ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

BENGT CALLES & LENA KULANDER
Erosive rains in Lesotho
Institute of Earth Sciences, Physical Geography,
Uppsala University, Norbyvagen 18 B, S-752 36 Uppsala, Sweden
Lesotho is justly regarded as one of the countries in the
world that has suffered most from soil erosion caused by
surface runoff of water. As part of a study on the effects of
soil erosion and efficiency of conservation measures, the
frequency of erosive rains was investigated. Erosive rains
were defined as having a higher intensity than 25 mm/h
based on 30-minute intensities. This measure conforms in
principle with the conventional definition of erosive rains
as being EI 30 >25 mm/h, e.g. total impact energy based on
the sum of rain drop energies during the most intensive 30
minutes of a rain.
Are precipitation conditions unusually harsh in Lesotho or
are other factors likely to be responsible for the present
status of a severely eroded landscape? The recurrence interval
of rains exceeding the threshold value gives valuable
information on the natural climatic conditions for erosional
processes and enhanced erosion as a result of man's activities
and is also a measure well suited for erosion hazard
assessment.
Background information on precipitation was available as
daily rainfall amounts at three sites covering the period
from 1880 to present. The information on rainfall intensities
is scarce and scattered in time. Apart from 3 major sites
shorter time series of recorded intensities at 4 four sites
were used. The two types of data sets give an areal coverage
of Lesotho's lowland and foothill regions which are the
principal agricultural regions of the country and where soil
erosion is a problem of social and economic importance.
In order to overcome the lack of long series of intensity
observations, the relationship between storm amount and
intensity was determined for the 7 sites. Mean relation-ships
using linear least square fit of regression lines through
the data sets were determined. A second step in the analyses
consisted of determination of the relationship between
total daily rainfall amount and amount in individual storms
during the same day. Combining these two procedures made
it possible to transform long series of daily amounts into
series of maximum 30-minute intensities.
The partial duration technique was applied to determine
the recurrence interval for rains of varying intensity. The
results show that the spatial variation over the studied area
in Lesotho is small. The average number of rains each year
exceeding 25 mm/h ranges from 2.2 to 1,5 during the wet
season and from 0.4 to 0.3 events each year during the dry
season. No major difference is apparent if the long series
on daily amounts and the shorter intensity series are compared.
A comparison of the computed recurrence intervals
with information from other regions in Africa and Europe
100
shows that the computed frequencies are not extremely
high. Hence it is likely that rainfall conditions cannot be regarded
as the only or major cause for the present situation.
The relatively scarcity of erosive event makes it justified to
doubt if the term erosive rains is a suitable measure for
estimating soil erosion. More emphasis should be put at
understanding and modelling the entire physical processes
on erosional processes resulting from antecedent conditions
and overland flow of water.
MARC CALVET
Rythmes et vitesses d'evolution morphogenetique
dans un orogene alpin.
Le cas des Pyrenees orientales franco-espagnoles
Departemenr de Geographie, Universite de Perpignan, 52,
Av. de Villeneuve 66860 Perpignan Cedex, France
Upres a 1562 Cnrs «Geodynamique des Milieux Naturels
et Anthropises», Universite Blaise Pascal, 29 Bd. Gergovia,
63037 Clermont-Ferrand Cedex 1, France
La pertinence des modeles d' evolution geomorphologique
sur Ie long terme suppose leur confrontation avec des
donnees quantifiees, bilans globaux de matiere, taux de
surrection et d' ablation, de facon asaisir ala fois rythmes
et vitesse d'evolution morphogenetique, L'approche se
doit d' etre multiscalaire, pour esperer depasser Ie hiatus
qui separe dynamique des formes de celIe des processus et
atteindre aune veritable geomorphologic dynamique globale.
Le troncon oriental des Pyrenees propose ala fois des
fosses a remplissages correlatifs neogenes et des surfaces
d'erosion culminantes, qui fournissent des reperes chronologiques
et geometriques de grande valeur pour toute approche
quantifiee, L'orogenese y apparait comme un phenomene
brutal et tres saccade, en plusieurs episodes dont
seuls les derniers, de plus en plus generalises et rapides depuis
Ie Miocene superieur, sont responsables du volume
montagneux actuel. On dernontre que 1'arasion de la chaine
a ete totale entre la fin de la tectogenese pyreneenne
vers 35 Ma. et 1'Aquitanien. Al'echelle d'un bloc comme Ie
Canigou, les conglomerats du Conflent prouvent la naissance
d'une montagne vers 19 Ma., 1'ablation d'au moins 1
a2 km de gneiss oeilles puis l'arasion totale de ce horst de
2000 m et large de 20 km, au Miocene moyen, en 6 a8 Ma.
seulement. La confrontation de ces donnees, brievete et caractere
saccade des phases de surrection, rapidite de l'arasion
dans Ie contexte bio-climatique du Paleogene superieur
et du Neogene, redonne quelque credibilite au modele
cyclique davisien: il s'applique non seulement aux domaines
de socle, OU on a tendance aIe cantonner, mais aussi
acertains orogenes recents, au moins ceux de type intracontinental.