations that could be mistaken for secular tectonic trends when observed over a time span of a few years.
1.
Introduction
A continuous gravity time series from an individual station often contains a significant signal associated with surface mass changes due to 1) the Newtonian attraction of the excess mass and 2) the response of the earth to this surface mass loading. The loads likely to be important are atmospheric pressure, tidal and non-tidal ocean mass fluctuations, and redistribution of water, snow and ice on the continents (van Dam and Wahr, 1998). Of these, the effects due to terrestrial water-storage variations are the least well understood. In this paper, we look at the effects of long-period (seasonal) and long-wavelength(> 100 km) water-storage changes on gravity time series. This regional effect is to be distinguished from the local water-storage variability caused by an individual precipitation event which may be very localizedin space and of very high frequency. The relationship between changes in the gravitational acceleration and local precipitation, local well level variations and near by soil moisture have been addressed in numerous investigations (Peter et al., 1995;Delcourt Honorez,1995; Richter, 1995; Crossley et al., 1998). These indicators of local water storage changes are superimposed on the long wavelength variations that we describe here. We use a model of global water storage variations together with a model of how the earth deforms in response to a surface load to predict gravity changes due to changes in continental water storage. We specifically look at the predicted water-storage induced gravity changes for locations where there are superconducting gravimeters.
250
Tonic M. van Dam, John M. Wahr, P. Chris D. Milly and Olivier Francis
2.
Calculations Storage
of
of
land
water
water
as
energy
recharge
a single a
excess
recharges
to
groundwater
Evaporation
evaporation
for types
sheets and
soil
Climatology work
texture.
(CMAP)
Milly,
errors
water-storage
formulation. temporal
Forcing
and
soil
so
the
and
estimates. we
regional
spatial
storage
determined
by
These
expect structure
determined
mass are
that
Prediction
1997).
The in
arise the of
the
water assumed
model
was
errors
in
will
tend
(time
forcing, to
realistic
maps
of
biome
Land-Surface Climatology
Net-
Analysis
of
1979-1998. and
space)
parameters, reproduce
reasonably
availignored.
physically
Merged
for
a rate negligi
water
Satellite
run
at
are
global
(CPC)
point
Elsewhere, and
not
Historical
wavelength
from
water-storage
availability. energy
is
rainfall
which
are
are
Global
model the
at
surface
Center
short
and
to
International
the
estimated
changes
from
model
Snowpack
capacity),
by
changes
the
by
impoundments
estimated
from
forced
Snowmelt
energy
surface-water
modeled
a global
conditions.
discharges
and
present
is
field
water
I dataset,
Arkin,
probably
reaches
a rate
Climate
model
accounting.
(i.e.,
parameters
Initiative
(Xie
However,
at
is synthesized
and
are
a rate
using
atmospheric
surface
mining
Model
records,
Considerable
is full
calculated
The
Groundwater
the
modeled,
(ISLSCP)
precipitation
derived
not
1•‹ grid.
near-surface
and at
from
was
energy-balance it
snowpack
glaciers.
Project
Precipitation
until
storage,
water
is and
1•‹ x
and
groundwater
dynamics
ice
a
groundwater.
Anthropogenic
Ice-flow
groundwater
and
store
depletes removes
ability.
on
mass-
soil-water
Storage
and
radiation, using
water
proportional
soil,
balance,
store
single
Water
snowpack,
downwelling
tracked
ble.
in
and
precipitation,
any
of Continental
and the
well
long (Shmakin
modelmodel period and
1999).
Summation of model storage outputs (snow, soil water and groundwater) provides our estimate of terrestrial water storage loading on a monthly time scale. Here we focus on results for 1994-1998. Because of the noted limitation of the model in areas of permanent ice cover, calculated storage is ignored in Greenland, Antarctica and other glaciated grid cells. At most locations, we find the load is dominated by an annually-varying signal. Over tropical areas in Africa and South America, and in the region of the Asian monsoon, the range (maximum-minimum) of water-storage load can be between 500 and 1000 mm. Vari ations between 1000 and 2000 mm are observed along the western coast of North America, extending from Alaska and into northern California. The RMS value of the load tends to be about one-fourth of the full load range, consistent with the load being dominantly sinusoidal (i.e. seasonal) but having significant interannual variability in amplitude. Estimates of the gravity changes are calculated by convolving Farrell's (1972) Green's functions with the surface load over the entire globe. In Figure 1, we show the full modeled range of gravity changes for a theoretical observation point located at every 2.5 degrees of latitude and 2.5 degrees of longitude. We calculate the change in gravity due to the elastic deformation of the solid earth and that due to the Newtonian attraction of the redistributed hydrological mass. The contribution from the excess mass directly below the station is crudely approximated by multiplying the local water-storage in centimeters by the factor -0.42 ,aGal/cm, determined by assuming the excess mass is an infinite plane of uniform density. The effect of the direct attraction of the mass on the gravity is typically
Gravity
Fig. 1. Range in the gravitational water storage from 1994-1998.
Changes
Due to Continental
acceleration
over the surface
Water
Storage
of the globe
251
due to changes
in continental
larger than the deformation induced acceleration effects by a factor of 3. In the Asian monsoon region, in tropical South America and Africa, and along the western coast of Canada and the southern coast of Alaska, the gravity changes can be as large as 40 ttGals. Over most of the continental areas, however, the expected ranges in gravity are between 10 and 20 pGals. In general, the map of the RMS value of gravity changes (not shown) resembles that of the gravity range, but reduced in magnitude by a factor of about 4. The time series from some selected locations (Bandung, Indonesia; Brasimone, Italy; Cantley, Canada; Wuhan, China; Kyoto, Japan; and Vienna, Austria) are shown in Figure 2. The predicted water-storage gravity signal is strongly annual in character with an annual amplitude that can vary significantly from year to year. At each station in the Figure, the gravity change has a peak-to-peak range of at least 5 to 10 iGals. However, in Bandung the gravity change can have a range up to 20 ,Gals. Maximum gravity variations and the RMS of predicted gravity changes are given in Table 1 for many of the continuously operating superconducting gravity sites around the globe. The maximum range values are between about 2 and 22 uGals and give an estimate of the signal that would be introduced into a gravity time series if regional water-storage
252
Tonic
M. van Dam,
John
M. Wahr,
P. Chris D. Milly and Olivier
Francis
Fig. 2. Modeled water-storage. induced gravity effects at the locations of some superconducting ass attraction and deformation effects are both included in these results. effects
were not accounted
epoch
measurements
average of scatter between
3.
out between that
could
for.
where
This
the
effect would
water-storage
observations.
The
be introduced
into a continuous
2 and 7 µGals
for the locations
RMS
be particularly
induced
listed
values
problematic
accelerations
would indication
gravimeters . M
in the
case of
not necessarily
provide
some
gravity
time series . The scatter
of the amount ranges
in the Table.
Secular Trends
Continuous gravity observations from a permanent superconducting gravimeter, would average out most of the water-storage-induced gravity variability after a few years. But because the loading contains power at very long periods, some residual linear trend will be present in the gravity observations, and that trend would be indistinguishable from a secular tectonic trend or an instrumental drift. In Table 1 we report the trend in gravity that would be obtained using the modeled gravity effects for the 3 year period, 1996-1998. Although the trend is small at most sites, less than 0.5 .uGals/yr, the trend is almost 2 µGals/yr at the Japanese stations and is on the order of 1,uGal/yr at many of the European sites. The apparent trend caused by water-storage-induced loading decreases as the length of the observing period increases. The last column in Table 1 shows the trend calculated using 5 years of the predicted water-storage-induced effects 1994-1998.After 5 years of observations, the trend at all sites is less than or equal to about 1 uGals/yr.
Gravity
Table
1. Statistics
gravimeter
4.
and trends
Changes
from modeled
Due to Continental
water-storage
Water
induced
Storage
gravity
changes
253
at some superconducting
sites.
Summary
In this paper we have used modeled monthly, global variations of long-period and longwavelength continental water storage to estimate the associated changes in gravity. We conclude that continental water loading may cause gravity changes of up to 20 iGals at the locations of superconducting gravimeters but can be significantlylarger over vast regions of the globe, with changes of up to 40 ,aGals predicted for some regions. The RMS variability of the gravity changes can be as large as 7 ,aGals at the specific locations investigated but can be as high as 11 uGals over other parts of the globe. These predicted gravity changes are far larger than the measurement repeatability that is routinely achieved with superconducting gravimeters and absolute gravimeters. Gravity residuals, however, routinely exhibit non-secular variability that is larger than the repeata bility, and which tends, to be of the same order of magnitude as the water-storage effects investigated in this paper. Water-mass loading can include long-term variability, which appears as an apparent secular signal over observing periods of a few years. Although these trends tend to be small at most superconducting gravimeters sites (usually less than 0.5 ,uGals/yr) over a three year period), the three year trend can be as large as 1.5 µGals/yr at some continuous gravity sites.
A comparison of these modeled results with actual superconducting gravimeter obser vations is necessary to verify the amplitude of these predicted effects and to validate the long-wavelength water storage model. Once the validation is performed, models like these can be used to adjust the gravimeter observations by the water-storage models.
254
Tonie M. van Dam,
John
M. Wahr,
P. Chris D. Milly
and Olivier
Francis
References
Crossley, D. J., S. Xu and T. van Dam (1998): Comprehensive Analysis of 2 Years of SG Data from Table Mountain, Colorado, Proceedings of the 13th International Symposium on Earth Tides, Jul. 22-25, 1997, Brussels, Belgium, 654-668. Delcourt-Honorez, M. (1995): Hydrological Effects on Local Gravity, Cahiers du Centre Europden de Gdodynamique et Seismologie, 11, "Non Tidal Gravity Changes Intercomparison between Absolute and Superconducting Gravimeters, Sep. 6-8, 1994, Walferdange, Luxembourg, 161-167. Farrell, W. E. (1972): Deformation of the Earth by Surface Loads, Rev, of Geophys.,10, 761-797. Peter, G., F. J. Klopping and K. A. Berstis (1995): Observing and Modeling Gravity Changes Caused by Soil Moisture and Groundwater Table Variations with Superconducting Gravimeters in Richmond, Florida, U.S.A., Cahiers du Centre Europden de Gdodynamique et Seismologie, 11, "Non Tidal Gravity Changes Intercomparison between Absolute and Superconducting Gravimeters, Sep. 6-8, 1994, Walferdange Luxembourg, 147-159. Richter, B. (1995): Cryogenic Gravimeters: Status Report on Calibration, Data Acquisition and Envi ronmental Effects, Cahiers du Centre Europden de Gdodynamique et Seismologie, 11, "Non Tidal Gravity Changes Intercomparison between Absolute and Superconducting Gravimeters, Sep. 6-8, 1994, Walferdange Luxembourg, 125-146 Shmakin, A. B. and P. C. D. Milly (1999): Evaluation of interannual variations in runoff from large river basins (abstract), IUGG XXII General Assembly, Abstracts, 36. van Dam, T. and J. M. Wahr (1998): Modelling Environmental Loading Effects: A Review, Geophys. J. Int., 100, 99-106. Xie, P. and P. A. Arkin (1997): Global Precipitation: a 17-year Monthly Analysis Based on Gauge Obser vations, Satellite Estimates and Numerical Model Outputs, Bull. Amer. Meteor. Soc., 78, 2539-2558.
