The long-running drought currently affecting California shows no signs of easing up. Declaring a state of emergency, as Governor Jerry Brown did in January 2014, would, to most people, have been the most extreme this situation could have reached. However, that was later followed by talk of a ‘megadrought’, predicting conditions ‘far worse than anything those regions have seen in the past several thousand years at least’.

Meanwhile, water resources dwindle, agriculture continues to suffer, and a new NASA study has concluded that the state has now missed a total of a whole year’s worth of rainfall within just the past three years.

The findings show that Californians can expect, within normal conditions, to experience as much as 30 per cent higher or lower annual rainfall, compared to the historical average, which is about 20 inches (53 cm) per year. These alternating conditions should balance out the state’s overall rainfall.

Between 2012 and 2014, however, California accumulated a deficit of almost 13 inches (33 cm). When added to the record-breaking drought in 2014, which saw another seven inches missing from the rainfall pattern, it makes the total deficit from 2012 to 2015 a full 20 inches, which Californians would normally expect to have falling on them within a single year.

California’s precipitation deficit from 2012 to 2014, as a per cent change from the 17-year average (Image: NASA/Goddard Scientific Visualization Studio)

The record dry conditions are a result of interference with ‘atmospheric rivers’, powerful flows of water vapour, which normally carry moisture in the atmosphere from the Pacific Ocean over the North American landmass. ‘When they say that an atmospheric river makes landfall, it’s almost like a hurricane, without the winds,’ says Andrey Savtchenko, lead author of the NASA study. ‘They cause extreme precipitation.’

Since 2011, however, high pressure systems over the Pacific have prevented these rivers from forming, and without the rivers, a huge source of Californian water is lost. That in itself isn’t an unusual situation, however the length of the drought, coupled with significant population growth in California, makes the impact more severe.

One possible outlet for the drought may come in the form of an El Niño event, one of which is currently building over the Pacific. ‘If this El Niño holds through winter, California’s chances to recoup some of the precipitation increase,’ says Savtchenko. ‘Most likely the effects would be felt in late 2015 to 2016.’ However, the evidence analysed in the study suggests that El Niño events have driven ‘at best only six per cent of precipitation variability in California in the past three decades’.

If that wasn’t a clear enough explanation of the dire situation facing California, a separate study from Indiana University observes that the 2012 to 2014 drought can be classified as ‘nearly a 10,000-year event’, while the ongoing lack of rainfall over the past year means that the overall 2012 to 2015 drought ‘has an almost incalculable return period and is completely without precedent’.