Groundwater Overdraft
--From Nabhan, Gary Paul and Andrew R. Holdsworth.1998.
State of the Sonoran Desert Biome: Uniqueness, Biodiversity,
Threats and the Adequacy of Protection in the Sonoran Bioregion.
Sponsored by The Wildlands Project. p. 36
Within the first quarter of the twentieth century, many of
the Sonoran Desert's Pleistocene aquifers moved from a state
of hydrologic balance to one of severe depletion, or overdraft.
By 1923, groundwater pumpage surpassed water recharge in Arizona.
With the continued development of relatively inexpensive and
more powerful mechanized pumps, groundwater overdraft increased
exponentially in agricultural areas of the Sonoran Desert.
In combination with water diversion, groundwater pumping has
affected nearly all river valleys in Arizona's portion of
the Sonoran Desert (Figure 5).
Large expanses of riparian forest and mesquite woodlands have
died as groundwater levels declined. While other biotic communities
are also affected by water table declines, the relationship
between their vegetation changes and lowering groundwater
levels is still largely unexplored (Bahre 1991). Nevertheless,
it is clear that groundwater pumping immediately outside protected
areas can devastate the vegetation within them (Nabhan and
Klett 1994), and ultimately effect faunas.
In the heart of agricultural areas groundwater declines
have been precipitous. In 18% of 56 groundwater basins in
all Sonoran Desert states (excluding California) there have
been groundwater declines of over 1 meter per year (Figure
6)'. In another 16% of the basins, groundwater declines have
ranged from 0.3 - 1 meter per year. In the Carefree sub-basin
northeast of Phoenix water levels in one area have dropped
over 3 meters per year, largely due to golf course development
(ADWR 1994b). In the area around Casa Grande, Arizona groundwater
levels have dropped up to 150 meters since 1920 (ADWR 1994a).
In the 1940s this agriculture-induced drawdown became the
principal cause of the death of the once extensive mesquite
bosque at Casa Grande National -Monument (Judd 1971). The
creation of the Costa de Hermosillo irrigation istrict was
the direct cause of the loss of the extensive mesquite bosques
in the delta of the Rio Sonora. In the Rio Yaqui and Rio Mayo
deltas, over a million hectares of coastal thornscrub, riparian
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'Descriptions of basin-wide groundwater level changes are
as representative as the number of wells that are monitored,
the conditions under which they are monitored and the period
of time over which they are monitored. Figure
6 shows levels measured over a wide variety of time periods
and some only until the late 1970s and early 1980s. Also,
groundwater level declines are not just a factor of the volume
and rate at which the water is pumped but also the geohydrological
factors surrounding the well and the rate of recharge. However,
the available information does provide the means for an initial
regionwide assessment of where groundwater overdraft has been
a more severe problem.
Forests, and mesquite woodlands have been lost (Búrquez and
Martínez-Yrízar 1997). In the upper San Pedro basin of southeastern
Arizona, water levels have declined an average of 0.4 meters
per year in the vicinity of a large cone of depression in
Sierra Vista, Arizona (Lacher 1994). While some efforts are
being made to prevent the growth of this cone of depression
and its lessening of surface water flows, negative impacts
on the life sustaining flows of the riparian area are likely
(Lacher 1994). Stromberg et al. (1996) suggests that just
a 0.3 meter decline in riparian zone water levels could reduce
key species such as rushes, and a 1 meter drop would eliminate
them, reducing willow coverage by 51 percent, and allowing
mesquite and tamarisk to expand, creating "desertification"
of riparian areas.
In the Sonoran Desert, areas with the highest levels of groundwater
extraction, soil compaction, land subsidence and associated
fissuring are secondary effects of groundwater overdraft.
These effects are of most concern in intensive agricultural
areas. Around Picacho Peak between Tucson and Phoenix, land
has subsided by one meter causing fissures which extend for
more than a kilometer. In the municipio of Caborca, Sonora
subsidence and fissuring are so extensive that roads have
been closed for fear that vehicles would disappear into subsurface
crevices (SARH, pers. Comm.) Fissuring can cause the abandonment
of agricultural fields (Anderson 1989) but then further delays
the recovery of these lands. In areas with natural vegetation,
fissuring changes runoff patterns which consequently could
alter the vegetation community.
For many areas of Arizona, the greatest damage from groundwater
overdraft has already occurred. Arizona groundwater pumping
peaked in 1974 at 5.7 million acre-feet but dropped to 3.2
million acre-feet in 1990. This reduction is due principally
to the statewide decline in irrigated cropland, but also to
above average precipitation and the use of Central Arizona
Project (CAP) water (de Kok 1997). As a result, groundwater
levels have leveled off and even rebounded in many Sonoran
Desert basins; 23% of basins have areas where groundwater
levels have increased (Figure 6).
Metropolitan sprawl has converted irrigated croplands to housing
subdivisions, but it remains unclear whether this necessarily
means that we are "saving water" over the long run.
The peak water withdrawal of 2.2 million acre-feet in the
late 1950s fell to less than one million acre-feet in the
late 1980s (de Kok 1997). However, the degree to which the
transfer of water from agriculture to urban use reduces long
term groundwater overdraft deserves careful analysis, taking
into account projections of population growth, per capita
water demand, and water pricing subsidies.
Reliance on the waters of the already over allocated Colorado
River is the lynch pin of most plans to reduce or cease Arizona's
overdraft of its Pleistocene aquifers. After such plans were
formulated, it became obvious that they will be difficult
to implement; further demands on surface water supplies and
prolonged aquifer overdraft seem likely. In the Phoenix Active
Management Area 2, projected water demands are expected to
be 17% higher in 2040 than in 1990 (ADWR 1994). The AMA's
goal is the attainment of safe yield (balance between groundwater
withdrawals and natural and artificial recharge) by 2025.
While safe-yield in the face of increased demand is expected
to be accomplished largely with CAP water, the means for achieving
this goal are still very uncertain. The Tucson AMA is also
projected to attain safe yield by 2025, but demand is anticipated
to be 30% higher, with overdraft expected to be 90,000 acre-feet
(67% of 1990 overdraft). This shortfall is due to the lack
of economic and management incentives to use renewable supplies
(ADWR 1994a). The goal for the 2 Active Management Areas (AMAs)
were established by the 1980 Arizona Groundwater Act in severely
water depleted areas. While the area of the Santa Cruz AMA
was separated from the Tucson AMA in 1994, the two are combined
for the purposes of this report.
Pinal AMA is planned phase-out of crop irrigation; agricultural
use will be extended as long as possible while still allowing
for municipal noninigation development. Water demands in Pinal
county are expected to grow 8% between 1990 and 2040. However
the lack of a municipal CAP allocation to one subbasin where
severe groundwater overdraft continues could imperil the long-term
economy of the Pinal AMA (ADWR 1994a), and hasten land subsidence
and fissuring.
(Bahre 1991).
(Nabhan and Klett 1994),
(ADWR 1994b).
(ADWR 1994a). de National -Monument (Judd 1971).
-Gary Paul Nabhan and Mark Klett Desert Legends(1994)
(Búrquez and Martínez-Yrízar 1997).
(Lacher 1994)
(Lacher 1994).
Stromberg et al. (1996)
(SARH, pers. Comm.)
(Anderson 1989)
(de Kok 1997)
(ADWR 1994).
(ADWR 1994a).
(ADWR 1994a)
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