Climatological Context for California's Ongoing Drought
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Climatological Context for California’s Ongoing Drought
John Abatzoglou
Back-to-back hot-dry years have some cases, these droughts have cata- 20th century average. Record-setting
left California parched. By late lyzed adaptation responses, as well as warmth, combined with dry condi-
summer of 2021, half of California mitigation efforts, to provide a buffer tions, allowed for an extra 3–4 inches
found itself in an exceptional against future droughts. Nonetheless, of evaporative demand (the amount
drought. I review the climatic recent drought impacts have mate- of potential moisture pulled by the
factors that led us into this deficit rialized in diverse sectors including atmosphere from a well-watered land
and provide its context relative to widespread tree mortality in the Sierra surface, sometimes referred to as
some of the state’s most notorious Nevada, rapid groundwater deple- potential evapotranspiration) rela-
droughts. Increasing atmospheric tion that has resulted in dry wells and tive to average levels for the late 20th
thirst, together with climate change, agricultural challenges in the Central century.
Valley, and combined low-flows and
has arguably supercharged recent These conditions set the stage for
warm river temperatures decimating
droughts, including California’s moisture-starved soils and vegeta-
salmon populations in the rivers of
ongoing drought. tion with the commencement of the
Northern California. The ongoing
2021 water year. The storm track
drought presents another stress test
door remained sealed through early
for the state and likely will facilitate
November, as the jet stream that is a
further adaptation plus mitigation
highway system for storms remained
efforts for future droughts.
well north of the state. A few meager
The Ongoing Drought storms visited the state throughout the
winter. Davis, California saw only 16
When did the ongoing drought
days with meaningful precipitation
start? A historic multi-year drought
(daily totals of at least a tenth of an
commenced during the 2012 water
inch)—tying a water-year record for
year (October 2011–September 2012).
futility with the infamous 1977 water
Several studies showed that the multi-
year. Notably absent were strong
year drought was not only the most
atmospheric rivers. The presence or
extreme in the modern climate record
absence of atmospheric rivers can
but also the most extreme in at least
make or break a water year, as such
the last 1,200 years based on tree-ring
events contribute up to 50% of the
data. Although the state had a couple
annual precipitation in parts of the
The Sacramento basin had the lowest of very wet years in 2017 and 2019
state. A moderate atmospheric river
precipitation and highest evaporative that ameliorated drought impacts
demand in the past four decades for the in late January brought solid precipi-
(e.g., reservoir levels), it is debatable
2021 water year. tation totals along the central Califor-
Photo credit: Juan A. Salgado/ as to whether a couple of wet years
nia coast and into the central Sierra
Shutterstock.com “ended” the drought based on other
Nevada. This system brought signifi-
lagging indicators such as ground-
cant lower-elevation snowfall accumu-
California’s climate is defined by water levels and vegetation mortality.
lations, with locations like Calaveras
wild year-to-year variability in pre- For this writeup, I will focus on the
Big Tree State Park receiving a record
cipitation. An average water year in ongoing drought beginning with the
76.5 inches of snowfall in a three-day
terms of accumulated precipitation 2020 water year.
period. This event softened the mount-
is quite rare in California. Many of
The 2020 water year was dry and hot. ing precipitation deficits in the San
the state’s ecosystems are adapted to
Statewide precipitation was 68% of Joaquin basin.
such volatility. Further, state infra-
20th century averages, with higher
structure—including reservoirs and A persistent ridge of high pressure
deficits in the Sacramento basin (60%
water conveyance systems—has in the northeastern Pacific kept the
of average). The state had its warm-
been designed to cope with historical storm track well north of the state,
est April–September since 1895, with
droughts. Recent droughts, however, and continued through the remainder
temperatures nearly 3.5ºF above the
have tested many of these systems. In of the winter. By the end of March,
2 Giannini Foundation of Agricultural Economics, University of California
Figure 1. Temperature and Precipitation Rankings for Water Year 2021
California—Precipitation California—Mean Temperature
October–March 2021 Percentile October–March 2021 Percentile
42oN 42oN
Record Record
Wettest Warmest
Much Above Much Above
40oN Normal 40oN Normal
Top 10% Top 10%
Above Above
Normal Normal
38oN Top 33% 38oN Top 33%
Near Normal Near Normal
36oN Below 36oN Below
Normal Normal
Bottom 33% Bottom 33%
Much Below Much Below
34oN Normal 34oN Normal
Bottom 10% Bottom 10%
Record Driest Record
Coldest
32 N
o 32 N o
124oW 122oW 120oW 118oW 116oW 124oW 122oW 120oW 118oW 116oW
California—Precipitation California—Mean Temperature
April–August 2021 Percentile April–August 2021 Percentile
42oN 42oN
Record Record
Wettest Warmest
Much Above Much Above
40oN Normal 40oN Normal
Top 10% Top 10%
Above Above
Normal Normal
38oN Top 33% 38oN Top 33%
Near Normal Near Normal
36 N
o
Below 36 N
o
Below
Normal Normal
Bottom 33% Bottom 33%
Much Below Much Below
34oN Normal 34oN Normal
Bottom 10% Bottom 10%
Record Driest Record
Coldest
32oN 32oN
124oW 122oW 120oW 118oW 116oW 124oW 122oW 120oW 118oW 116oW
Note: (Top row) Rankings of October 2020–March 2021 cumulative precipitation and mean temperature relative to the 1895–2021 period.
(Bottom row) Rankings of April 2021–August 2021 cumulative precipitation and mean temperature relative to the 1895–2021 period.
Source: West Wide Drought Tracker. Available at http://www.wrcc.dri.edu/wwdt/.
most of the state had well below-av- winter temperatures. By early April, Spring continued the streak of months
erage water-year precipitation, with a nearly all of the state was in drought— with below-normal precipitation.
few isolated locations in Ventura and per the U.S. Drought Monitor—with Record low April–August precipita-
Marin counties having their driest about 5% in exceptional drought. tion was seen for much of the western
start to a water year since at least 1895 Snowpack decreased rapidly in April, slopes of the Sierra Nevada northward
(Figure 1). Autumn and winter tem- with the onset of anomalously warm into the Klamath basin (Figure 1). Just
peratures were warm, but not unusual temperatures. Unfortunately, the as the storm track door opened late in
in the context of the past couple decrease in spring snowpack was autumn, it closed early in the spring.
decades for the state. Consequentially, not well reflected in spring stream- Statewide, the 2021 water year was the
late winter snowpack (60% of average) flow, leading to a sizable reduction third driest since 1895 and the driest
largely reflected precipitation deficits. in state water resource allocations. It since 1977—receiving about half of
The snow drought was not as acute as is hypothesized that snowmelt infil- the 20th century average rainfall (47%
in recent winters, such as water year tration into parched soils reduced the of average in the Sacramento basin).
2015 that saw below-normal mountain amount of water available for runoff. Cumulative precipitation deficits since
precipitation and extremely warm
Giannini Foundation of Agricultural Economics, University of California 3
Figure 2. Drought Rankings for Water Year 2021
a. Rankings of July 2021 PDSI Relative to b. Scatterplot of Water Year Precipitation and Evaporative
the 1895–2021 Period of Record.* Demand for the Sacramento Basin for 1980–2021.
62
Water Year Evaporative Demand (Inches)
1980–2011, 2016–2019
60
2012–2015
Other 58 2020
56
2021
Bottom 25 54
52
Bottom 10
50
48
Lowest
46
44
10 20 30 40 50 60 70
Water Year Precipitation (Inches)
Note: *Red denotes the lowest PDSI on record, with orange and yellow denoting PDSI values in the lowest 10 and 25 years on record, respectively.
Source: West Wide Drought Tracker, gridMET. Available at: http://www.wrcc.dri.edu/wwdt/.
October 2019 have left parts of the A combination of dry conditions and 1895—highlighting the important role
state missing more than a year’s worth low cloud cover in spring, and record- of extreme evaporative demand in
of precipitation. Such numbers have setting summer temperatures, were 21st century droughts.
been reflected in low—and in some responsible for a very thirsty atmo-
Another way to benchmark the
cases record-low (Oroville)—reservoir sphere. This exceptional atmospheric
ongoing drought is to contextualize
levels by late summer. thirst has further taxed sparse soil
water-year precipitation and annual
and vegetative moisture, allowing for
Some have viewed drought as being evaporative demand, given their
worsening drought conditions.
entirely driven by precipitation short- combined influence on drought.
falls. This view presumes little change Benchmarking the Ongoing Constraining the period of analysis to
in drought in California after April, Drought water years 1980–2021, and focusing
given the nominal precipitation that on the Sacramento basin, we see a
Where does this drought rank rela-
falls from May–September. However, moderate negative correlation sug-
tive to many of the state’s infamous
the data do not bear out that view. gesting that high-demand years tend
droughts? The answer depends on
Between early May and late August, to co-occur with low-precipitation
how we define drought. Based on
the percent of the state in exceptional cool seasons (Figure 2b). Water year
the Palmer Drought Severity Index
drought rose from 5% to nearly 50%. 2021 was unique: it was both the
(PDSI)—a widely used drought index
Drought isn’t defined solely by water driest water year and had the highest
that tracks normalized soil moisture
supply. The demand side can be demand for the Sacramento basin
anomalies based on precipitation
particularly important. Climate-based of any year in the past four decades.
and evaporative demand—July 2021
drought monitoring typically consid- Water year 2020 was close behind—it
values set records for much of North-
ers some type of demand in addition was the third driest water year and
ern California (Figure 2a). Most of the
to precipitation. had the third highest demand. Both
rest of the state had PDSI values in
years were drier and had higher
Significant increases in evaporative the ten driest years; conditions in 2014
demand than any two consecutive
demand have been observed across were more acute in the San Joaquin
years in the 2012–2015 drought.
California and much of the western and Tulare basins. PDSI values for the
United States over the past several Sacramento basin during July 2021 Furthermore, while the 1976 and 1977
decades. Evaporative demand in were the lowest since at least 1895— water years had similar cumulative
2021 for much of Northern California topping individual values during the precipitation deficits to the 2020 and
is the highest it has been in at least 2012–2015 period. By comparison, 2021 water years, evaporative demand
the last four decades—over 8 inches water-year precipitation for the Sacra- during the current, ongoing drought
above the late 20th century average. mento basin was the third lowest since is at least 4 inches higher. A hallmark
4 Giannini Foundation of Agricultural Economics, University of California
of recent droughts is the acute atmo- seasonal drying of soils and vegeta- Suggested Citation:
spheric thirst tied to the shifting base- tion in montane environments, and an Abatzoglou, John. 2021.
lines of temperature and evaporative advancement in the timing of runoff “Climatological Context for
demand. Increased atmospheric thirst that further decouples water supply California’s Ongoing Drought.”
not only depletes soil and vegetated and demand in California’s Mediter- ARE Update 25(1): 2–5. University of
moisture in natural lands, but can ranean climate. The latter acts as a tax California Giannini Foundation of
also translate into increased irrigation on the surface water balance—like Agricultural Economics.
demands for agricultural lands. adding a couple of extra straws to a
drink. Whereas in wet years, an extra
Influence of Human-Caused couple of straws sucking surface water Author’s Bio
Climate Change may have negligible impacts, the extra John Abatzoglou is an associate
straws pulling from the half-empty professor in the Department of
Can we blame this drought on climate
glasses we experience in dry years Management of Complex Systems
change? Not exactly. The predomi-
intensifies impacts. at UC Merced. He can be reached at
nant driver of droughts in California
is shortfalls in precipitation—some- Studies estimate that human-caused jabatzoglou@ucmerced.edu.
thing inherent to the state’s climate. warming made the 2012–2015 drought
Yet, there is mounting evidence to in California approximately 8–27%
suggest that climate change has For additional information, the
worse. Furthermore, studies show
increased evaporative demand and author recommends:
that human-caused climate change
supercharged droughts. The state has effectively doubled the severity of Abatzoglou, J.T., D.J. McEvoy, and
has warmed nearly 3ºF over the past the “megadrought” that the broader K.T. Redmond. 2017. “The West
five decades, consistent with changes Wide Drought Tracker: Drought
southwestern United States has been
simulated by climate models forced by Monitoring at Fine Spatial Scales.”
in since the turn of the century—turn-
Bulletin of the American Meteorological
known human activity (i.e., human- ing a significant long-term drought Society 98(9): 1,815–1,820. Available
caused greenhouse gas emissions). into potentially the worst in at least at: https://wrcc.dri.edu/wwdt/.
1,200 years. Given the elevated evapo-
Changes in precipitation are less clear. Williams, A.P., J.T. Seager, J.T. Abat-
rative demand in the two most recent
There is a non-significant decline in zoglou, B.I. Cook, J.E. Smerdon, and
years, it is likely that human-caused
annual precipitation over the past E.R. Cook. 2015. “Contribution of
climate change has its fingerprints on
century, yet this decline is entirely due Anthropogenic Warming to Cali-
the ongoing drought.
to the past decade that has been punc- fornia Drought During 2012–2014.”
tuated by severe drought. We observe Conclusion Geophysical Research Letters 42:
declines in autumn precipitation and a 6,819–6,828.
delayed onset of seasonal precipitation As we end the 2021 water year, the
Williams, A.P., E.R. Cook, J.E. Smer-
that result in a seasonal compression question that we all want to know
don, B.I. Cook, J.T. Abatzoglou, K.
of the wet season. Several generations is when this drought will end. The
Bolles, S.H. Baek, A.M. Badger, and
of climate models agree on one thing state may play host to a conga line of B. Livneh. 2020. “Large Contribution
regarding changes in precipitation drought-busting atmospheric rivers from Anthropogenic Warming to an
for the state: they agree to disagree. this upcoming winter or may be left Emerging North American Mega-
While climate models do not suggest high and dry. Improvements in sub- drought.” Science 368(6488): 314–318.
any robust changes in annual precip- seasonal-to-seasonal forecasting may
itation, they show a tendency for less help aid in seasonal water-resource
precipitation in the shoulder seasons decision making. The long-term
of spring and autumn and more pre- prospects suggest further increases
cipitation in mid-winter. in evaporative demand with a warm-
ing climate that will tilt the odds for
A few studies have quantified the acute drought conditions similar to
influence of human-caused climate the ongoing drought. This aridifica-
change on recent extreme droughts tion will not be without wet and very
in California and the broader south- wet years. The optimist in me hopes
western United States. Warming exerts the upcoming water year will be one
direct control on the mountain snow- of these wet years and bring some
pack storage efficiency and evapora- reprieve to the ongoing drought. The
tive demand. The former results in pessimist in me says we should pre-
reduced spring snowpack, hastened pare for lean years ahead.
Giannini Foundation of Agricultural Economics, University of California 5
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