Sheet n°281 - November 2007
Overgrazing accelerating soil erosion
in northern Mexico
n the countries of the
South, erosion is a process often exacerbated
by the high rainfall that
affects these regions
during the wet season.
Recent research conducted in Mexico by the IRD
and its partners (1) and
continued as an observation system by one of
them since the end of the
IRD-CENID RASPA programme, has led to better informed assessment
of the role of overgrazing
and tree clearance in
soil degradation (2). This
overexploitation of space
was shown to occur on
all the mountain slopes of
the western Sierra Madre
whereas the influence of
gully erosion was marginal. The soils in this northern region of Mexico
are being eroded at the
rate of several millimetres
each year. Larger-scale
investigations demonstrated that the current
land degradation strongly
reduces soils’ rainwater
storage capacity. This
new review of the state
of health of the soils of
this area of Mexico arouses cause for concern,
but it should be useful
for identifying more easily
the areas most vulnerable
to erosion in order better to protect them from
this process, particularly
by overgrazing control
measures.
© IRD/Jean-Louis Janeau
I
Visible results of overgrazing in northern Mexico.
Every year in the world an estimated
20 million hectares of arable land
are rendered infertile simply owing
to water-induced erosion. It is
therefore crucial to understand how
these processes arise in order better to
protect the layer of a few tens of metres
of fertile soil essential for plant growth
and therefore for sustaining agriculture.
In the North of Mexico, about ten years
ago IRD teams studied the erosion
phenomenon which affects this region
where pastoral activities and tree felling
aggravate the process.
As part of field studies conducted from
1993 to 2000 on the mountain crests
of the western Sierra Madre and in
the more arid regions in the south of
the Chihuahua Desert, the scientific
team established a soil classification
according to climatic and topographic
characteristics. They used a rainfall
index type hydrological model which
gives real-time simulation of the
humidity state of soil on the basis of
a range of parameters including soil
humidity, runoff rate, water storage
capacity. This measurement method
also takes into account the volume of
rain collected at a given moment and
the time lapsed since it fell. This model,
named NAZASM, with reference to
the Rio Nazas drainage basin where
it was first used in 1999, provides a
means of classifying each soil type
according to the processes that allow
infiltration of water but also its runoff on
the surface. From pine and oak forests
of the Sierra’s mountains, to the bare
expanses of the Chihuahua Desert
located at 1000 m altitude, the rainfall
was determined, varying locally from
1000 mm to less than 200 mm. The
NAZASM model led to the assessment
that only the drainage basins of the
sub-humid zone of the western Sierra
Madre had the capacity to let the
rainwater pass through the soil before
part of it could flow into water courses.
The soil degradation associated with
overgrazing could substantially reduce
this storage capacity. In the other,
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CONTACTS :
Luc DESCROIX
Laboratoire d'étude des
transferts en hydrologie et
environnement
Address :
LTHE (UMR IRD/CNRS/
INPG/université Joseph
Fourier)
1023-1025 rue de la piscine
Domaine universitaire
BP 53
38041 Grenoble cedex 9
France
TEL : +33 (0)4 76 82 51 19
FAX : +33 (0)4 76 82 52 86
EMAIL : luc.descroix@ird.fr
WEBSITE : http://www.lthe.
hmg.inpg.fr
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Gäelle COURCOUX
+33 (0)1 48 03 75 19
presse@ird.fr
INDIGO, IRD PHOTO
LIBRARY :
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REFERENCES :
DESCROIX L., VIRAMONTES
D., ESTRADA J.,
GONZALEZ BARRIOS
J.L., ASSELINE J.
Investigating the
spatial and temporal boundaries
of Hortonian and
Hewlettian runoff
in Northern Mexico,
Journal of Hydrology,
2007 346, 144– 158
DOI :10.1016/
J.JHYDROL.2007.09.009
DESCROIX L.,
GONZÁLEZ BARRIOS
J.L., VIRAMONTES D.,
POULENARD J., ANAYA E.,
ESTEVES M., ESTRADA
J. Gully and sheet
erosion on subtropical mountain slopes:
Their respective roles
and the scale effect,
Catena, (2008) 325–339
DOI :10.1016/
J.CATENA.2007.07.003
KEY WORDS :
Erosion, runoff, Mexico,
soil
the soil infiltration capacity was most
often lower than the rainfall rate. That
is translated by the formation of runoff
on the soil surface and this accentuates
the process of water erosion.
Other
more
local-scale
results
presented by the same team showed
that surface-type erosion, or sheet
erosion, which applies to all the surface
considered, was the cause of almost
all the soil losses affecting the western
Sierra Madre. The proportion of fertile
land lost by flooding-induced gully
erosion was estimated at 2% of total
erosion, even though a large amount
of the material dragged down from the
mountain slopes themselves were in
fact transported through the gullies so
formed. These original measurements
suggest that, in this northern area of
Mexico, it is the soils of all the drainage
slopes that lose several millimetres
every year. In this part of the country,
with its steep slopes, this process
was found to be the consequence
of livestock’s trampling of vegetation
combined with the sheer intensity of
rainfall events. The intense grazing
pressure exerted by cattle, which eat
mainly grassy and herbaceous plants,
means that the unpalatable species
that the livestock leave alone no longer
have any competition and eventually
take over the whole of the space.
This overgrazing therefore causes the
grassland ecosystem to be replaced by
thorn scrub and pine, less effective for
holding in place the fine layer of fertile
soil. In the space of around ten years,
pressure from ever-growing herds and
tree-felling for timber have contributed
greatly to changes in the landscape. In
the western Sierra Madre, practically
all the pasture land located at 500 to
2500 m altitude are already degraded.
Yet paradoxically it was in the valley
bottoms where there was least rainfall
that sheet erosion was most intense.
These zones were also the site of the
most degraded pastures and where
runoff was strongest.
These observations provide scientists
with a better understanding of the
functioning of soil and the erosion
processes in regions like northern
Mexico where it is becoming especially
intense. This type of approach
offers a better way of assessing soil
degradation. It should in the long term
offer easier identification of the places
most vulnerable to erosion and runoff,
a capability useful for devising control
measures to limit certain detrimental
practices such as tree clearance or
over-intense grazing practices.
Grégory Fléchet - DIC
Translation : Nicholas Flay
(1) These research studies were conducted
jointly with the Mexican institutions: CENID
RASPA (Centro Nacional de Investigación
Disciplinaria en la Relación Agua Suelo
Planta Atmósfera, and the research centre
of INIFAP (Instituto Nacional de Investigación
Forestal y Agro-Pecuaria) in Gomez Palacio
(Durango State).
(2) In geology, a soil corresponds to the layer
of fertile earth resulting from weathering of a
surface rock under the influence of climate,
vegetation or living organisms.
©IRD/Luc Descroix
Sheet n°281 - November 2007
For further information arid or semi-arid type terrains studied,
Trends in sheet erosion with time
Pinus Quercus Grasses Soil
Stones Rock mother
Grégory Fléchet, coordinator
Délégation à l’information et à la communication
Tél. : +33(0)1 48 03 76 07 - fax : +33(0)1 40 36 24 55 - fichesactu@ird.fr