Look At This
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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).
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