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San Joaquin Valley, California: Largest human alteration of the Earth's surface

Authors:
Largest human alteration of the Earth’s surface
Devin Galloway and Francis S. Riley
U.S. Geological Survey, Menlo Park, California
Mining ground water for agriculture has enabled the San
Joaquin Valley of California to become one of the world’s
most productive agricultural regions, while simulta-
neously contributing to one of the single largest alterations of the
land surface attributed to humankind. Today the San Joaquin Valley
is the backbone of California’s modern and highly technological
agricultural industry. California ranks as the largest agricultural
producing state in the nation, producing 11
percent of the total U.S. agricultural value.
The Central Valley of California, which
includes the San Joaquin Valley, the Sacra-
mento Valley, and the Sacramento-San
Joaquin Delta, produces about 25 percent
of the nations table food on only 1 percent
of the country’s farmland (Cone, 1997).
In 1970, when the last comprehensive sur-
veys of land subsidence were made, sub-
sidence in excess of 1 foot had affected
more than 5,200 square miles of irrigable
land—one-half the entire San Joaquin
Valley (Poland and others, 1975). The
maximum subsidence, near Mendota, was
more than 28 feet.
Approximate location of maxi-
mum subsidence in United
States identified by research
efforts of Joseph Poland (pic-
tured). Signs on pole show ap-
proximate altitude of land
surface in 1925, 1955, and
1977. The pole is near bench-
mark S661 in the San Joaquin
Valley southwest of Mendota,
California.
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San Joaquin Valley
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Mendota
SAN JOAQUIN VALLEY, CALIFORNIA
Mining Ground Water24
Since the early 1970s land subsidence has continued in some loca-
tions, but has generally slowed due to reductions in ground-water
pumpage and the accompanying recovery of ground-water levels
made possible by supplemental use of surface water for irrigation.
The surface water is diverted principally from the Sacramento-San
Joaquin Delta and the San Joaquin, Kings, Kern and Feather Rivers.
Two droughts since 1975 have caused surface-water deliveries in
the valley to be sharply curtailed, and demonstrated the valleys
vulnerability to continued land subsidence when ground-water
pumpage is increased.
The history of land subsidence in the San Joaquin Valley is inte-
grally linked to the development of agriculture and the availability
of water for irrigation. Further agricultural development without
accompanying subsidence is dependent on the continued availabil-
ity of surface water, which is subject to uncertainties due to climatic
variability and pending regulatory decisions.
Land subsidence in the San Joaquin Valley was first noted in 1935
when I. H. Althouse, a consulting engineer, called attention to the
possibility of land subsidence near the Delano (Tulare-Wasco)
area. The process was first described in print by Ingerson (1941, p.
4042), who presented a map and profiles of land subsidence based
on comparison of leveling of 1902, 1930, and 1940. Four types of
subsidence are known to occur in the San Joaquin Valley. In order
of decreasing magnitude they are (1) subsidence caused by aquifer-
system compaction due to the lowering of ground-water levels by
sustained ground-water overdraft; (2) subsidence caused by the
hydrocompaction of moisture-deficient deposits above the water-
table; (3) subsidence related to fluid withdrawal from oil and gas
fields; and (4) subsidence related to crustal neotectonic move-
ments. Aquifer-system compaction and hydrocompaction have
significantly lowered the land surface in the valley since about the
1920s, and our review of the subsidence problems there is limited
to these two primary causes.
THE SAN JOAQUIN VALLEY IS PART
OF A GREAT SEDIMENT-FILLED TROUGH
The San Joaquin Valley comprises the southern two-thirds of the
Central Valley of California. Situated between the towering Sierra
Nevada on the east, the Diablo and Temblor Ranges to the west,
and the Tehachapi Mountains to the south, the valley occupies a
trough created by tectonic forces related to the collision of the Pa-
cific and North American Plates. The trough is filled with marine
sediments overlain by continental sediments, in some places thou-
sands of feet deep, deposited largely by streams draining the
mountains, and partially in lakes that inundated portions of the
valley floor from time to time. More than half the thickness of the
continental sediments is
composed of fine-grained (clay, sandy clay,
sandy silt, and silt) stream (fluvial) and lake (lacustrine) deposits
susceptible to compaction.
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Tulare Lake Bed
Buena Vista Lake Bed
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25
San Joaquin Valley, California
The valley floor, comprising about 10,000 square miles, is arid to
semiarid, receiving an average of 5 to 16 inches of rainfall annually.
Most of the streamflow in the valley enters from the east side in
streams draining the western Sierra Nevada, where much of the
precipitation occurs as snow. The San Joaquin River begins high in
the Sierra Nevada and descends onto the valley floor, where it takes
a northerly flow path toward the Sacramento-San Joaquin Delta.
On its course northward to the Delta it collects streamflow from
the central and northern portions of the valley. The southern valley
receives streamflow from the Kings, Kaweah, and Kern Rivers,
which issue from steeply plunging canyons onto broad, extensive
alluvial fans. Over many thousands of years, the natural flow of
these rivers distributed networks of streams and washes on the
slopes of the alluvial fans and terminated in topographically closed
sinks, such as Tulare Lake, Kern Lake, and Buena Vista Lake.
Streams draining the drier western slopes and Coast Ranges adja-
cent to the valley are intermittent or ephemeral, flowing only epi-
sodically. Precipitation and streamflow in the valley vary greatly
from year to year.
Pumping for irrigation altered the ground-water budget
Ground water occurs in shallow, unconfined (water table) or par-
tially-confined aquifers throughout the valley. Such aquifers are
particularly important near the margins of the valley and near the
toes of younger alluvial fans. A laterally extensive lacustrine clay
known as the Corcoran Clay is distributed throughout the central
and western valley. The Corcoran Clay, which varies in thickness
from a feather edge to about 160 feet beneath the present bed of
Tulare Lake, confines a deeper aquifer system that comprises fine-
grained aquitards interbedded with coarser aquifers. Most of the
subsidence measured in the valley has been correlated with the
distribution of ground-water pumpage and the reduction of water
levels in the deep confined aquifer system.
Meltwater from the Sierra
snowpack recharges ground
water in the San Joaquin Val-
ley and supplies surface wa-
ter during the dry summer
months.
(California Department of Water Resources)
Mining Ground Water26
Under natural conditions before development, ground water in the
alluvial sediments was replenished primarily by infiltration through
stream channels near the valley margins. The eastern-valley streams
carrying runoff from the Sierra Nevada provided most of the re-
charge for valley aquifers. Some recharge also occurred from pre-
cipitation falling directly on the valley floor and from stream and
lake seepage occurring there. Over the long term, natural replenish-
ment was dynamically balanced by natural depletion through
ground-water discharge, which occurred primarily through evapo-
transpiration and contributions to streams flowing into the Delta.
The areas of natural discharge in the valley generally corresponded
with the areas of flowing, artesian wells mapped in an early USGS
investigation (Mendenhall and others, 1916). Direct ground-water
outflow to the Delta is thought to have been negligible.
Today, nearly 150 years since water was first diverted at Peoples Weir
on the Kings River and more than 120 years after the first irrigation
colonies were established in the valley, intensive development of
ground-water resources for agricultural uses has drastically altered
the valleys water budget. The natural replenishment of the aquifer
systems has remained about the same, but more water has dis-
charged than recharged the aquifer system; the deficit may have
amounted to as much as 800,000 acre-feet per year during the late
1960s (Williamson et al., 1989). Most of the surface water now being
imported is transpired by crops or evaporated from the soil. The
amount of surface-water outflow from the valley has actually been
PREDEVELOPMENT
POSTDEVELOPMENT
Corcoran clay
(confining layer)
Deep
aquifer
system
Shallow
aquifer
system
Water
table
Potentiometric surface in
deep aquifer system
Clayey
lenses
River
Slough
Water
movement
Fresh water
recharge
Sierra Nevada
Coast Ranges
Subsidence area
Pumping wells
Bedrock
Marine deposits
Ground water flowed from
the mountains toward the
center of the valley where it
discharged into streams or
through evapotranspiration.
Ground water flows generally
downward and toward
pumping centers.
27
San Joaquin Valley, California
reduced compared to predevelopment conditions. Ground water in
the San Joaquin Valley has generally been depleted and redistributed
from the deeper aquifer system to the shallow aquifer system. This
has created problems of ground-water quality and drainage in the
shallow aquifer system, which is infiltrated by excess irrigation water
that has been exposed to agricultural chemicals and natural salts
concentrated by evapotranspiration.
A STABLE WATER SUPPLY IS DEVELOPED FOR IRRIGATION
In the San Joaquin Valley, irrigated agriculture surged after the 1849
Gold Rush and again in 1857, when the California Legislature
passed an act that promoted the drainage and reclamation of river-
bottom lands (Manning, 1967). By 1900, much of the flow of the
Kern River and the entire flow of the Kings River had been diverted
through canals and ditches to irrigate lands throughout the south-
ern part of the valley (Nady and Laragueta, 1983). Because no sig-
nificant storage facilities accompanied these earliest diversions, the
agricultural water supply, and hence crop demand, was largely lim-
ited by the summer low-flows. The restrictions imposed by the need
for constant surface-water flows, coupled with a drought occurring
around 1880 and the fact that, by 1910, nearly all the available sur-
face-water supply in the San Joaquin Valley had been diverted,
prompted the development of ground-water resources.
The first development of the ground-water resource occurred in
regions where shallow ground water was plentiful, and particularly
where flowing wells were commonplace, near the central part of the
valley around the old lake basins. Eventually, the yields of flowing
wells diminished as water levels were reduced, and it became neces-
sary to install pumps in wells to sustain flow rates. Around 1930, the
development of an improved deep-well turbine pump and rural
electrification enabled additional ground-water development for
irrigation. The ground-water resource had been established as a
reliable, stable water-supply for irrigation. Similar histories were
repeated in many other basins in California and throughout the
Southwest, where surface water was limited and ground water was
readily available.
Overhead and flood irriga-
tion supply water to a wide
range of crops.
(California Department of
Water Resources)
By pumping the vast reserves
of ground water, farmers have
developed the San Joaquin
Valley into a major agricultural
region.
(California Department of Water Resources)
Mining Ground Water28
WATER WITHDRAWAL CAUSED LAND SUBSIDENCE
Shortly after the completion of the Delta-Mendota Canal by the
U.S. Bureau of Reclamation in 1951, subsidence caused by with-
drawal of ground water in the northern San Joaquin Valley had
begun to raise concerns, largely because of the impending threat to
the canal and the specter of remedial repairs. Because of this threat
to the canal, and in order to help plan other major canals and engi-
neering proposed for construction in the subsiding areas, the
USGS, in cooperation with the California Department of Water
Resources, began an intensive investigation into land subsidence in
the San Joaquin Valley. The objectives were to determine the causes,
rates, and extent of land subsidence and to develop scientific crite-
ria for the estimation and control of subsidence. The USGS con-
currently began a federally funded research project to determine
the physical principles and mechanisms governing the expansion
and compaction of aquifer systems resulting from changes in aqui-
fer hydraulic heads. Much of the material presented here is drawn
from these studies.
In 1955, about one-fourth (almost 8 million acre-feet) of the total
ground water extracted for irrigation in the United States was
pumped in the San Joaquin Valley. The maximum changes in water
levels occurred in the western and southern portions of the valley,
in the deep confined aquifer system. More than 400 feet of water-
Extraction of ground water for
irrigation in the San Joaquin Valley
Total ground-water extractions
for irrigation, 1955
0
4
8
1900 1920 1940 1960
0
20
40
Pumpage
(millions of
acre-feet)
Pumpage
(millions of
acre-feet)
United States California San Joaquin Valley
(Joseph F. Poland, U.S. Geological Survey,
written communication, ca 1957)
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0 to 40
Change in water-table altitude
from 1860 to spring 1961
0 to 40
40 to 100
Greater than 100
0 to 40 Rise
Change in water level in the deep confined
aquifer system from 1860 to spring 1961
0 to 40
40 to 120
Decline
120 to200
200 to 300
Greater than 300
Ground-water level change (feet)
Ground-water level change (feet)
Rise
Decline
(Williamson and others, 1989)
29
San Joaquin Valley, California
level decline occurred in some west-side areas in the deep aquifer
system. Until 1968, irrigation water in these areas was supplied
almost entirely by ground water. As of 1960, water levels in the deep
aquifer system were declining at a rate of about 10 feet per year.
Western and southern portions of the valley generally experienced
more than 100 feet of water-level decline in the deep aquifer sys-
tem. Water levels in the southeastern and eastern portions of the
valley were generally less affected because some surface water was
also available for irrigation. In the water-table aquifer, few areas
exceeded 100 feet of water-level decline, but a large portion of the
southern valley did experience declines of more than 40 feet. In
some areas on the northwest side, the water-table aquifer rose up to
40 feet due to infiltration of excess irrigation water.
Accelerated ground-water pumpage and water-level declines, prin-
cipally in the deep aquifer system during the 1950s and 1960s,
caused about 75 percent of the total volume of land subsidence in
the San Joaquin Valley. By the late 1960s, surface water was being
diverted to agricultural interests from the Sacramento-San Joaquin
Delta and the San Joaquin River through federal reclamation
projects and from the Delta through the newly completed, massive
State (California) Water Project. Less-expensive water from the
Delta-Mendota Canal, the Friant-Kern Canal, and the California
Aqueduct largely supplanted ground water for crop irrigation.
Ground-water levels began a dramatic period of recovery, and sub-
sidence slowed or was arrested over a large part of the affected area.
Water levels in the deep aquifer system recovered as much as 200
feet in the 6 years from 1967 to 1974 (Ireland and others, 1984).
When water levels began to recover in the deep aquifer system,
aquifer-system compaction and land subsidence began to abate,
although many areas continued to subside, albeit at a lesser rate.
During the period from 1968 to 1974, water levels measured in an
observation well near Cantua Creek recovered more than 200 feet
while another 2 feet of subsidence continued to accrue. This appar-
ent contradiction is the result of the time delay in the compaction
By 1971 the growing use
of imported surface-water
supplies surpasses the use
of local ground-water sup-
plies, but the effects of
drought reverse this trend
in 1977.
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(Modified from Poland
and others, 1975)
0 40 Miles
0 40 Kilometers
Land subsidence from 1926 to 1970
Less than 1
1 to 4
4 to 8
8 to 12
12 to 16
16 to 24
Greater than 24
Decline
Subsidence (feet)
Ground water
Surface water
0
600
1200
1970 1974 19761972
no data
no data
1968
Annual total
water use
(millions of
acre-feet)
Mining Ground Water30
of the aquitards in the aquifer system. The delay is caused by the
time that it takes for pore-fluid pressures in the aquitards to equili-
brate with the pressure changes occurring in the aquifers, which
are much more responsive to the current volume of ground-water
being pumped (or not pumped) from the aquifer system. The time
needed for pressure equilibration depends largely on the thickness
and permeability of the aquitards. Typically, as in the San Joaquin
Valley, centuries will be required for most of the pressure equilibra-
tion to occur, and therefore for the ultimate compaction to be real-
ized. Swanson (1998) states that Subsidence is continuing in all
historical subsidence areas. . . , but at lower rates than before. . . .
Since 1974, land subsidence has been greatly slowed or largely ar-
rested but remains poised to resume. In fact, during the severe
To supplement local ground-
water supplies, the California
Aqueduct (left) conveys water
from the Delta to the dry south-
ern valleys.
When water levels recover,
compaction and land subsid-
ence can abate.
400
200
600
12
0
1960 1970 1980 1990
Depth to
water
(feet below
land surface)
Compaction
(inches)
Deep well
During the droughts of 1976 –77 and
1987–91, deliveries of imported water
to the west side of the San Joaquin
Valley were cut back. More ground
water was pumped to meet the de-
mand, resulting in a drop in the water
table and consequent compaction.
Some elastic expansion of the aquifer
system has occured, but the com-
pacted materials can never return to
their pre-compacted thickness.
no data
Drought Drought
no data
(Modified from Swanson, 1998)
(California Department of Water Resources)
31
San Joaquin Valley, California
droughts in California in 197
6
77 and 198791, diminished deliv-
eries of imported water prompted some water agencies and farm-
ers, especially in the western valley, to refurbish old pumping
plants, drill new wells, and begin pumping ground water to make
up for cutbacks in the imported water supply. The decisions to re-
new ground-water pumpage were encouraged by the fact that
ground-water levels had recovered nearly to predevelopment levels.
During the 197677 drought, after only one-third of the peak an-
nual pumpage volumes of the 1960s had been produced, ground-
water levels rapidly declined more than 150 feet over a large area
and subsidence resumed. Nearly 0.5 feet of subsidence was mea-
sured in 1977 near Cantua Creek. This scenario was repeated dur-
ing the more recent 198791 drought. It underscores the sensitive
dependence between subsidence and the dynamic state of im-
ported-water availability and use.
That a relatively small amount of renewed pumpage caused such a
rapid decline in water levels reflects the reduced ground-water stor-
age capacitylost pore spacecaused by aquifer-system compac-
tion. It demonstrates the nonrenewable nature of the resource
embodied in the water of compaction. It emphasizes the fact that
extraction of this resource, available only on the first cycle of large-
scale drawdown, must be viewed, like more traditional forms of
mining, in terms not only of its obvious economic return but also
its less readily identifiable costs.
In the major subsiding areas,
subsidence has continued ex-
cept for a slight leveling off in
the mid 1970s.
12
6
0
1930 1940 1950 1960 1970
Subsidence
(volume measured,
millions of acre-feet)
Major subsiding areas
in the San Joaquin Valley
A
A
B
B
C
C
S
a
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J
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a
q
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R
.
Fresno
Cantua Creek
Bakersfield
18
36
0
Estimated cumulative
pumpage, in area A
(millions of acre-feet)
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A
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E
S
(Modified from Poland and others, 1975)
A 1to3 ratio between subsi-
dence and pumpage in area A
reflects the portion of total
pumpage that was initially de-
rived from water of compaction.
Mining Ground Water32
Hydrocompaction
Compaction near the surface
Hydrocompactioncompaction due to wetting is a
near-surface phenomenon that produces land-surface
subsidence through a mechanism entirely different from
the compaction of deep, overpumped aquifer systems.
Both of these processes accompanied the expansion of irri-
gated agriculture onto the arid, gentle slopes of the alluvial
fans along the west side and south end of the San Joaquin
Valley. Initially, the distinction between them, and their
relative contributions to the overall subsidence problem,
were not fully recognized.
In the 1940s and 50s farmers bringing virgin valley soils
under cultivation found that standard techniques of flood
irrigation caused an irregular settling of their carefully
graded fields, producing an undulating surface of hollows
and hummocks with local relief, typically of 3 to 5 feet.
Where water flowed or ponded continuously for months,
very localized settlements of 10 feet or more might occur
on susceptible soils. These consequences of artificial wet-
ting seriously disrupted the distribution of irrigation water
and damaged pipelines, power lines, roadways, airfields,
and buildings. In contrast to the broad, slowly progressive
and generally smooth subsidence due to deep-seated aqui-
fer-system compaction, the irregular, localized, and often
rapid differential subsidence due to hydrocompaction was
readily discernible without instrumental surveys. Recogni-
tion of its obvious impact on the design and construction
of the proposed California Aqueduct played a major role
in the initiation in 1956 of intensive studies to identify,
characterize, and quantify the subsidence processes at
work beneath the surface of the San Joaquin Valley.
The mechanisms and requisite conditions for hydrocom-
paction, initially known as near-surface subsidence, were
investigated by means of laboratory tests on soil cores from
depths to 100 or more feet, and by continuously flooded
test plots equipped with subsurface benchmarks at various
depths and, in some cases, with soil-moisture probes.
The combined field and laboratory studies demonstrated
that hydrocompaction occurred only in alluvial-fan sedi-
ments above the highest prehistoric water table and in
areas where sparse rainfall and ephemeral runoff had never
MECHANISMS OF COMPACTION
WERE ANALYZED
Hydrocompaction produces an
undulating surface of hollows and
hummocks with local relief, typically
of 3 to 5 feet. In this view of a
furrowed field, the hollows are filled
with irrigation water.
Hydrocompaction caused surface
cracks and land subsidence at experi-
mental Test Plot B, Fresno County.
33
San Joaquin Valley, California
Mudflow containing
hydrocompactible sed-
iments, western Fresno
County (1961)
Prewetting a new section of
the California Aqueduct to
precompact shallow deposits
susceptible to hydrocompaction
(near toe of Moreno Gulch, 1963)
penetrated below the zone subject to summer desiccation
by evaporation and transpiration. Under these circum-
stances the initial high porosity of the sediments (often
enhanced by numerous bubble cavities and desiccation
cracks) is sun-baked into the deposits and preserved by
their high dry strength, even as they are subjected to the
increasing load of 100 or more feet of accumulating over-
burden. In the San Joaquin Valley, such conditions are
associated with areas of very low average rainfall and infre-
quent, flashy, sediment-laden runoff from small, relatively
steep upland watersheds that are underlain by easily erod-
able shales and mudstones. The resulting muddy debris
flows and poorly sorted stream sediments typically contain
montmorillonite clay in proportions that cause it to act,
when dry, as a strong interparticulate bonding agent.
When water is first applied in quantities sufficient to pene-
trate below the root zone the clay bonds are drastically
weakened by wetting, and the weight of the overburden
crushes out the excess porosity. The process of densifying
to achieve the strength required to support the existing
overburden may reduce the bulk volume by as much as 10
percent, the amounts increasing with increasing depth and
overburden load.
Most of the potential hydrocompaction latent in anoma-
lously dry, low-density sediments is realized as rapidly as
the sediments are thoroughly wetted. Thus the progression
of a hydrocompaction event is controlled largely by the
rate at which the wetting front of percolating water can
move downward through the sediments. A site underlain
by a thick sequence of poorly permeable sediments may
continue to subside for months or years as the slowly
descending wetting front weakens progressively deeper
deposits. If the surface water source is seasonal or inter-
mittent, the progression is further delayed.
Localized compaction beneath a water-filled pond or ditch
often leads to vertical shear failure at depth between the
water-weakened sediments and the surrounding dry mate-
rial. At the surface this process surrounds the subsiding
flooded area with an expanding series of concentric ten-
sional fissures having considerable vertical offseta
severely destructive event when it occurs beneath an engi-
neered structure.
The hazards presented by hydrocompaction are somewhat
mitigated by the fact that the process goes rapidly to com-
pletion with the initial thorough wetting, and is not subject
to reactivation through subsequent cycles of decreasing
and increasing moisture content. However, if the volume
of water that infiltrates the surface on the first wetting cycle
is insufficient to wet the full thickness of susceptible depos-
its, then the process will propagate to greater depths on
subsequent applications, resulting in renewed subsidence.
Also, an increase in the surface load such as a bridge foot-
ing or a canal full of water can cause additional compaction
in prewetted sediments.
Studies undertaken in the mid-1950s
led to a better understanding of hy-
drocompaction and to the identifica-
tion of long reaches of the California
Aqueduct route that were underlain
by deposits susceptible to hydro-
compaction. Construction of the
aqueduct through these reaches was
preceded by prewetting, and thus
compacting to a nearly stable state,
the full thickness of susceptible de-
posits beneath the aqueduct align-
ment. These measures added more
than two years and tens of millions
of dollars to the cost of the project.
Mining Ground Water34
MANY COSTS OF LAND SUBSIDENCE ARE HIDDEN
The economic impacts of land subsidence in the San Joaquin Valley
are not well known. Damages directly related to subsidence have
been identified, and some have been quantified. Other damages
indirectly related to subsidence, such as flooding and long-term
environmental effects, merit additional assessment. Some of the
direct damages have included decreased storage in aquifers, partial
or complete submergence of canals and associated bridges and pipe
crossings, collapse of well casings, and disruption of collector drains
and irrigation ditches. Costs associated with these damages have
been conservatively estimated at $25,000,000 (EDAW-ESA, 1978).
These estimates are not adjusted for changing valuation of the dol-
lar, and do not fully account for the underreported costs associated
with well rehabilitation and replacement. When the costs of lost
property value due to condemnation, regrading irrigated land, and
replacement of irrigation pipelines and wells in subsiding areas are
included, the annual costs of subsidence in the San Joaquin Valley
soar to $180 million per year in 1993 dollars (G. Bertoldi and S.
Leake, USGS, written communication, March 30, 1993).
... Land subsidence is a geological phenomenon affecting a large number of cities worldwide, frequently caused by excessive extraction of groundwater, oil, and gas in addition to mining (mostly coal) or natural causes, [1][2][3][4][5] with damaging effects to infrastructures and buildings such as towers, bridges, railways, and agricultural areas. [6][7][8][9][10] Land subsidence caused by overexploitation of groundwater resources is one of the most common types around the world. For instance, San Joaquin Valley in California is one of the most famous plains in the world where land surface has subsided roughly 9 m from 1925 to 1977 due to withdrawal of groundwater. ...
... The backward pass, on the other hand, starts at the output layer by passing the error signals backward through the network, layer by layer, and recursively computing the δ (the local gradient) for each neuron according to Eq. (7). It then applies a correction, Δω ji ðnÞ, to the synaptic weight, ω ji ðnÞ, which is defined by the delta rule [Eq. ...
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... On the western side of the valley, in communities such as Alpaugh, the Corcoran clay layer plays a converse role. This impermeable layer requires that CWSs relying on groundwater drill deeper wells (Galloway and Riley 2006), but at these deeper levels wells are likely to draw naturally arsenic-laden water (Welch et al. 2000;Gao et al. 2007). ...
... Cross-section of the valley, with Corcoran Clay layer on the left (west) and the shallower aquifers on the right (east). Adapted from Galloway and Riley (2006). representation to draw on for infrastructure improvements. ...
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