Sierras Lost Water Weight, Grew Taller During Drought

Loss of water from
the rocks of California’s Sierra Nevada
caused the mountain range to rise nearly
an inch (24
millimeters) in height during the drought years from October
2011
to October 2015, a new NASA study finds. In the two
following years of more abundant
snow and rainfall, the
mountains have regained about half as much water in the
rock
as they had lost in the preceding drought and have fallen about
half an
inch (12 millimeters) in height.

“This
suggests that the solid Earth has a greater capacity to
store water than previously
thought,” said research scientist
Donald Argus of NASA’s Jet Propulsion
Laboratory in Pasadena,
California, who led the study. Significantly more water was
lost from cracks and soil within fractured mountain rock during
drought and gained during heavy precipitation than hydrology
models show.

Argus is giving a talk on the new finding today
at the
American Geophysical Union’s fall conference in New Orleans.

The research
team used advanced data-processing techniques on
data from 1,300 GPS stations
in the mountains of California,
Oregon and Washington, collected from 2006
through October
2017. These research-quality GPS receivers were installed as
part of the National Science Foundation’s Plate Boundary
Observatory to measure
subtle tectonic motion in the region’s
active faults and volcanoes. They can monitor
elevation
changes within less than a tenth of an inch (a few
millimeters).

The team found
that the amount of water lost from within
fractured mountain rock in 2011-2015
amounted to 10.8 cubic
miles of water. This water is too inaccessible to be used
for
human purposes, but for comparison, the amount is 45 times as
much water as
Los Angeles currently uses in a year.

JPL water
scientist Jay Famiglietti, who collaborated on the
research, said the finding solves
a mystery for hydrologists.
“One of the major
unknowns in mountain hydrology is what
happens below the soil. How much
snowmelt percolates through
fractured rock straight downward into the core of the mountain?
This is one of the key topics that we addressed in our study.”

Earth’s surface
falls locally when it is weighed down with
water and rebounds when the weight
disappears. Many other
factors also change the ground level, such as the
movement of
tectonic plates, volcanic activity, high- and low-pressure
weather
systems, and Earth’s slow rebound from the last ice
age. The team corrected for
these and other factors to
estimate how much of the height increase was solely
due to
water loss from rock.

Before this study,
scientists’ leading theories for the growth
of the Sierra were tectonic uplift
or Earth rebounding from
extensive groundwater pumping in the adjoining
California
Central Valley. Argus calculated that these two processes
together only
produced a quarter of an inch (7 millimeters) of
growth — less than a third of
the total.

Famiglietti said the techniques developed for
this study will
allow scientists to begin exploring other questions about
mountain groundwater. “What does the water table look like
within mountain
ranges? Is there a significant amount of
groundwater stored within
mountains? We just don’t have
answers yet, and this study identities a set
of new tools to
help us get them.”

A paper on the
research, titled “Sustained water loss in
California’s
mountain ranges during severe drought from 2012
through 2015 inferred from
GPS,” was published in the
Journal
of Geophysical Research: Solid Earth.

News Media Contact

Alan Buis
Jet Propulsion Laboratory, Pasadena, California
818-354-0474
Alan.Buis@jpl.nasa.gov

Written by Carol Rasmussen
NASA’s Earth Science News Team

2017-318