California Drought
2012-2016 California Drought: Historical Perspective
On January 17, 2014, California State Governor Jerry Brown declared a drought state of emergency. On April 2, 2017, Governor Brown lifted the drought emergency, but declared that California must continue water conservation efforts. With the official conclusion of the most recent drought, which spanned water years 2012 through 2016, it is timely to compare it with other historic California droughts and also to consider some of the lingering impacts.
California's Historic Droughts
“Water year” is defined as starting on October 1 of the preceding year and ending on September 30 of the water year (e.g. Water Year 2018 starts on October 1, 2017, and ends on September 30, 2018). Hydrologically, “water year” is a useful timeframe because the majority of precipitation in Western states occurs from late fall to early summer. Thus, water years are useful to delineate dry and wet periods.
From a runoff perspective under the Sacramento and San Joaquin Valley Water Year Indices, the majority of years in these six periods were classified as “dry” or “critical.” The amount of runoff in these two valleys is an important agricultural drought indicator, given the vast agricultural production in California’s Central Valley. Historical scientific records of other related stressors, such as Delta salinity, also indicate that these periods were times of ecological stress.
In addition, WYs 1924 and 1994 were also extremely dry, from both runoff and precipitation perspectives indicating that these are meteorological droughts. Water Year 1924 ranks as 3rd driest out of 117 years, from a runoff perspective, and driest out of 122 years, from a precipitation perspective. In WY 1924, California only received 9.94 inches precipitation resulting in 26.9 million acre-feet (MAF) runoff statewide. Water Year 1994 ranks as 6th driest out of 117 years, from a runoff perspective, and 9th driest out of 122 years, from a precipitation perspective. In WY 1994, California received 15.13 inches precipitation resulting in 33.7 MAF runoff statewide. By contrast, the statewide mean precipitation is 22.45 inches for the period from 1901 through 2000. The lowest statewide runoff on record is 15.5 MAF in WY 1977; the highest is 201.7 MAF in WY 1983.
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Runoff and precipitation conditions for California’s six historical droughts. The most severe drought both in terms of precipitation and runoff was the drought of 1976-77. However, because it was just a two-year drought, the water supply impacts were not as severe as those associated with the longer duration droughts because shorter droughts can be partially mitigated by surface and groundwater storage.
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Cumulative runoff for the six historic droughts. The longer the time period of impaired runoff, the more severe the drought impacts.
A Tale of Three Droughts
Because of their duration and severity for both lack of rainfall and runoff, the 1928-34 drought, which lasted seven years, and the 1987-92 drought, which lasted six years, are compared to the 2012-16 five-year drought, to assess similarities and differences.
Runoff is an important parameter in assessing drought impact severity. The amount of runoff is dependent upon many factors including the amount, location and type of precipitation (rain or snow); rainfall rates; the amount of base flow (i.e. contribution of groundwater to streamflow); antecedent soil moisture conditions; the amount of empty surface water reservoir storage; the magnitude to which groundwater aquifers are drawn down; watershed geology and topography; the level of urbanization in the watershed; and the amount and type of landscape and cultivated plant cover. A related story discusses the role of snowfall, rainfall, and reservoirs in buffering drought impacts, and indirectly describes the impact of snowfall, rainfall, and reservoirs on runoff.
The Statewide Cumulative Runoff graph shows that cumulative statewide runoff in year five of the drought was 204.5 MAF for the WY 1987-92 drought, 221 MAF for the WY 2012-16 drought and 258.3 MAF for the WY 1928-34 drought. This might indicate that the 1987-92 drought was the most severe, but this is not necessarily the case. Following is a closer look at each of the three droughts.
WY 1928-1934 Drought
This seven-year drought predated the construction of many of the water projects in California including the Federal Central Valley Project and State Water Project. Because the 1928-34 drought constituted the first major drought on record in California, it served as the basis for early reservoir operations planning and the development of shortage criteria for water supply contracts.
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This graph shows a direct relationship between annual precipitation and runoff, i.e. when precipitation decreases or increases so does runoff. In looking at the cumulative runoff graph, this drought appears a little less severe than the 1987-92 and 2012-2016 droughts; however, if such a drought reoccurred today, its seven year duration, as compared to six and five years, respectively, for the other two droughts, might make the impacts of such a drought more severe.
WY 1987-1992 Drought
This six-year drought occurred when most major reservoirs in California had been constructed. Despite carryover storage in reservoirs to buffer drought impacts, a drought water bank was initiated in 1991 to make water available for sale to areas of extreme need.
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This graph does not show as direct a relationship between annual precipitation and runoff as the 1928-34 drought. The statewide runoff decreased from 43.2 to 40.8 MAF in WYs 1987 to 1988 and from 34.7 to 33.5 MAF in WYs 1990 to 1991 despite increases in statewide precipitation from 13.97 to 18.38 inches and 15.02 to 17.03 inches, respectively, for the same sets of water years. This decrease in runoff could be due to such factors as a mismatch between the location of rainfall and the most productive watersheds for runoff, depleted soil moisture, or empty reservoirs filling.
WY 2012-2016 Drought
This five-year drought, which is the most recent, has well-documented agricultural (e.g. extremely agricultural surface water allocations), physical (e.g. groundwater depletion-related subsidence) and environmental impacts (e.g. fish mortality). Surface and groundwater withdrawals were used to mitigate water supply impacts. Water transfers were a primary tool to move water to areas of need, such as permanent crops.
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Similar to the 1987-92 drought, this graph shows that precipitation and runoff are not always directly related on an annual basis. Annual statewide runoff decreased from 49.9 to 45.3 MAF from WY 2012 to 2013, despite an increase in statewide precipitation of 16.44 to 16.95 inches during the same timeframe. Given that both Sacramento Valley and San Joaquin Valley runoff increased during this period, the filling of smaller reservoirs in watersheds outside the Central Valley rather than the larger ones associated with the State and Federal projects, may have caused the statewide decrease in runoff.
References
USGS integrated drought science
Historical Sacramento and San Joaquin Valley Water Year Type Index
California Department of Water Resources
Evidence of ecological stresses in 1928-34 drought: high Delta salinity in early dry years, such as 1924, was an impetus in the construction of the Central Valley Project
Bureau of Reclamation
Annual Runoff by State
USGS WaterWatch
Statewide Precipitation
NOAA National Centers for Environmental Information