Impact of California's Drought on its Agriculture

AgMRC Renewable Fuels Monthly Report
August 2015

Don HofstrandDon Hofstrand
Retired Agricultural Extension Economist

California consistently leads the nation in agricultural output.  However, the drought, currently in its fourth year, is taking its toll.  Research at UC-Davis indicates the drought will cost California’s agricultural economy $1.84 billion in 2015.  In addition, an estimated 10,100 seasonal jobs directly related to farm production will be lost. Due to the water shortage, an estimated 542,000 acres of land will be idled. If the drought continues, agricultural production and employment will continue to decline.  So, how did this happen and what is the prognosis for the future?

U.S. Drought Monitor

California’s Water Supply

Much of California’s water supply comes from a circuitous route in the Pacific Ocean.  Prevailing winds that blow west along the equator push warm water into the western Pacific where the heat generates storms which push the moist air high into the atmosphere.  Here the jet stream, which blows east, catches the moist air and moves it towards North America.  

When the moist air reaches the Sierra Nevada mountains in eastern California, it precipitates as rain and snow.  Snow pack builds up during the winter which slowly melts in the spring and summer producing a steady flow of water coming down the mountainside into the Sacramento-San Joaquin River Delta.  This huge delta east of San Francisco is the major source of water for agriculture in the Central Valley. 

Californias average precipitation


Due to California’s heavily managed surface water, about half of the water flowing into the delta is held to maintain wetlands and keep saltwater from flowing up into the delta.  The other half of the water is distributed to users. About 20 percent goes to urban areas and 80 percent to farmers to irrigate much of California’s nine million acres of farmland. 

This process of supplying water to California agriculture is currently blocked by the formation of a high pressure ridged over the eastern Pacific.  The ridge deflects the jet stream north so the moist air does not reach the Sierra Nevada mountains. Rather the moist air reaches western Canada and Alaska.

High pressure ridges in the eastern Pacific are not uncommon.  Most of them dissipate in a few weeks.  However, this high pressure ridge, nicknamed the “Ridiculously Resilient Ridge,” has persisted for well over a year and seems resistant to break-up, in-turn becoming a major cause of California’s drought. 

Not all of California’s agricultural surface water is fed by the Sierra Nevada.  Agricultural irrigation in the Inland South comes from the Colorado River (the eastern boundary of southern California) which is fed upstream by snowpack in the Rocky Mountains. Less snowpack is reducing the amount of water coming down the Colorado River.  

California’s Agriculture

California agriculture is quite diverse in the types of crops that are grown.  As shown in Table 1, California agriculture is primarily composed of dairy, fruit, nuts and vegetables.  It is the sole U.S. grower of many fruits, nuts and vegetables including almonds, dates, olives, pistachios and walnuts.  The common ingredient of California agriculture is that virtually all of it is produced under irrigation including 100 percent of the land used for vegetables and berries and 98 percent of the land in orchards.  So, water for irrigation is the life-line of California agriculture.

Table 1. Rankings of California Commodities by Value
Commodity Rank Value (B$)
Milk and Cream 1 $6.89
Grapes (wine & table grapes) 2 $4.45
Almonds 3 $4.35
Cattle & Calves 4 $3.30
Nursery 5 $2.55
Berries 6 $2.12
Hay 7 $1.78
Lettuce 8 $1.45
Walnuts 9 $1.36
Tomatoes 10 $1.17

Source: Climate Risks to California Agriculture

California’s crops require large amounts of water.  California produces about 80 percent of the world’s almonds. To produce one almond it takes about one gallon of water.  Almonds consume three times more water than Los Angeles.  Vineyards and many vegetables consume less water than almonds.  However, as a leading state in dairy and beef, cows require huge amounts of water. It isn’t the amount of water that cows consume directly. Rather it is the large amounts of water required to produce the alfalfa and other crop needed to feed the cattle

California’s secret water-weapon is groundwater.  Due to the drought, the surface water shortage is expected to reach almost 8.7 million acre feet (an acre foot of water is equivalent to an acre of land covered by one foot of water).  About 70 percent of this deficiency in surface water supplies is made up by pumping an additional 6 million acre feet of ground water.

Although surface water is highly controlled through a convoluted maze of regulations, there is very little regulation of the use of ground water.  Farmers can generally sink wells at will to water thirsty crops.

Irrigating with the use of ground water is often just a one-time fix.  The recharge of ground water is very slow. Once the water has been taken it cannot be easily replenished.  So wells have to go deeper to access water at lower levels.  Groundwater levels have dropped by up to 100 feet in recent years.  Some wells are as deep at 1,500 feet sourcing water that may have originated as rain 10,000 years ago. 

Many of these groundwater sources contain salt and other contaminants that are problematic for growing crops.  High-salinity water is causing substantial damage to California’s almond industry.  Also, once the salt is deposited on the soil surface, it is difficult to get rid of. 

Pumping groundwater is not only depleting underground aquifers, it is destroying them.  As pumping depletes water in layers of underground clay, the clay collapses and can no longer hold water.  So these layers cannot be recharged during wet periods. 

The collapse is evidenced by the sinking of the land surface (called subsidence).  Due to the practice of over-pumping of groundwater, large areas of the Central Valley have sunk almost 30 feet since early in the last century.  However, subsidence has gotten worse in recent years.  Land in the San Joaquin Valley is sinking by almost two inches per month in some locations.  In the Sacramento Valley, one location was sinking about one-half inch per month.

Subsidence is causing damage to infrastructure such as roads, bridges, canals, etc.  More importantly, it is a sign of the danger of overusing groundwater and depleting aquifers that are not available for later use unless recharged.  Even worse, it is a sign of the destruction of the ability of underground layers to once again hold groundwater. 

Relief from California’s Drought

Relief may be coming for California in the form of an El Nino.  An El Nino occurs when the easterly equatorial winds that push the warm Pacific water west tend to dissipate or even reverse.  El Nino, meaning “The Christ Child”, occurs when the traditionally extremely dry western side of South America (e.g. Peru) turns wet, usually about Christmas, due to the shifting of the winds from easterly to westerly.  This same effect may bring rain to California during the winter.  The current El Nino is strong and expected to get stronger.  So its impact may be greater than normal.  

The Role of Climate Change

What is the role of global warming in California’s drought?  Climate scientists differ on whether global warming is a cause of the current drought.  Some believe the resilient high pressure ridge discussed above is impacted by global warming. Others believe that natural weather variations have caused the lack of rain.  However, there is general scientific consensus that rising temperatures are making the drought worse.

Although there is no clear trend in changes in precipitation, California’s average temperature has increased by about 2.5 degrees Fahrenheit since the beginning of the 20th Century.  Through evaporation, the higher temperatures pull more moisture out of soils and plants making already dry conditions even worse.  In a sense, temperature rise is pulling already scant rainfall back into the atmosphere. Recent research from the Earth Institute at Columbia University concludes that global warming is causing from 8 to 27 percent of the current drought.  Although this is a broad range, the “most likely” scenario is that the impact is between 15 to 20 percent.

Global warming driven higher temperatures will impact California’s agriculture in other ways. As temperatures rise, precipitation in the Sierra Nevada mountains will fall as rain rather the snow.  The rain will lead to flooding rather than the gradual release of snow melt during spring and summer when California’s agriculture needs water.  Even the snowpack that exists will melt earlier (5 to 30 days earlier than fifty years ago). So the timing and flow of water will coincide less with agriculture’s needs.

Crops need to be grown in a certain temperature range to produce maximum yields. As California warms, the prevailing temperatures become out of sync with crop needs.  An example is orchard crops that need a period of time each year below 45 degree Fahrenheit to rest and rejuvenate.  Warming temperatures will threaten these crops.  To offset this impact, crop could be moved to different parts of the state.  However, these new areas may not have the irrigation and processing infrastructure or the proper soils.  Moreover, perennial crops require long-term commitments of time and money.  So, moving these crops to a new location is much more difficult that with annual crops.

Global warming may have a positive impact through the process of “carbon fertilization”.  Plants use photosynthesis (taking in carbon dioxide and emitting oxygen) to build the plant’s structure.  Higher levels of carbon dioxide in the atmosphere (which causes global warming) may also improve the efficiency of water uptake of certain crops.  This phenomenon has been found in some crops.  However, it is not known if any California’s crops react in this manner.        

California’s Agriculture Future

A continuation of this warming trend into the future will cause more moisture losses and push California into a state of persistent aridity.  By around 2060 California may be experiencing something akin to a permanent drought.

During a drought, the occasional occurrence of wet years is intermingled with the dry years.  However, the moisture from the wet years is not enough to offset the drought. The occurrence of a strong El Nino this winter may give the impression that things have returned to normal for California agriculture.  But over time, warmer temperatures will counteract the ability of an El Nino to overcome dry conditions. This will lead to a “new normal” for California agriculture.  California farmers and agribusinesses are known to be innovative and creative.  These skills will be tested in future years as California agriculture needs to adapt to this new normal.  

References and More Information