PUB00052

• Year: 1998
• Title: Layperson's Guide to Groundwater
• CWCB Section: Interstate & Federal
• Author: California Water Education Foundation
• Description: Layperson's Guide to Groundwater
• Publications - Doc Type: Other

Extractio1/ (~lgrol!ndU'ater
in the A111e/ol'c Valfey has
cal/sedfissures on Rogers
LaJ..:ehed af E(hl'ards Air
F'orc(' Base. which serres
as a rummy for the space
shuttle (lnd other airemjt,
Be/mI'. Mel MUn/let.
oilfield 11IaJlagel: stonds
iJl a 2.000-foot-hmg
.fissure thatforlned in
January /99/.

~~~-',
i.........

,-

'-

-

11

~

The 1987-1993 drought showed California the
enormous value of well-stocked groundwater basins.
When water imports from the Sacramento River to
the San Joaquin Valley were cut, valley growers
turned to aquifers that had significantly recovered
from earlier drawdowns. But the drought also
reminded California of the enormous consequence
of overdralt and lowered groundwater levels as parts
of the San Joaquin Valley experienced renewed land
subsidence that many property owners thoughf had
been relegated fa the history books.

Land subsidence was a widespread problem earlier
in the century - when pumping technologies had
developed enough to increase production. but before
surface water systems were developed to meet
growing water demands. Subsidence can occur
where too much water is pumped and the fine-grain
clay sediments are drained. When drained, the

~

..:.~

",-'" ""'-r . ...- . -
-="~~-...~-
-~-~J~- ,'- - _ .. .;~"." . -"..
>6. _____ - . ..'-.

~ $
. "
--~, !
'.

t ~

"<Ii f>.i..

weight of the soil above the aquiter compacts the
previously saturated clays and the soil sinks. In some
cases, subsidence can affect how much water an
aquifer can hold.

The most severely affected areas were in southern
and western portions of the San Joaquin Valley. Land
near Mendota sank by nearly 30 leet. and a broad
area down the west side at the valley to Kellleman
City in Kings County sank by up to 25 teet. On the
eastern side of the valley. Tulare and Wasco area
farms sank 12 teet. At the extreme southern end
of the valley, the ground sank 8 feet. Even in the
weller Sacramento Valley, in the Zamora and Knights
Landing areas at Yolo County, the land sank by 4
to 6 teet. Between 1925 and 1980, approximately
one-hall at the valley - or about 5.400 square miles
of the San Joaquin Valley alone - subsided at least
1 foot.

Subsidence has damaged buildings, aqueducts, well
casings, bridges and highways. As the land sinks,
flooding problems are aggravated. In all, subsidence
has resulted in millions of dollars in damage, Half of
all of the subsidence that has occurred in the nation
has occurred in California.

I
I

Land subsidence had been largely arrested - until
the dry years in the late 1980s and early 1990s
increased pumping demands on the aquifers and the
water fable in the Central Valley fell by as much as
100 feet. Portions of the San Joaquin Valley began
to sink again - by 8 feet in some areas. For some
water managers, these drought-related events are
an indication of long-term problems. DWR estimates
that by the year 2010, water demands in the San
Joaquin Valley will increase by 8 percent to 9 percent
from 1980 demands - without additional surface
water imports. That increase would require a
combined extraction and overdraft of 2.5 million
acre-feet.

Subsidence is not only a problem in the Central
Valley. Urban development in the Mojave Desert and
Antelope Valley northeast of Los Angeles has
increased groundwater extractions to the point that
the land has began to sink and fracture. At Edwards
Air Force Base, where the space shuttle lands and
the military has tested airplanes for decades, one of
the largest fissures - 12 feet deep, 4 leet wide and
2,000-feet long - took a runway out of service and
raised questions about the integrity of the dry lake
bed that is used in base operations.