The picture above shows large scale spray irrigation of desert farmland in the state of Utah (Allan and Warren 1993). Irrigated agriculture is the largest consumer of water in all of the Southwestern states. The water used for irrigation in the Southwestern United States is obtained from both surface and groundwater sources (El-Ashry and Gibbons 1986). The ecological impacts of the extensive use of both of these water sources will be addressed in this section of our web page.
The main impacts of irrigated agriculture
in the Southwest are due to the building of dams and the diversion of water,
the waterlogging of soils, the overdraft of groundwater resources, and
the salinization of soil and water. "Consumptive" use is a term used
for water which is lost after its use, rather than immediately returned
to its source for reuse. Consumptive use of water by irrigation can
reach as high as 90% in the Southwest, and the table below lists the irrigated
acreage for all the Southwestern states and the percentage of the total
consumptive use of water that is accounted for by irrigation practices
(El-Ashry and Gibbons 1986).
Irrigation accounts for the majority of the consumptive use of water in the Southwest. This is because relatively more of the water diverted from rivers, streams, and aquifers evaporates from the soil or transpires from crops instead of returning to its source for reuse (El-Ashry and Gibbons 1986).
Building Dams and Diverting Water
One of the major reasons dams and reservoirs are built is to facilitate the diversion of water for irrigation. The building of dams leads to the flooding of the canyons and land behind the dam, the fragmentation of the habitat above and below the dam, the disruption of the natural level of water flow, an increase in water temperature, the build up of silt causing decreased nutrient availability downstream, and the creation of a barrier to movement along the river corridor (migratory fishes). For example, when the Colorado leaves Glen Canyon Dam the water is clear and no longer reflects the name colorado (color red). In addition, the water is around 50 degrees Ferenheit due to the fact that the water sent downstream is drawn from the bottom of Lake Powell (Wallace 1975).
The Colorado's native riparian species have adapted to the river's annual cycle of late spring flooding and lowered flow levels later in the season, so it is clear that habitat is drastically reduced and altered by water damming and diversion. Cohen (1997) also notes the importance of water quantity in determining water quality, and the affect that a habitat's water quality can have on aquatic and riparian species. Irrigated agriculture's need for the regulation and damming of rivers has been the single most important cause of the degradation and destruction of riparian habitat (Rosenberg at al. 1991 in Cohen 1997). Since the late 1900's numerous fish species have gone extinct due to the human activities of dam building and water diversion (Schoenherr 1986).
The diversion of water for irrigation
can be extremely harmful to the endemic species of the Southwestern United
States. The West used to be a lot less arid a long time ago, and
many springs and ponds contained populations that became isolated as the
land dried up (Hendrickson and Kubly 1984).
These isolated populations have evolved into endemic species restricted
to a particular spring or river segment (Cooperrider et al. 1995).
An example of this is the Devil's Hole pupfish found only in one tiny spring
at the edge of a 100 square km oasis in Southern Nevada (Cooperrider et
al. 1995). The diversion of waters is essentially the removal of available
habitat for fish and wildlife. In 1993, the U.S. Fish and Wildlife
Service included a large portion of the Colorado river within its designation
of critical habitat for endangered freshwater fish, and this led to the
development of a Multiple Species Conservation Program (MSCP) for the lower
Colorado River (Cohen 1997).
Waterlogging of Soils
some Desert Pupfish
Waterlogging is generally defined
as occurring in areas where the water table lies less than 2m from the
ground surface. When waterlogging reaches the root zone of plants,
yields are drastically decreased because the roots need air to survive
and they cannot survive in anaerobic conditioned (Wolman 1987). Many
agricultural areas in the southwest have poor drainage and when irrigation
water is applied the water table rises, resulting in waterlogging and severe
soil salinization. Many areas like the San Joaquin Valley in California,
have a tight layer of material that blocks the draining water's passage
downwards, and therefore the salty water builds up (Sheridan 1981).
In the San Joaquin Valley, crop yields have declined 10% with financial
losses of $31.2 million annually since 1970 because of high saline water
tables, and the losses are expected to rise to $321.3 million by 2000 if
no action is taken to stop the accumulation.
Groundwater overdraft occurs when
water is removed from an underground reservoir at a rate faster than it
is naturally being replenished. Groundwater overdraft is a significant
problem because it results in land subsidence and desertification.
In Arizona, land subsidence due to groundwater overdraft is a serious problem
because the water supporting the land is being mined. Arizona is
mining groundwater at almost twice the natural replenishment rate, and
90% of this water is used by irrigation (Wolman 1987). Arizona has
become the first state to limit the pumping of ground water (Wolman 1987).
Land subsidence in Arizona has resulted in more than 75 cracks in the earth,
and some fissures have been measured to as much as 25 ft wide and 60 ft
deep (Sheridan 1981). This
picture to the right shows a crack in the earth in Arizona caused by the
overdraft of groundwater (Sheridan 1981). This crack is at least
50 feet deep. Land subsidence ranges from as little as 20-30 cm to
up to 900 cm (in some parts of California), and from zones 10 square km
in size to 13500 square km (also in California) (Wolman 1987).
Ground water overdraft is also occurring in other states in the Southwest.
For example, most parts of western Texas rely heavily on the mining of
the Ogallala aquifer for irrigation water (El-Ashry and Gibbons 1986).
Each year between 5 and 8 million acre-feet of water are removed from the
Ogallala aquifer, and replaced by only 372,000 acre-feet. The mining
of groundwater for irrigation will eventually result in the abandonment
of irrigated cropland when the water runs out, resulting in desertification.
Abandoned fields lack any natural ground cover, and soil erosion on abandoned
fields is a very common problem (Sheridan 1981).
Salinization of Soil and Water
In the Southwestern United States,
the salinization of soil as well as both ground and surface waters is also
caused by irrigation. Salinization of soil is detrimental because
it causes habitat degradation, depressed populations of native plants and
animals, and desertification when salinity becomes high enough. The
surface waters and ground waters of the Southwest are naturally saline
due to leaching from the surrounding rocks, but irrigation has drastically
increased the amount of leaching. This pie graph shows the percentage
of salt contributed by different sources to the total saline content of
the waters of the Colorado River at Hoover Dam (M and I stands for municipal
and industrial) (Law and Hornsby 1982).
This pie graph makes it clear that irrigated agriculture is by far the single largest source of man made salinity in the Colorado River Basin. The salinity problem in the water is a result of two processes: salt loading - the addition of soluble salts to the river, and salt concentration - caused by the reduction in the volume of river water as a result of evaporation, transpiration by plants and water diversions (Miller et al. 1986). The principle dissolved ions in the Colorado are calcium, magnesium, sodium, sulfate, chloride, and bicarbonate. When chemically combined these dissolved salts are what produce high salinity levels in the Colorado river (Feldman 1991).
Salinization is a problem that irrigated agriculture has had since the first large human civilizations in Mesopotamia. The fall of the ancient civilization in the fertile crescent is attributed to both waterlogging and soil salinization (Rhoades 1990).
Salt loading of surface and ground water occurs as the irrigation water which did not evaporate or get absorbed by plants percolates down into the soil. Research has shown that under present irrigation practices plants use only about 1/2 of the water which is applied to irrigated fields (El-Ashry and Gibbons 1986). The excess water drains into the soil and either absorbs soluble minerals becoming saline or it displaces saline groundwater; the end result is the addition of water with high saline concentrations into the river system, or the groundwater storage (Miller et al. 1986).
In the case of the Colorado River, the largest river in the Southwestern United States, most of the salt loading occurs in the upper basin states (Utah and Colorado), and results in damages in the lower basin (Arizona, New Mexico, and California). Return flow from irrigation carries salts away from the irrigated lands and back to the surface or groundwater. The drainage water can also carry away fertilizers, pesticides, and other chemicals (Miller et al. 1986). The annual salinity of the Colorado River in 1982 ranged from 50 milligrams per liter at the headwaters of the Colorado in Colorado, to 900 milligrams per liter at Imperial Dam the most southern diversion point in the United States. The salinity of New Mexico's Pecos River increases from 760 milligrams per liter to 2,020 milligrams per liter in just 30 miles, and in Texas the Rio Grande River increases from 870 milligrams per liter to 4,000 milligrams per liter in 75 miles (El-Ashry and Gibbons 1986).
Salt concentration increases when water is taken out of a river, so there is less downstream flow to dilute the saline water. This occurs directly when water is removed for irrigation, but also when water evaporates from reservoirs. Of the 13.5 million acre-feet of average annual flow in the Colorado, 2.25 million acre-feet evaporate from large reservoirs such as lake Mead and lake Powell, and another 2.1 million acre-feet are consumed by upper basin agriculture (Feldman 1991).
Fish and wildlife are greatly affected by irrigation practices in the Southwest. The Colorado River system supports the largest list of threatened and endangered species in the United States (Miller et al. 1986). Of the 31 fishes listed as endangered by the U.S. Fish and Wildlife Service in 1981, 23 are desert species (Hendrickson and Kubly 1964). Toxic elements in agricultural runoff pose extensive environmental problems (El-Ashry and Gibbons 1986). Selenium, manganese, aluminum, iron, zinc, and copper may all be toxic to some fish. However, it is important to note that the effect of the increased salinity on native fish species is likely to be low, because these species evolved in an environment with naturally high levels of salinity. Studies conducted by the U.S. Fish and Wildlife Service's Colorado Fishery Project found that three native species, Colorado squawfish, bonytailed chub, and humpback chub, are all tolerant of high salt levels in the Colorado (Miller et al. 1986).
All irrigation water contains salts and salt concentrations in soils increase as irrigation water evaporates from the surface or is transpired by plants. The salts then accumulate in the soil (Wolman 1987). This salinization of the soil increases continuously and when it reaches a high enough level, the land can no longer support life (Wolman 1987). David Sheridan in his book Desertification of the United States calls soil salinization one of the "deadliest" forms of desertification. The Bureau of Reclamation estimates that salinity caused $91 million in total damages in 1983 and predicts a $267 million annual loss by 2010 (Miller et al. 1986).
Soil salinity results in a general decrease in plant growth rate, depending on how salt tolerant the species is, because as salinity increases so does the energy that must be used to extract water from the soil (Rhoades 1990). Stewart et al. (1977 in Rhoades 1990) found that salinity affects crop growth, because it results in reduced water uptake and transpiration.
The control of the Colorado's salinity is the responsibility of the national government. The 1974 Colorado River Basin Salinity Control Act maintains the salinity level of the river at 879 milligram per liter or lower at Imperial Dam. This Act also resulted in the construction of a multi-million dollar desalting plant in Yuma, Arizona in order to meet treaty obligations with Mexico (Feldman 1991). When large-scale diversion and salinization of the Colorado began Mexico suffered even more then the lower basin states, and a treaty was made up to improve international water relations.
So far, it seems like the Bureau of Reclamation has relied on expensive, structurally complex projects such as the building of the huge desalinization plant at Yuma to deal with the problem of salinization in the southwest. The new emphasis for the Bureau should be on reducing deep percolation, minimizing salt leaching, switching to agricultural crops that need less water, and retiring highly saline and marginally productive lands (El-Ashry and Gibbons 1986). Other alternatives include installing special drains, ditch linings, sprinkler and drip irrigation systems, leveling farms to achieve more uniform water application, and decreasing the amount of land devoted to agriculture in the Southwestern United States.
As a final note, it is important
to realize that salinity control projects can also be harmful to fish and
wildlife. Many projects involve reducing seepage of irrigation water from
canals and ditches. This practice does reduce salt loading, but it
can also reduce or eliminate riparian habitat that relies on the seepage.
Species that have been listed by the U.S. Fish and Wildlife Service as
possibly affected by salinity control projects include the blackfooted
ferret, Colorado squawfish, humpback chub, peregrine falcon, bald eagle,
whooping crane, Yuma clapper rail, and the Mesa Verde cacti and hookless
cacti (Miller et al. 1986). Canal lining can also be hazardous to
deer and antelope, which may be unable to cross the structures safely (Miller
et al. 1986).