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| North America Desertification characteristics. |
NORTH AMERICA
Overgrazing has left a lasting imprint on the arid lands of North America, and
little has been accomplished in attempts to restore the land to its original
productivity. Wind and water erosion are extensive and salinization and
waterlogging-- of varying degrees of severity--are common in nearly all of the
irrigated valleys. At present, desertification probably has been stabilized, with
some improvement in plant cover of rangelands and in control of erosion and
waterlogging over the past 30 years. Salinization of irrigated land is still
occurring and the dryland saline seep problem is getting worse.
The overgrazing that destroyed or severely altered the original grass cover of
the rangelands of Mexico and the United States began in Mexico after the
Spanish conquest and spread into the U.S. Southwest. By the early part of the
l9th century, overgrazing was already a fact on both sides of the international
boundary. With the explosive expansion of cattle numbers in the Southwest
when the railroads arrived, range carrying capacities were greatly exceeded,
and that situation continued well into the 20th century. The increased gully
(arroyo) formation that occurred in overgrazed rangelands during the latter part
of the l9th century has been associated with range deterioration, erosion,
salinization, and waterlogging first received attention from research
organizations in the late 1800s and early l900s. Many of the basic principles for
controlling desertification were established at that time. However, application of
the principles did not become widespread until the 1940s and 195Os. Even
though solutions are available now, much remains to be done to put them into
practice.
Desertification Characteristics
There are about 450 million hectares in the arid regions of Canada, the United
States, and Mexico. Approximately two-thirds of that total is moderately
desertified and less than one-third severely desertified, with a considerable
area of slightly desertified land and four small delineations of very severely
desertified land (Figure 5).
In general, rangelands are moderately desertified in the north and severely
desertified in the south. The dividing line approximates the ecotone between
sagebrush (cool climate) and creosote bush (hot climate). Desertification is the
result of a combination of overgrazing and water erosion. The four delineations
of very severely desertified land on the map are in rangelands. Three of those
four areas in New Mexico, Texas, and Chihuahua suffered first from
overgrazing, then from wind erosion. The result is small hummocks of
mesquite-topped sand dunes separated by completely barren blow-outs. The
fourth-- and largest--of the very severely desertified areas is on the Navajo
Indian Reservation of northern New Mexico and Arizona. Overgrazing by sheep
has removed most of the perennial grasses and exposed the easily eroded,
shale-derived soils to extensive sheet and gully erosion. It is unlikely that any of
those four areas will recover naturally in the next hundred or more years, even if
grazing is completely prohibited.
Wind erosion on dry-farmed lands is the second most extensive desertification
process. The ravages of the drought-inspired Dust Bowl of the 1930s were but
an extreme example of the damage wind erosion has caused in the western
Great Plains, the Pacific Northwest and elsewhere. Wind erosion of cultivated
land, whether dry farmed or irrigated, has been a problem ever since the
original sod cover was broken in the dry regions. It is an ever-present threat,
worsening during droughts and lessening during wet periods, but always
occurring some-where every year. While sandy soils are the most susceptible,
even line textured soils will blow when conditions are right, and along with the
dust goes organic matter and nutrient elements. The introduction of sprinkler
irrigation systems, especially the center pivot systems, has enabled previously
suitable rolling sandy lands to be cropped successfully. If and when those soils
are abandoned, for reasons of economy or shortage of water, the United States
will face an even greater wind erosion threat than it has had to cope with in the
past.
Accelerated water erosion has been especially serious on overgrazed
rangelands and on cropland in the Palouse region of the Pacific Northwest and
in the highlands of Mexico. Normal geologic erosion has been severe in the
Badlands of South Dakota and in the shale materials of Wyoming, Colorado,
Utah, and New Mexico but man has managed to make matters worse through
overgrazing and cultivation.
While salinity and waterlogging problems are found in nearly every irrigated
valley, the only areas large enough to be shown are in the San Joaquin Valley of
California, the contiguous Imperial and Mexicali valleys, and the lower valley of
the Rio Grande. Land degradation is more severe in the Mexicali Valley than in
the adjoining Imperial Valley because (1) the salinity of the irrigation water is
worse where Mexico diverts Colorado River water than where the Imperial
Valley does, and (2) the Mexicali Valley does not have the intensive drainage
network that the Imperial Valley has. Waterlogging is more of a problem in the
northern states of the United States, whereas the combination of waterlogging
and salinization is more serious in the south. Serious salinity problems are
associated with marine shales in the numerous small irrigated valleys in the
upper Colorado River watershed.
The special problem of saline seeps in nonirrigated croplands is widespread in
the states of Montana, South Dakota, and North Dakota and in the prairie
provinces of Manitoba, Saskatchewan, and Alberta (Vander Pluym, 1978). The
affected areas are too scattered to be delineated on the map.
Soil compaction and soil crusting are problems associated with medium-and
fine-textured soils. Man-induced compacted subsoil layers are especially
common in fine-textured irrigated soils, and in medium-to moderately
coarse-textured dryland soils. Surface crusting occurs on irrigated, dryland, and
rangeland soils.
Magnitude of Desertification
Nearly 90 percent of North American arid lands are moderately and severely
desertified (Table 6). The status of the U.S. rangelands does not appear to have
been much improved in recent years, and less than 20 percent is producing
anywhere near its potential. It is likely that the same condition prevails in Mexico.
Data on the status of wind and water erosion in the arid regions are not
available. A 1979 report of the U.S. Soil Conservation Service said that 58
percent of the nation's cropland needed conservation treatment of an
unspecified character. That figure was down from 64 percent in 1967. The
major conservation treatment undoubtedly would be water erosion control.
An indication of the magnitude of the wind erosion problem can be obtained
from annual surveys conducted by the U.S. Soil Conservation Service for the
Great Plains. Since 1935, surveys have been made of the amount of land
damaged by wind erosion in susceptible parts of the Great Plains. Annual totals
have varied from about 400,000 hectares to a maximum of 6,000,000 hectares.
The lowest amount of land damage occurred during the wet 1940s and 1960s;
the highest was in 1955 during a protracted drought. In any one year, the
percentage of damaged crop-land varied from 0.6 to 9 percent of the total
cropland surveyed. There is no discernible trend upward or downward in the
area damaged; the dryness of the particular year seems to determine how bad
wind erosion will be.
Wind erosion in the prairie provinces of Canada--and the associated loss in soil
fertility--has long been recognized as a serious problem (Soil Research
Laboratory Staff, 1949). Soil and crop conditions are similar on both sides of
the international boundary in the northern Great Plains, and so are erosional
problems. Wind erosion is much less of a problem than water erosion in the arid
regions of Mexico, where sandy soils are less prevalent in cropped areas than
is the case in Canada and the United States.
Salinization is estimated to affect adversely about 25 percent of the irrigated
land in the United States and a somewhat greater percentage in Mexico.
Dryland saline seepage is a Canada-United States problem, with about 0.8
million hectares severely affected and perhaps another 2.7 million slightly or
moderately affected. Waterlogging may affect about the same area as
salinization but the two problems are not always found together.
Techniques for controlling desertification in North America are well-known.
Recognition of the seriousness of the problem varies from year to year, usually
depending on the amount of notoriety droughts or floods receive. In the United
States, where soil conservation has been given considerable financial support
at the federal level, only about one-third of the nation's cropland has been
adequately treated against soil erosion. Much less than one-third of the
depleted rangeland has been improved. The situation may be better in Canada
but probably is worse in Mexico.
The reasons for the relatively poor performance in combating desertification are
numerous. High among them is the emphasis on short-term versus long-term
benefits (Boggess et al., 1979), inability to finance improvements, lack of
clear-cut proof that control is profitable in the immediate future, and the absence
of a public sense of urgency toward controlling land degradation (General
Accounting Office, 1977).