Climate and Water Supply – Another Dry Winter and Water Shortages?
By Richard Bell, MWDOC Principal Engineer
Overview. Because of the critical water supply conditions in the State and the prospect
for water shortages and allocations from the Metropolitan Water District of Southern
California, we plan to provide a continuing update through this winter and spring on
climate and water supply conditions.
As most readers are keenly aware, the western States have been in the throes of a
significant drought since 2000, with some relief seen in the Upper Colorado River Basin,
but in California, conditions remain dry.
Last year, after above average precipitation in the fall and early
winter, the weather changed resulting in the driest spring on
record and below normal water supply. Reservoirs were recently
at their lowest levels since the extreme dry year of 1977. To the
left is a recent photograph of Lake Oroville.
Compounding the drought situation, is the recent U.S. Fish &
Wildlife Biological Opinion that implements a 2008 Federal Court
decision ordering increased protection for the Delta Smelt through
cutbacks in diversions from the Central Valley Project and State
Water Project. The following chart shows the historical and recent low levels of Delta
Smelt “abundance”.
On top of this decision, the California Fish & Game Commission recently ruled on listing
the Longfin Smelt as a Candidate Species. The species by species approach to
endangered species protection has caused a major impact on the State Water Project’s
ability to meet its contracted water obligations.
The California Department of Water Resources estimates that protections for both
species as proscribed by the Court and fishery agencies would result in a substantial
49% reduction in supply under average conditions from allowable yield under SWRCB
Water Rights Decision 1641. Under wet conditions, only 59% of the prior average yield
would be able to be diverted and under dry conditions, the yield would be 25% of prior
average yield. SWRCB D-1641 was issued in 1999 after a decade of water right
hearings that culminated in the Bay-Delta Water Quality Control Plan. That decision was
subsequently reaffirmed by the Court of Appeal in February 2006. The ruling for Delta
Smelt occurred one year later and the Longfin Smelt controls will go into effect upon
release of the biological opinion, possibly later this year.
With continuing drought in northern California, and with regulatory fish protections
controlling Delta pumping, the prospect of supply shortages and imported water
allocations - rationing by another name - by Metropolitan Water District as early as July
2009 is likely. DWR has established a Drought Water Bank to help mitigate water
supply shortages. The ability to buy our way out of shortages from agricultural transfers
is limited by diversion restrictions and considerations for food and fiber production and
the economy.
So what are the climatologists predicting for this year? Nothing simple here – lot’s of
factors: a continuing, but weaker La Nina condition, a continuing multi-decadal cooling in
the northern Pacific Ocean (Pacific Decadal Oscillation), likely continuing global
warming, further increased Arctic ice cap melting, effects on ocean current circulation
patterns, and climate change effects on hydrology. Read more to see what is being
predicted for this year and into the future.
Ocean and Climate Indicators. There are three ocean indicators that have received
attention over the past few years that have influenced climate and water supply
conditions. These are the El Nino and La Nina tropical Pacific Ocean sea surface
temperature cycles, the Pacific Decadal Oscillation, and the Madden Julian Oscillation.
Following is a brief overview of these conditions and what they may be telling us about
the coming winter/spring.
Most persons are familiar with “El Nino”, a condition where the tropical Pacific Ocean
sea surface temperature warms which leads to heavier rainfall conditions throughout the
southern part of the United States. When the tropical Pacific Ocean cools, it is known as
a “La Nina” condition. The following graphic, courtesy of NASA, shows the La Nina
condition in November 2007 as denoted by the blue, cooler water in the tropical Pacific
Ocean. A similar condition is now developing.
La Nina conditions tend to lead to drier conditions in central and southern California and
the southwest. At this time, it looks like we may be entering a La Nina condition. The
following graphic shows historical anomalies in precipitation during weak to moderate La
Nina conditions. Note that California has been generally drier than normal under these
conditions.
Another, longer term and more persistent condition is the periodic waning of northern
Pacific Ocean surface temperature between warm and cool phases. This cyclical
pattern, termed “Pacific Decadal Oscillation” (PDO), when fully established, appears to
have had an effect on past storms patterns and western water supply. When the ocean
surface is cooler, it results in reduced evaporation and cooler air masses. This may
result in less intense storms and may be a factor in shifting storm tracks further
northwards. Historical periods of prolonged cooling have corresponded with drier
conditions in both the Upper Colorado River Basin and southern California, while
precipitation in northern California has remained generally in the normal range. Unlike
El Nino and La Nina tropical sea surface warming and cooling events, which last one to
two years, the PDO tends to be persistent, lasting for several decades.
PDO Cool Phase
PDO Warm Phase
The PDO index (Mantua, University of Washington) turned negative in September 2007
and has remained cooler to date. Scientists believe the northern Pacific Ocean may
have entered a cooling phase in 1998, but it hasn’t fully developed yet and has oscillated
between warmer and cooler conditions in recent years. Currently, the PDO is negative,
with a November 2008 index of -1.25. Negative values represent cooler temperatures
(blue) and positive values represent warmer temperatures (red).
Pacific Decadal Oscillation Index -Blue indicates below normal and red
above normal sea surface temperatures. Source: Joint Institute for the
Study of Atmosphere and Ocean, University of Washington and NOAA.
A cool regime from 1947 to 1977 was associated with a lengthy dry period in the
Colorado River Basin and southern California. The recent Colorado River drought has
occurred over this a period of generally cooler PDO conditions, but its occurrence is
likely more due to a general northerly trend of storm tracks over this period and a more
stable western high pressure ridge.
The Madden Julian Oscillation (MJO) is a 30-60 day pattern of tropical rainfall which
leads to storm tracks that can bring extreme precipitation events in the Pacific Northwest
and to California. It tends to occur during periods of neutral ENSO conditions and
features a northeastward progressive circulation pattern from the tropics to the mid
latitudes of the North Pacific. It is often termed the “Pineapple Express” when these
storm tracks drop over warmer parts of the Pacific and often have passed over the
Hawaii Islands. These warm, moisture ladened air masses can bring extreme flooding
to the Pacific Northwest and California. Earlier this year, before the tropical Pacific
Ocean began to cool, there was some indication that a MJO could be developing, but
since that time, a La Nina condition appears to be re-occurring, and the likelihood of
extreme lower latitude progression of these storm tracks has likely been reduced.
Combining a PDO with La Nina condition, where both the northern Pacific Ocean and
tropical Pacific Ocean are simultaneously cool, suggests drier conditions. So far this
year we have had two periods of early winter storms interspersed by longer dry periods,
which has resulted to date in below average precipitation in northern California.
Currently, northern California precipitation and snowpack conditions are at 67% of
normal. Interestingly, the first winter storms looked like a MJO pattern that tracked more
northerly and primarily impacted Alaska. However, wetter conditions in the Rocky
Mountains have occurred to date.
Storm track and precipitation pattern data for the period October 16, 2008 through
January 13, 2009 is shown below. Note that the tracks over the Pacific Coast have
occurred mostly over higher latitudes, which is the main factor for lower than normal
precipitation along California.
Storm Tracks and Precipitation Patterns, 10, 30 and 90 day periods. Ref.
NOAA
Climate Predictions. NOAA scientists utilize various models and data trending
techniques to provide monthly short-term climate predictions. The following December
18 NOAA chart shows the predicted precipitation conditions for January through March,
2009. It shows continuing drought in the southwest, but neutral conditions in the
northern watersheds. Red indicates warmer temperatures and browns indicate drier
conditions. It does not yet take into account the developing La Nina like conditions.
Jan-Mar 2009 Predicted Temperature
2009 Precipitation
Jan-Mar Predicted
Long-Term Climate Change. What effect has atmospheric and ocean warming had on
water supply? Over the last century, observations have shown a general rise in
temperatures in many western watersheds. Data shows a general higher elevation and
reduction in snowpack accumulation, earlier and reduced runoff, and more frequent
increased storm intensity. Over the last century, the total precipitation and runoff trend
has been slightly downward in the Colorado River Basin and as well as in southern
California, but has remained about the same in northern California.
Looking ahead, nearly all the Global Circulation-Ocean Coupled Models (“GCM’s”)
indicate a general polar shift in storm tracks in the mid-latitudes that some scientists
believe will lead to increasing aridity in the southwest and suspect that recent drier
conditions are the result of these effects. A warming world in the southwest and in
northern California will result in increased evapotranspiration rates and water demands,
higher elevation snow levels, reduced snowpack storage, and reduced precipitation and
water supply.
Current Water Supply Conditions and “Our Prediction”. California Department of
Water Resources publishes weekly updates of hydrologic data collected throughout the
State. Snow surveys are conducted monthly beginning with the first snow survey in late
December. A useful indicator of water supply conditions in the primary watershed areas
in the northern Sierra is the 8-Station Index, which is shown below through January 7.
Historical extreme and average conditions are shown as well as the current precipitation
total. Precipitation is currently tracking slightly below average. The first snow survey
was completed on January 8 which showed snow water accumulation at 67% of normal
in the northern Sierra Mountains and about normal in the southern Sierra Mountains.
So, what do we think is in store for us? If we consider the current trend, the developing
La Nina condition, and if it stays weak, and the cool phase PDO situation, that would
suggest that water supply conditions in northern California will most likely be trending
below normal. So, until things change, it looks like the drought will continue.
8-Station Index of Northern Sierra Precipitation – Dec 18
Snow water content conditions in western States.
Snow Water Content - January 8, 2009
.