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| Titel |
Erosion measurements at various scales in a semi arid mountainous catchment - case of the Rheraya watershed, High Atlas, Morocco |
| VerfasserIn |
A. Cheggour, V. Simonneaux, E. Roose |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250031544
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| Zusammenfassung |
| Erosion is a critical phenomenon in North Africa, under the combined effects of aggressive
rainfall and soil fragility, increased by the grazing pressure on rangelands. However
measurements of actual erosion rates are rare, especially in mountainous areas. Siltation of
dams is estimated at more than 60 million m3 annually in Morocco, which corresponds to a
decrease of 0.5% of the storage capacity. The Rheraya watershed (225 km2) is located in a
semi-arid climat, in the High Atlas of Morocco. In order to assess erosion processes at
various scales, three types of measurements were achieved on this area, namely rainfall
simulation tests one square meter, erosion plots on 150 m2, and catchment’s discharge and
associated sediments measurements.
Rainfall simulation experiments were achieved on 27 sites, measuring runoff and
sediment charge. The turbidity was correctly measured thanks to the development of a new
runoff collector which doesn’t disturb the soil. In the scope of spatial extrapolation, we
searched for indicators obtained from ground description variables and/or by laboratory
tests on soil samples, which were well correlated with infiltration and turbidity of
the simulations. For the various soils present in the study area, the results show a
large variability of infiltration (from 1 to 70 mm h-1) and turbidity (from 3 to 325
g.l-1). Analysis showed that infiltration is correlated mainly with texture and soil
surface opening, and that turbidity is related to the surface of bare soil exposed to
runoff.
Six erosion plots of about 150 m2, located on various soil and land cover conditions, were
measured during four years. The observations showed very rare runoff events in the main part
of the watershed, producing a low sediment load (between 0.015 and 2.5 t.ha1.year1).
Conversely, runoff was much more frequent on silty badlands, producing about 95% of the
watershed sediment (350 t.ha-1.year-1) despite their area was only 1% of the watershed.
There was a significant linear relation between simulation turbidity and erosion plot turbidity.
However, there was a great difference between infiltration estimates from the two types
of measurements. Plot infiltrations estimates were only between 3 and 5 mm/h,
but they were significantly correlated to the one from test, through an exponential
relation.
Finally, an estimate of the overall erosion at catchment’s scale was achieved from plots
values extrapolated using a soil map, and gave about 3 to 4 t.ha-1.year-1. A good correlation
was found between this watershed scale estimate and the catchment’s exportation, indirectly
validating the significance of both measurements. Moreover, both estimates were about the
same, showing a sediment delivery ratio around one.
Keywords: erosion, rainfall simulation, erosion plot, sediment exportation |
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