Groundwater is being depleted in California’s Central Valley at a rapid rate, according to a team of researchers analyzing data from the Gravity Recovery and Climate Experiment (GRACE) satellite in a paper published in the journal Geophysical Research Letters. Famiglietti et al. warn that the current rate of groundwater depletion in the Central Valley may be unsustainable and could have “potentially dire consequences for the economic and food security of the United States.”
The authors used 78 months (October, 2003–March, 2010) of data from the Gravity Recovery and Climate Experiment satellite mission to estimate water storage changes in California’s Sacramento and San Joaquin River Basins. They found that the basins are losing water at a rate of 31.0 ± 2.7 mm yr-1 equivalent water height, equal to a volume of 30.9 km3 for the study period, or nearly the capacity of Lake Mead, the largest reservoir in the United States.
They used additional observations and hydrological model information to determine that the majority of these losses are due to groundwater depletion in the Central Valley. Their results show that the Central Valley lost 20.4 ± 3.9 mm yr-1 of groundwater during the 78-month period, or 20.3 km3 in volume.
Facing significant
cuts in managed surface water allocations during periods of
drought, farmers, in particular those in the drier San Joaquin
Valley, are forced to tap heavily into groundwater reserves
to attempt to meet their irrigation water demands – this in a
region where groundwater dependence is already high. Under
these conditions, groundwater use rates exceed replenishment
rates, and groundwater storage and the water table drop.
Given the naturally low rates of groundwater recharge in the
San Joaquin Valley, combined with projections of decreasing
snowpack [Cayan et al., 2006] and population growth, continued
groundwater depletion at the rates estimated in this
study may become the norm in the decades to come, and may
well be unsustainable on those time scales.—Famiglietti et al.
Resources
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Famiglietti, J. S., M. Lo, S. L. Ho, J. Bethune, K. J. Anderson, T. H. Syed, S. C. Swenson, C. R. de Linage, and M. Rodell (2011), Satellites measure recent rates of groundwater depletion in California’s Central Valley, Geophys. Res. Lett., 38, L03403, doi: 10.1029/2010GL046442