483
Validation for Terrestrial Water Storage Variations
and Famiglietti 2001). The twin satellites were launched on March 17, 2002, as a
collaborative mission of the U.S. (National Aeronautics and Space Administration)
and German (German Aerospace Center) space agencies. The GRACE mission
deployed two identical satellites apart with approximately 220 km in identical orbits
at approximately 500 km high. The satellites are continuously tracking each other
using the K-Band Microwave Ranging System. Their separation and the rate, which
vary as they pass over a not uniformly distributed gravity field, are measured with a
precision of 10 μm. Other onboard instruments include accelerometers to distinguish
velocity changes due to factors other than the gravitational perturbation of the earth
(e.g., atmospheric drag and solar pressure). For more information on GRACE, see
the comprehensive reviews by Ramillien et al. (2008) and Schmidt et al. (2008).
Monthly, seasonal, and interannual variations in gravity on land are largely due to
corresponding changes in vertically integrated TWS (Tapley et al. 2004; Wahr et al.
2004), and atmospheric pressure change. The latter has been removed from GRACE
time-variable gravity solutions during the dealiasing process of GRACE data processing (Bettadpur 2007). By exploiting the unique relationship between changes in
gravity field and changes in mass at the earth’s surface, the month-to-month gravity
variations obtained from GRACE can be inverted into global estimates of vertically
integrated TWS with a spatial resolution of a few hundred kilometers and larger,
with higher accuracy at larger spatial scales (Swenson et al. 2003; Wahr et al. 2004).
This has allowed, for the first time, observations of variations in the total TWS (i.e.,
the sum of snow, vegetation water, surface water, soil moisture, and groundwater)
from large river basins (Swenson et al. 2003; Chen et al. 2005; Seo et al. 2006)
to continental scales (Wahr et al. 2004; Ramillien et al. 2005). This movement is
deemed as new approaches to remotely estimate river discharge (Syed et al. 2005,
2009), evapotranspiration (Rodell et al. 2004), and storage variations in groundwater (Yeh et al. 2006; Rodell et al. 2007; Strassberg et al. 2007; Rodell et al. 2009;
Famiglietti et al. 2011), snow water storage (Frappart et al. 2005), and river storage
(Frappart et al. 2008; Kim et al. 2009). More advances were geared toward monitoring extreme hydrologic events (Andersen et al. 2005; Chen et al. 2009), and validating and improving water balance in global land surface hydrologic models (Niu and
Yang 2006; Swenson and Milly 2006; Lo et al. 2010). However, while most of the
studies above acknowledged that GRACE is monitoring the total TWS, a critical
evaluation of the accuracy of GRACE water storage data and the potential for separating GRACE signals into individual TWS components has yet to be conducted.
The objective of this chapter is to explore the potential for GRACE to observe
TWS variations and to develop a framework for its validation. The validation is
based on the comparisons between GRACE TWS estimates and other estimates
derived using the other independent approaches, including in situ direct observations, land surface modeling, and combined land–atmosphere water balance computations. In this chapter, focus has been laced on the comparison of GRACE TWS
estimates in Illinois with long-term observed water storage data provided by the
Illinois State Water Survey (ISWS), including soil moisture, groundwater depth, and
snow water equivalent. It is followed by a comparison of GRACE TWS estimates
with the model simulation of TWS from a LSM, the Minimal Advanced Treatment
of Surface Interaction Runoff (MATSIRO; see Takata et al. 2003) over the 20 large
Validation for Terrestrial Water Storage Variations
and Famiglietti 2001). The twin satellites were launched on March 17, 2002, as a
collaborative mission of the U.S. (National Aeronautics and Space Administration)
and German (German Aerospace Center) space agencies. The GRACE mission
deployed two identical satellites apart with approximately 220 km in identical orbits
at approximately 500 km high. The satellites are continuously tracking each other
using the K-Band Microwave Ranging System. Their separation and the rate, which
vary as they pass over a not uniformly distributed gravity field, are measured with a
precision of 10 μm. Other onboard instruments include accelerometers to distinguish
velocity changes due to factors other than the gravitational perturbation of the earth
(e.g., atmospheric drag and solar pressure). For more information on GRACE, see
the comprehensive reviews by Ramillien et al. (2008) and Schmidt et al. (2008).
Monthly, seasonal, and interannual variations in gravity on land are largely due to
corresponding changes in vertically integrated TWS (Tapley et al. 2004; Wahr et al.
2004), and atmospheric pressure change. The latter has been removed from GRACE
time-variable gravity solutions during the dealiasing process of GRACE data processing (Bettadpur 2007). By exploiting the unique relationship between changes in
gravity field and changes in mass at the earth’s surface, the month-to-month gravity
variations obtained from GRACE can be inverted into global estimates of vertically
integrated TWS with a spatial resolution of a few hundred kilometers and larger,
with higher accuracy at larger spatial scales (Swenson et al. 2003; Wahr et al. 2004).
This has allowed, for the first time, observations of variations in the total TWS (i.e.,
the sum of snow, vegetation water, surface water, soil moisture, and groundwater)
from large river basins (Swenson et al. 2003; Chen et al. 2005; Seo et al. 2006)
to continental scales (Wahr et al. 2004; Ramillien et al. 2005). This movement is
deemed as new approaches to remotely estimate river discharge (Syed et al. 2005,
2009), evapotranspiration (Rodell et al. 2004), and storage variations in groundwater (Yeh et al. 2006; Rodell et al. 2007; Strassberg et al. 2007; Rodell et al. 2009;
Famiglietti et al. 2011), snow water storage (Frappart et al. 2005), and river storage
(Frappart et al. 2008; Kim et al. 2009). More advances were geared toward monitoring extreme hydrologic events (Andersen et al. 2005; Chen et al. 2009), and validating and improving water balance in global land surface hydrologic models (Niu and
Yang 2006; Swenson and Milly 2006; Lo et al. 2010). However, while most of the
studies above acknowledged that GRACE is monitoring the total TWS, a critical
evaluation of the accuracy of GRACE water storage data and the potential for separating GRACE signals into individual TWS components has yet to be conducted.
The objective of this chapter is to explore the potential for GRACE to observe
TWS variations and to develop a framework for its validation. The validation is
based on the comparisons between GRACE TWS estimates and other estimates
derived using the other independent approaches, including in situ direct observations, land surface modeling, and combined land–atmosphere water balance computations. In this chapter, focus has been laced on the comparison of GRACE TWS
estimates in Illinois with long-term observed water storage data provided by the
Illinois State Water Survey (ISWS), including soil moisture, groundwater depth, and
snow water equivalent. It is followed by a comparison of GRACE TWS estimates
with the model simulation of TWS from a LSM, the Minimal Advanced Treatment
of Surface Interaction Runoff (MATSIRO; see Takata et al. 2003) over the 20 large