From the half-million acres of parched and idle farmland to the swaths of receding reservoirs, the signs of California's historic drought are obvious. Less obvious is just how historic the ongoing drought actually is when compared with previous dry spells in the Golden State's history. A CNBC analysis of annual Palmer Drought Severity Index recordings, founded by meteorologist Wayne Palmer as a way to factor in both precipitation and temperature to track soil moisture, shows the nearly four-year drought is in fact the worst in over a century. The PDSI is used by the National Oceanic and Atmospheric Administration as its main drought index. (Tweet This)



A PDSI recording of 0 reflects normal conditions, while -1 is considered to be the official drought threshold. Any recording of -4 and below signifies extreme drought.

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Aiguo Dai, atmospheric and environmental sciences professor at the University of Albany, said PDSI paints a more complete picture than rainfall alone and does a better job of showing how bad things have actually gotten. "California has been gradually warming over the last 30 years," he said. "It's an important part of the equation, because a warmer temperature means water evaporates more quickly, worsening the dryness from the precipitation deficit." The current drought has averaged a reading of -3.67 over the last three years, nearly twice as bad as the second-driest stretch since 1900, which occurred in 1959. Ironically, that drought was battled by current Gov. Jerry Brown's father, Pat Brown. The former governor approved the California Water Project, a $1.8 billion initiative that directed water to Southern California and contributed to a boom in agriculture and population. Now his son faces an even more serious problem.