491
Validation for Terrestrial Water Storage Variations
Soil moisture data were collected by the ISWS from 1981 to the present at 19 stations using the neutron probes technology. Weekly to biweekly measurements of soil
wetness were taken at 11 soil layers with a resolution of about 20 cm down to 2 m
below the surface, and no data were collected below 2 m. Sixteen of these 19 sites
covering the period of GRACE data (2003–2008) were used in this study. The data
on soil porosity, field capacity, and permanent wilting point were also provided in
this dataset, which enables the estimation of the water-holding capacity of soil layers
(Hollinger and Isard 1994; Yeh et al. 1998). The data on groundwater depth consists
of 19 wells scattered relative uniformly over Illinois, which are used to monitor the
unconfined silt loam aquifers. These aquifers are relatively shallow, and the average
depth to the water table ranges between 1 and 10 m below the surface. Ten out of 19
wells with the complete records from 2003 to 2008 were used in this study. For more
details on the Illinois hydroclimatologic datasets, see the work of Yeh et al. (1998)
and Yeh and Famiglietti (2008, 2009).
Twenty-five-year (1985–2009) monthly anomalies of observed Illinois state-average
soil moisture and groundwater storage are plotted in Figure 20.4. For deriving water
storage from soil moisture and groundwater depth measurements, the porosity was
provided by ISWS for each of the 19 soil moisture monitoring stations, and the
specific yield (S y ) was determined as 0.08 for the dominant soil type of silt loam
in Illinois following the estimate by Yeh et al. (1998). The signatures of hydrologic
extremes such as the late spring drought in 1988 and the summer flood in 1993 are
clearly shown in Figure 20.4. Illinois has experienced another severe drought in
100
50
–50
Anomaly (mm)
Anomaly (mm)
–100
–150
1985
1990
1995
(a)
(b)
2000
2005
2010
1985
1990
1995
2000
2005
2010
0
150
100
50
–50
–100
–150
0
FIGURE 20.4 Twenty-five-year (1985–2009) average monthly anomalies of observed (a)
soil moisture and (b) groundwater storages in Illinois.
Validation for Terrestrial Water Storage Variations
Soil moisture data were collected by the ISWS from 1981 to the present at 19 stations using the neutron probes technology. Weekly to biweekly measurements of soil
wetness were taken at 11 soil layers with a resolution of about 20 cm down to 2 m
below the surface, and no data were collected below 2 m. Sixteen of these 19 sites
covering the period of GRACE data (2003–2008) were used in this study. The data
on soil porosity, field capacity, and permanent wilting point were also provided in
this dataset, which enables the estimation of the water-holding capacity of soil layers
(Hollinger and Isard 1994; Yeh et al. 1998). The data on groundwater depth consists
of 19 wells scattered relative uniformly over Illinois, which are used to monitor the
unconfined silt loam aquifers. These aquifers are relatively shallow, and the average
depth to the water table ranges between 1 and 10 m below the surface. Ten out of 19
wells with the complete records from 2003 to 2008 were used in this study. For more
details on the Illinois hydroclimatologic datasets, see the work of Yeh et al. (1998)
and Yeh and Famiglietti (2008, 2009).
Twenty-five-year (1985–2009) monthly anomalies of observed Illinois state-average
soil moisture and groundwater storage are plotted in Figure 20.4. For deriving water
storage from soil moisture and groundwater depth measurements, the porosity was
provided by ISWS for each of the 19 soil moisture monitoring stations, and the
specific yield (S y ) was determined as 0.08 for the dominant soil type of silt loam
in Illinois following the estimate by Yeh et al. (1998). The signatures of hydrologic
extremes such as the late spring drought in 1988 and the summer flood in 1993 are
clearly shown in Figure 20.4. Illinois has experienced another severe drought in
100
50
–50
Anomaly (mm)
Anomaly (mm)
–100
–150
1985
1990
1995
(a)
(b)
2000
2005
2010
1985
1990
1995
2000
2005
2010
0
150
100
50
–50
–100
–150
0
FIGURE 20.4 Twenty-five-year (1985–2009) average monthly anomalies of observed (a)
soil moisture and (b) groundwater storages in Illinois.