A lack of water caused California’s Sierra Nevada mountain range to rise nearly an inch (24 millimeters) in height during the drought years from October 2011 to October 2015, a NASA study has revealed.
It shows that Earth is far better than thought at storing water than researchers had thought.
During 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, the team say.
Researchers had previously believed groundwater pumping has contributed to the changes – but now say the Earth is far better at storing water than they thought
California’s forests are seeing a continued die-off of trees even a year after last year’s heavy rains ended the state’s crippling drought, the U.S. Forest Service has warned.
It says 27 million trees died over the past 13 months after five dry years left them severely dehydrated and vulnerable to bark beetle attack.
The unprecedented casualties, which run rampant across the Sierra Nevada as well as parts of the coast, have turned patches of forest into a somber rust color for mile after mile. The mortality is so great in some places that foresters have closed roads and campgrounds for fear of the dried-out, lifeless trees falling on people.
Tourists to Yosemite and Sequoia national parks, meanwhile, have been stunned by the unexpectedly grim views.
‘This suggests that the solid Earth has a greater capacity to store water than previously thought,’ said Donald Argus of NASA, 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.
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.’
HOW THEY DID IT
The researchers 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).
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?
The granite cliffs of the Yosemite Valley in California, Sierra Nevada, USA in winter. 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.
‘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.