F A L L 2 0 1 2
Cali fornia Depar tment of Water Resources
1416 ninth St reet , Sacramento , C A 95814
w w w.water. ca . gov/droug ht
Drought in California
California Depar tment of Water Resources
Natural Resources Agenc y
State of California
Definition
California
Drought
Cause &
Prediction
Impact
Groundwater
Preparation
0
10
20
30
40
1900
1910
1920
1930
1940
1950
1960
1970
YEAR
IN
C
H
ES
Red line Denotes 11-year running mean
Western Regional
Climate Center
1980
1990
2000
2010
Defining Drought
2 0 1 2 D R O U G H T B R O C H U R E
2
There are many ways that drought can be defined. Some ways can
be quantified, such as meteorological drought (period of below
normal precipitation) or hydrologic drought (period of below average
runoff), others are more qualitative in nature (shortage of water
for a particular purpose). There is no universal definition of when a
drought begins or ends. Drought is a gradual phenomenon.
Impacts of drought are typically felt first by those most dependent on annual
rainfall, such as ranchers engaged in dryland grazing or rural residents relying on
wells in low-yield rock formations. Drought impacts increase with the length of
a drought, as carry-over supplies in reservoirs are depleted and water levels in
CALIFORNIA STATEWIDE PRECIPITATION
OCT–SEPT (WATER YEAR)
CONTENTS
Defining Drought
2
Definition
4
Droughts in California
6
Drought Causation and Prediction
8
Drought Impacts from a Water

Use Perspective
10
Droughts and groundwater
11
Preparing for Droughts and

Mitigating Drought Impacts
Defining Drought
2 0 1 2 D R O U G H T B R O C H U R E
3
Through water year 2012, Colorado River inflow into Lake
Powell has been below average in 10 of the past 13 years,
resulting in reduced storage levels in Lakes Mead and Powell.
The Colorado has historically been a highly reliable water
supply for Southern California despite long-term drought,
thanks to its large reservoir storage capacity. Interim
guidelines adopted in 2007 for Lower Basin shortages and
coordinated operations of Lakes Mead and Powell help reduce
the risk of shortages to California.
ground water basins decline. Hydrologic impacts of drought
to water agencies may be exacerbated by other factors
such as regulatory requirements to protect environmental
resources or to satisfy the rights of senior water right holders.
from a water use
perspective, drought is
best defined by its impacts
to a particular class of
water users in a particular
location. In this sense,
drought is a very local
circumstance. Hydrologic
conditions constituting a
drought for water users
in one location may not
constitute a drought for water users in a different part of the
state or with a different water supply. California’s extensive
system of water supply infrastructure— reservoirs, managed
groundwater basins, and inter-regional conveyance
facilities—mitigates the effect of short-term (single year)
dry periods. Individual water suppliers may use criteria
such as rainfall/runoff, amount of water in storage, decline
in groundwater levels, or expected supply from a water
wholesaler to define their water supply conditions.
Criteria used to identify statewide drought conditions—
such as statewide runoff and reservoir storage—do not
address these localized circumstances. And although
California’s water supply infrastructure provides a means
to mitigate impacts for some water users, other types of
impacts (increased wildfire risk, stress on vegetation
and wildlife) remain.

California’s extensive
system of water supply
infrastructure—
reservoirs, managed
groundwater basins, and
inter-regional conveyance
facilities—mitigates the
effect of short-term
(single year) dry periods.
DroUgHTS In CAlIfornIA
2 0 1 2 D R O U G H T B R O C H U R E
4
Drought played a role in shaping California’s early
history, as the so-called great Drought in 1863-
64 contributed to the demise of the cattle rancho
system, especially in Southern California.
Subsequently, a notable period
of extended dry conditions was
experienced during most of the 1920s
and well into the 1930s, with the latter
time including the Dustbowl drought
that gripped much of the United States.
three twentieth century droughts were
of particular importance from a water
supply standpoint – the droughts of
1928-35, 1976-77, and 1987-92.
The 1928-35 Dustbowl drought
established hydrologic criteria widely
used in used in designing storage
capacity and yield of large northern
California reservoirs. The 1976-77
drought, when statewide runoff in 1977
hit an all-time, low served as a wake-
up call for California water agencies
that were unprepared for major cut-
backs in their supplies. forty-seven of
the State’s 58 counties declared local
drought-related emergencies at that
time. Probably the most iconic symbol
of the 1976-77 drought was construction of an emergency
pipeline across the San rafael Bridge to bring water obtained
through a complex system of exchanges to Marin Municipal
Water District in southern Marin County. The 1987-92
drought was notable for its six-year duration. Twenty three
counties declared local drought emergencies. Santa Barbara
experienced the greatest water supply reductions among the
larger urban areas. In addition to adoption of measures such
as a 14-month ban on all lawn watering, the city installed a
temporary emergency desalination plant and an emergency
pipeline was constructed to make State Water Project
supplies available to southern Santa Barbara County.
It is important to recognize that a period of historically
recorded hydrology of little more than a century does not
represent the full range of the climate system’s natural
variability. Paleoclimate information, such as streamflow
reconstructions based on tree-ring data, shows that natural
variability can be far greater than that observed in the historical
record. These reconstructions have identified droughts
prior to the historical record that were far more severe than
today’s water institutions
and infrastructure were
designed to manage.
The Colorado river Basin,
an important source of
Southern California’s
water supply, has been
particularly well studied;
its streamflow reconstructions show multidecadal periods
when flows were below the long-term average.
MULTI-YEAR
DROUGHTS OF
LARGE-SCALE
EXTENT SINCE
1900
(Based on statewide
runoff )

1918–1920

1923–1926

1928–1935

1947–1950

1959–1962
1976–1977

1987–1992

2000–2002

2007–2009
Droughts in California
The 1928-35 Dustbowl
drought established
hydrologic criteria widely
used in used in designing
storage capacity and
yield of large Northern
California reservoirs.
Emergency pipeline constructed during 1976-77 drought
to bring water to southern Marin County.
Fl
ow
, M
ill
io
n
A
cr
e-
Fe
et
YEAR
Long-term Streamflow Reconstruction from Tree-ring Data Observed Flow
Annual Flow
10 Running Year Mean
20 Running year Mean
25
20
800
1000
900
1100
1200
1300
1400
1500
1600
1700
1800
1900
2000
2005
15
15
5
0
762
DroUgHTS In CAlIfornIA
2 0 1 2 D R O U G H T B R O C H U R E
5
Some 5,000-6,000 years ago these trees were growing on lands
now submerged by Lake Tahoe, illustrating centuries-long periods
drier than present conditions. National Geographic submersible
shown inspecting tree stumps still rooted in place on the lakebed.
Photo courtesy of National Geographic.
RECONSTRUCTED COLORADO RIVER
STREAMFLOW AT LEE FERRY
Most of California’s moisture originates in the
Pacific ocean. During the wet season, the
atmospheric high pressure belt that sits off
western north America shifts southward, allowing
Pacific storms to bring moisture to California.
on average, 75 percent of the state’s average annual
precipitation occurs between november and March, with half
of it occurring between December and february. A few major
storms more or less shift the balance between a wet year and
a dry one. A persistent high pressure zone over California
during the peak winter water production months predisposes
the water year to be dry.
Drought Causation
and Prediction
On average, 75 percent of the state’s average
annual precipitation occurs between November
and March, with half of it occurring between
December and February.
Mount Shasta in 2008 at the end of the water year.
The impacts of climate change, such as the shift
in timing of spring runoff in the Sierra Nevada, are
becoming increasingly discernible in analysis of
hydroclimate data. Efforts to predict drought must
evaluate the natural climate variability seen in
historical and paleoclimate records, together with
changed conditions such as increased warming.
The ability to reliably predict drought conditions at
seasonal or annual timescales is very limited. The status of
El niño-Southern oscillation (EnSo) conditions is presently
the only factor that offers a hint of predictive capability
for precipitation in California. EnSo is a periodic shifting
of ocean-atmosphere conditions in the tropical Pacific that
ranges from El niño (warm phase) to neutral to la niña (cold
phase). la niña conditions tend to favor a drier outlook for
Southern California, but do not typically show significant
correlation with water year type for northern and Central
California. The predictive capabilities provided by EnSo
events are related to the strength of an event; stronger
events yield better predictive signals. In any individual year,
interactions with other climate patterns or forcings may affect
the outcome that would otherwise be expected from EnSo
conditions alone. How other factors such as the Madden-
Julian oscillation, Pacific Decadal oscillation, north Atlantic
oscillation, or Arctic oscillation modulate the expression of
EnSo conditions remains a subject for research.
GLOBAL-LEVEL TOP 10 WARMEST
YEARS SINCE 1880
Source: National Climate Data Center

1
2005

2
2010

3
1998

4
2003

5
2002

6
2006

7
2009

8
2007

9
2004

10
2001
0
2
5
IN
C
H
ES
7
9
Jan.
Feb.
Mar.
Apr.
May
June
MONTHS
July
Aug.
Sept.
Oct.
Nov.
Dec.
0
2
5
7
9
1
2
3
4
5
6
7
8
9
10
11
12
DroUgHT CAUSATIon AnD PrEDICTIon
2 0 1 2 D R O U G H T B R O C H U R E
7
MONTHLY DISTRIBUTION OF STATEWIDE PRECIPITATION,
SHOWING WET, AVERAGE, AND DRY YEARS
KEY
1923–1924
1982–1983
Average
Satellite image of atmospheric river reaching West Coast.
Atmospheric river storms – storms fueled by concentrated
streams of water vapor from the Pacific Ocean – are big
contributors to annual water supply conditions.
A few major storms more or less shift the balance between
a wet year and a dry one.

Image courtesy NOAA Hydrometeorology Testbed.
DroUgHT IMPACTS froM A WATEr USE PErSPECTIvE
2 0 1 2 D R O U G H T B R O C H U R E
8
Even a single dry year can pose problems
for activities that are wholly dependent on
unmanaged water supplies, such as dryland
farming or livestock grazing.
Some unmanaged recreational uses can also be affected,
such as rafting in rivers where flows are not controlled
by reservoir releases. Single dry year impacts to the natural
environment can often be seen in the form of increased
wildfire risk, a risk that increases in multiple dry years.
Damages associated with wildfires and loss of timber
resources can be one of the largest economic impacts of
drought, and California faces increasing risk of damages
as urban development encroaches on the urban/wildland
interface. California’s most devastating urban/wildland
fire episodes (oakland hills in 1991, Southern California
in 2003, Southern California in 2007) occurred during a drought
or in a year immediately following a multi-year drought,
when dry vegetation created conditions favorable for
massive fire outbreaks.
Multiple dry years predictably create problems for
small water systems in at-risk areas. Urban water suppliers,
particularly those serving larger metropolitan areas, normally
provide highly reliable supplies for their customers, as they
have the resources and the revenue base to prepare for and
respond to drought impacts. The majority of serious water
Drought Impacts from a
Water Use Perspective
SIZE DISTRIBUTION OF
CALIFORNIA PUBLIC WATER SYSTEMS
California Department of Public Health data
Transient nCWS
CWS, large

(3300+/Wholesaler)
CWS, large
(1000 – 3300)
CWS, large
(500 – 999)
CWS, Small
(100 – 499)
CWS, Small
(25 – 99)
CWS, Small
(<25)
non-Transient nCWS
KEY
CWS = Community Water System
NCWS = non-community Water System
Non-transient NCWS = serves 25 or more of
the same non-resident individuals, at least
6-month out of the year; e.g. schools,
places of employment, etc.
Transient NCWS = serves 25 or
transient individuals per day, for
any 60-days out of the year; e.g
rest stops, campgrounds, etc.
DroUgHT IMPACTS froM A WATEr USE PErSPECTIvE
2 0 1 2 D R O U G H T B R O C H U R E
9
supply problems during droughts (e.g. inability to maintain
fire flows, need for truck haulage of water) are experienced
by small water systems. Although small systems serve
a low percentage of California’s total population, they
constitute the majority of the state’s public water systems.
Small systems tend to be located outside the state’s major
metropolitan areas, often in lightly populated rural areas
where opportunities for interconnections with another
system or water transfers are nonexistent. Small systems
also have limited financial resources and rate bases
that constrain their ability to undertake major capital
improvements. Most small system drought problems stem
from dependence on an
unreliable water source,
commonly groundwater
in fractured rock systems
or in small coastal terrace
groundwater basins.
Historically, particularly
at-risk geographic areas
have been foothill areas of the Sierra nevada, Coast range,
and inland Southern California mountains, and the north
and Central Coast regions.
In the irrigated agriculture sector, the largest at-risk
area has been the west side of the San Joaquin valley,
particularly the area supplied by Central valley Project
south-of-Delta exports. Central valley Project contractors
in this area received 100 percent of their supplies in
only three years during the 23-year period from 1990
through 2012, and 75 percent or better of their supplies
in only eight of those years, due to combined impacts
of dry conditions and environmental regulatory
requirements. The impacts of reduced supplies were
evident in the 2007-09 drought, when growers abandoned
permanent plantings such as orchards and vineyards due
to water shortages.
Some avocado growers in Southern California stumped
orchards as a short-term measure to reduce water use
while keeping the trees alive, in hopes of improved
future water supplies after the 2007-09 drought.

Orchard on Westside of San Joaquin Valley
abandoned during 2007-09 drought .
Even a single dry year
can pose problems for
activities that are wholly
dependent on unmanaged
water supplies, such
as dryland farming or
livestock grazing.
DroUgHTS AnD groUnDWATEr
2 0 1 2 D R O U G H T B R O C H U R E
10
Some users of managed surface water supplies have the
ability to increase their use of groundwater when those
surface supplies are reduced.
An increase in the number of new wells being drilled or of existing
wells being deepened is typical during droughts; private residential
wells represent the single largest category of new or deepened wells.
As with small water systems, residential well problems are common in
fractured rock groundwater production areas.
Increased groundwater use is reflected in declining groundwater
levels; in groundwater basins not experiencing overdraft, a pattern
of water level drawdown during dry conditions and recovery during
wet conditions is typically seen. groundwater level decline in
overdrafted basins is typically exacerbated by drought.
Data availability limitations make it difficult to
assess drought impacts on groundwater at
statewide or large regional scales in a
near real-time manner, as can be
done for surface water.
Groundwater basins as defined by DWR are shown
in blue. Areas outside these basins are often fractured
rock groundwater zones, where groundwater production
capability is uncertain.
Droughts and
Groundwater
GROUNDWATER BASINS
Guidebook to Assist Urban Water
Suppliers to Prepare a 2010 Urban
Water Management Plan
California
Department of Water Resources
PrEPArIng for DroUgHTS & MITIgATIng DroUgHT IMPACTS
2 0 1 2 D R O U G H T B R O C H U R E
11
California’s extensive system of statewide and
regional-scale water infrastructure greatly enhances
the state’s drought resilience by providing the
capacity for facilitating water transfers and exchanges.
lessons learned from past droughts and from
disasters such as earthquakes and wildfires have
fostered system interconnections among the state’s
major water utilities, helping enable
mitigative measure such as transfers.
over more than three decades, California’s
voters have authorized substantial amounts
of state financial assistance to local urban
and agricultural water agencies, funding
projects — such as water conservation,
water recycling, or groundwater storage –
that are tools for drought preparedness. In
recent years, the 2002 Integrated regional
Water Management Act established state
policy of encouraging local agencies to work
cooperatively to manage local and imported
water supplies to improve their quantity,
quality, and reliability. In 2002 and 2006
the voters approved two bond measures
which specifically authorized a combined
$1.5 billion for water supply-related
integrated regional water management
planning and projects.
Drinking water supplies are additionally
covered by statutory and administrative
provisions. California Water Code Sections 10610 et seq.
require that public water systems providing water for
municipal purposes to more than 3,000 customers or serving
more than 3,000 acre-feet annually prepare an urban water
management plan and update it every five years. The plans
Preparing for Droughts
& Mitigating Drought
Impacts
The Urban Water
Management Planning
Act was adopted in
1983, setting in motion
a process of continuing
refinements and updates
to local plans for
ensuring service area
water supply reliability.
must include a water shortage contingency analysis that
addresses how systems would respond to supply reductions
of up to 50 percent, and must estimate supplies available
in a single dry year and in multiple dry years. The plans
must also address systems’ responses to catastrophic
supply interruptions. Although smaller water systems are
not covered by these requirements, state
drinking water regulations require that the
systems demonstrate technical, financial,
and managerial capacity (including having
an emergency response plan) as part of
being eligible for financial assistance.
In the agricultural sector, individual
water users (i.e., growers) are eligible for
a variety of programs authorized by the
farm Bill and administered through the U.S.
Department of Agriculture. Programs range
from risk management programs (crop
insurance) to disaster financial assistance
for drought impacts or prevented planting.
Many managed water supplies have
associated environmental regulatory
requirements that provide dry year
protections such as mandated instream
flows for fishery purposes. operations
of the State Water Project and federal
Central valley Project in the Sacramento-
San Joaquin river Delta, for example, are
intensively managed to meet water quantity and quality
requirements for fish species of special concern. Major
wildlife refuges in the Central valley have been guaranteed
specific quantities of water since the 1990 passage of the
Central valley Project Improvement Act.
Even a single dry year can pose problems
for activities that are wholly dependent
on unmanaged water supplies, such as dryland
farming or livestock grazing.