The idea is that, “The record-breaking California drought is so bad that monitoring stations used to study earthquakes can detect the drying ground rising up. Measurements of these subtle movements, made using GPS instruments, suggest that the western United States is missing some 62 trillion gallons of water, enough to cover the entire region six inches deep.”
We have a huge network (over 1000 measuring points) in the US and many of them are focussed on us – especially the San Andreas Fault. The activity is predictably unpredictable, of course. Every once in a while someone will see a slight pattern and then scientists rally round to figure it out. Dr. Adrian Borsa, from Scripps Institution of Oceanography, noticed one of these little patterns: ‘Most of the stations have been gradually rising in the last couple of years, just when the region was drying out.”
Apparently their instruments are so sensitive that, when water, which increases gravitational pressure on the crust, drains away, the decreased pressure allows the earth to shift upwards ever so slightly. This is a slightly different result than what happens when we drain aquifers – then the ground subsides, because the surface collapses to fill in the space left behind. I am not a scientist, so this is a super-simplified explanation.
Suffice to say that when Dr. Borsa and his buddies got together and made the necessary measurements, and most of these stations in southern California had begun to rise. By March of 2014, nearly all of the stations from here through Washington and Idaho registered on his instruments. They mapped the results with maps of deviation from normal precipitation and, BINGO!, found good correlations.
How big of a difference? On average 0.15 inches, with up to half an inch in the mountains.