The current California drought is considered to be the state’s worst ever, based on more than 100 years of record-keeping. This winter produced the smallest Sierra Nevada snowpack ever measured, meaning there is too little melting snow to refill important reservoirs.

It is commonly stated that this drought started in 2012. In fact, it may have begun earlier: statewide precipitation has been below normal since 2007, except in 2010.

The resulting shortages prompted Gov. Jerry Brown earlier this year to order the first mandatory conservation order: All residents of the state are required to cut their water use by 25 percent. In addition, hundreds of farmers have been cut off from irrigation water provided by state and federal agencies. This has caused heavy pumping of groundwater and created a lucrative market for anyone with extra water for sale.

The superficial cause of the drought is that winter storms simply aren’t reaching California. The jet stream, a current of high-velocity wind that flows several miles above the earth, normally sweeps across California and regularly delivers storms.

For reasons that still aren’t clear, the jet stream has since 2012 detoured around California to the north, diverting storms in a long loop toward Canada and the Midwest. The detour is caused by a ridge of high-pressure air that forms off the California coast. This isn’t unusual, but it normally lasts only a few weeks. Instead, this high-pressure ridge has been exceptionally stubborn, holding for months. Climate change may be to blame, but recent studies have been inconclusive.

Whatever the cause, it certainly marks a new extreme in weather variability. California already has the most widely variable climate in America. Some scientists say there is no such thing as “normal” weather in California, only a succession of extremes. A severe drought can be followed the next year by deadly floods.

Saving the Bay – Moving California’s Water Supply

Consider the geography. California encompasses a swath of land equivalent to the distance from Chicago to Atlanta. No one would suggest those cities have much weather in common.

When storms do come, California is ready with one of the most heavily engineered water-management systems in the world. This plumbing network is so extensive that it can collect rain that fell on redwood forests near the Oregon border and distribute it as far as 800 miles away to irrigate avocado orchards near San Diego.

The Sierra Nevada mountain range is the backbone of this system. It forms a bulwark up to 14,000 feet high across the state’s eastern boundary. These high mountains are great at snatching moisture out of Pacific storms. To take advantage of this natural phenomenon, water agencies have built hundreds of reservoirs along the Sierra front over the past century to capture storm runoff.

About 50 percent of all the freshwater consumed in California comes from this mountain runoff. It eventually runs into two rivers that drain the Sierra, the Sacramento and San Joaquin. They join to form the Sacramento-San Joaquin Delta in the heart of the state’s Central Valley.

The Delta is the largest estuary on the West Coast of the Americas all the Americas, from Prudhoe Bay to Cape Horn. State and federal agencies exploited this by building two giant pump and canal systems in the Delta. These distribute water to cities and farms all over the southern two-thirds of the state. It is primarily these diversions that have supported California’s phenomenal population growth and agricultural productivity.

Groundwater is also important, providing about 38 percent of the state’s water in typical years. In dry years this increases to 46 percent or more, because surface water becomes harder to get. As a result, aquifer levels have dropped sharply during this drought, especially in the San Joaquin Valley, where groundwater is an important backup for many farms.

In short, California has abundant water supplies. But it’s not enough when Mother Nature doesn’t deliver. All those dams, canals and pumps don’t make water. They only move it.