Water Resources in the Desert Southwest - Design with the Desert: Conservation and Sustainable Development - page 81

including decadal-scale climatic variation, groundwater pumping, and changing water
use for agriculture and riparian vegetation.
Groundwater levels fluctuate naturally and in response to water development. Riparian
vegetation is known to cause diurnal fluctuations of as much as 1 ft during the grow-
ing season. Natural fluctuation in water levels may exceed 5-10 ft over a period of years,
and low-frequency positive trends may be present owing to recharge beneath rivers or at
mountain fronts, or in response to decreases in pumping. Negative trends usually signal
effects of human withdrawals of groundwater, but persistent drought, channel downcut-
ting, or increasing use by riparian plants may also cause water-level declines.
In the arid and semiarid southwestern United States, groundwater has been an important
source of supply for agriculture, industry, and public use. Groundwater withdrawals in
Arizona escalated rapidly in the middle part of the twentieth century when large-capacity
turbine pumps became available 27 and electricity was brought to rural areas suitable for
agriculture. 28-30 Approximately 80% of groundwater withdrawal in Arizona is used for
agriculture, 28 although municipal use is increasing greatly owing to the rapid increase in
the state's population.* The volume of pumped groundwater statewide peaked around
1984 for several reasons. First, the landmark Arizona Groundwater Management Act,
passed in 1980, regulates groundwater in Active Management Areas (AMAs) around
major metropolitan areas, such as Tucson and Phoenix. ∗ Pumping intensified just prior
to this Act, which created an artificial high spike as water users attempted to justify
their allocations. Introduction of CAP water from the Colorado River, beginning in 1983,
delivered significant amounts of water for irrigation, municipal uses, and groundwater
recharge. 29 Finally, agricultural priorities shift with commodity markets, which can lead
to less water-intensive crops or suspension of farming. 30
Much of the large-scale development of groundwater in Arizona that began in the mid-
twentieth century was in basin-fill aquifers The conventional wisdom at that time was
that the “safe yield” of an aquifer was the rate of annual recharge and that an aquifer with
pumping exceeding safe yield was in a state of “overdraft.” Nonetheless, pumping in many
of the desert-basin aquifers greatly exceeded the rate of annual recharge. Furthermore,
undesired consequences came about in many cases, even at pumping at rates less than
the safe yield of an aquifer. Various methods have been developed to assess change in
groundwater conditions and address the question of safe yield, 31 but the most important
characteristic is the rate of decline in groundwater levels. Major problems associated with
large-scale pumping included falling water tables and increased pumping lift over large
areas; loss of water available to connected streams, springs, and wetlands; land subsidence
and surface fissuring29; 29 ; and degraded water quality.
Groundwater development on the Colorado Plateau has lagged that of development in
the basins in both timing and magnitude of withdrawals, but the needs for agriculture,
public supply, and industry have increased in recent decades. Major consequences of
groundwater development on the plateau have included loss of base flow in streams and
deepening water tables.
As previously discussed, reduced water availability to streams, springs, wetlands, and
riparian plants is a consequence of groundwater pumping both in the alluvial basins and
in the Colorado Plateau parts of the southwestern deserts. Concerns for preserving sur-
face-water flows and riparian systems stem from the need to protect surface-water rights,
as well as aquatic and terrestrial ecosystems associated with desert streams. As illustrated
in Figure 4.5, the effects of pumping on a groundwater-dependent riparian system can be
* http://www.azwater.gov/dwr/WaterManagement/Content/AMAs/default.htm (accessed February 9, 2009). 30