influence of water stored in the unsaturated soil zone. For aerial surveys, corrections
must also be made for water stored in vegetation. For satellite gravity surveys, one
must correct the water effects of the atmosphere in addition to the vegetation.
At present, there is only one satellite mission that can directly estimate the
quantity of groundwater stored deep beneath the Earth’s surface. It accomplishes
this by measuring the Earth’s gravity field which is influenced by the quantity of
groundwater below the surface. As groundwater is consumed or recharged, waterinduced gravity anomalies change through time. The Gravity Recovery and Climate
Experiment (GRACE) mission, launched in 2002, consists of two identical satellites orbiting in unison at the same altitude but with a separation of 220 km. The two
satellites monitor the relative position of each other using microwave-ranging
instruments. As they pass over a gravity anomaly, the leading spacecraft
approaches and speeds up due to higher angular momentum. This causes the two
satellites to increase separation. After the first spacecraft passes the anomaly, it
slows down again; meanwhile the following satellite accelerates, then decelerates
over the same point. By recording satellite separation, gravity is mapped; cycles of
growing/shrinking separation indicate the size and strength of the gravity anomalies, from which the quantity of groundwater can be estimated [49].
Due to atmospheric variability and observational errors, accurate estimates of
water mass can only be obtained for regions that are several hundreds of kilometers
or more in scale [52]. Thus, estimating water storage changes in large aquifer
systems is currently feasible [53]. Rodell and Famiglietti [54] used computer
simulations and GRACE performance parameters to show that groundwater storage
changes as small as 9 mm could be measured in the US High Plains aquifer system.
As shown in Fig. 3, in the High Plains the magnitude of annual groundwater storage
changes averaged 19.8 mm between 1987 and 1998. They expected that the
20
0
–20
–40
–60
–80
–100
–120
1987–91 1988–92 1989–93 1990–94 1991–95 1992–96 1993–97 1994–98
Four Year Period
Change In Groundwater Storage,mm
Fig. 3 Four-year changes in the central USA. High Plains aquifer water storage with error bars
that represent the total uncertainty in GRACE-derived estimates. Reprinted from Rodell and
Famiglietti [54] with permission from Elsevier
42
V. Klemas and A. Pieterse
must also be made for water stored in vegetation. For satellite gravity surveys, one
must correct the water effects of the atmosphere in addition to the vegetation.
At present, there is only one satellite mission that can directly estimate the
quantity of groundwater stored deep beneath the Earth’s surface. It accomplishes
this by measuring the Earth’s gravity field which is influenced by the quantity of
groundwater below the surface. As groundwater is consumed or recharged, waterinduced gravity anomalies change through time. The Gravity Recovery and Climate
Experiment (GRACE) mission, launched in 2002, consists of two identical satellites orbiting in unison at the same altitude but with a separation of 220 km. The two
satellites monitor the relative position of each other using microwave-ranging
instruments. As they pass over a gravity anomaly, the leading spacecraft
approaches and speeds up due to higher angular momentum. This causes the two
satellites to increase separation. After the first spacecraft passes the anomaly, it
slows down again; meanwhile the following satellite accelerates, then decelerates
over the same point. By recording satellite separation, gravity is mapped; cycles of
growing/shrinking separation indicate the size and strength of the gravity anomalies, from which the quantity of groundwater can be estimated [49].
Due to atmospheric variability and observational errors, accurate estimates of
water mass can only be obtained for regions that are several hundreds of kilometers
or more in scale [52]. Thus, estimating water storage changes in large aquifer
systems is currently feasible [53]. Rodell and Famiglietti [54] used computer
simulations and GRACE performance parameters to show that groundwater storage
changes as small as 9 mm could be measured in the US High Plains aquifer system.
As shown in Fig. 3, in the High Plains the magnitude of annual groundwater storage
changes averaged 19.8 mm between 1987 and 1998. They expected that the
20
0
–20
–40
–60
–80
–100
–120
1987–91 1988–92 1989–93 1990–94 1991–95 1992–96 1993–97 1994–98
Four Year Period
Change In Groundwater Storage,mm
Fig. 3 Four-year changes in the central USA. High Plains aquifer water storage with error bars
that represent the total uncertainty in GRACE-derived estimates. Reprinted from Rodell and
Famiglietti [54] with permission from Elsevier
42
V. Klemas and A. Pieterse