ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

vegetation, de frequence et de distribuition des inondations,
par la diversitee e distribuition des corixos, baias, uazantes,
cordilheiras et lagoas et par les traces laissees par l'ecoulement
superficiel.
Les regions sous 1'influence directe des crues du fleuve Taquari
a W et du fleuve Negro au S, delimitent la Nhecolandia;
elles presentent un systeme complexe reliant des
corixos aux baias et uazantes pendant les hautes eaux. La
region «Vazante do Corixao» est une large bande recouverte
par de la savane herbacee qui accompagne celle du
Taquari; elle presente tres peu de lagoas et est soumise a
l' action de l'elevation du niveau de l'eau des corixos et de
la nappe phreatique. La Nhecolandia plus humide a rest,
presente un ecoulement anastomose des corixos et des vazantes,
souvent marques par des forets-galeries, et une absence
presque totale de cordilheiras et lagoas; en haute saison
elle recoit l'influence de la montee des eaux des corixos
et de la nappe phreatique. Le trait principal de la
Nhecolandia centrale est Ie grand developpement du systeme
baia-vazante-cordilheira-lagoa, avec la tres grande
densite de celles-ci. Les crues sont dues sourtout al'elevation
de la nappe phreatique et en moindre partie, par les
eaux des corixos et vazantes et l' aspect anastomose de
ceux-ci est peu prononce, II est possible de distinguer,
vers Ie nord, une sons-region au Ie systeme cordilheiraslagoas
est plus restreint, traverse par des uazantes et corixos
plus larges. -,
NELSON F. FERNANDES & CARLA B. SANTI
The convex hilltops of Southeastern Brazil and the
question of dynamic equilibrium: insights from
hillslope curvature and numerical modelling
Departamento de Geografia, Universidade Federal do Rio de Janeiro,
Ilha do Fundao, 21941-590 Rio de janeiro;Brasil
Convex hilltops are typical features of many soil-mantled
landscapes around the world. These forms have been frequently
interpreted as characteristic of well-developed
landscapes, as the ones prevailing in most of the tropics
nowadays. They have also been suggested as representing
evidences supporting the classical idea of dynamic equilibrium'
implicitly proposed by Gilbert and later well organized
by Hack. Although the origin of these convex hillslopes
has been attributed to the work of diffusive (slope-dependent)
processes like creep, rainsplash and biogenic activity,
a number of numerical models have shown that the
hillslope curvature also depends on the baselevel conditions
associated, in general, with the incision rate at the base
of the profile. In addition, some of the models have
shown that the relaxation time of these convex hillslopes,
when evolving under diffusive processes and responding to
typical climatic and tectonic oscillations that have occurred
along the Quaternary, may be much longer than the fre-
quency of these oscillations, prevailing these hillslope profiles
to attain the dynamic equilibrium condition. This
study focus on the geomorphological meaning of the
smooth, well-rounded, convex hilltops observed in
southeastern Brazil, based both on field evidence and numerical
experiments.
Detailed hillslope profiles were surveyed in the field from
convex hilltops in three different areas of southeastern
Brazil, inside the state of Rio de Janeiro (Mage, Campos e
Pira'). These areas were selected because they include a variety
of bedrock conditions, incision rates, soil types, hillslope
convexities, climatic conditions, vegetation and erosional
processes. In addition, these factors are relatively
homogeneous inside each area. Based on the curvature of
the profiles we characterized the ratio between the incision
rate and the diffusion coefficient for these areas. It is assumed
here that where landscapes are close to dynamic equilibrium
this ratio will be relatively constant along the hillslopes
in that area.For the sites where the lowering rate is
known, this procedure allowed us to estimate the diffusion
coefficient of the landscape, a basic parameter of mathematical
models of hillslope evolution. The values obtained
in the field will be used to estimate the relaxation time of
the hillslopes in these areas in response to climatic and tectonic
changes.
The results show that hillslope curvature does not vary significantly
inside each area studied. For example, the ratio
between the incision rate and the diffusion coefficient for
the hillslopes in the Mage site varies for a factor smaller
than four. This range is extremely small since it is known
from the literature that both the incision rate and the diffusion
coefficient may vary, at least, three orders of magnitude.
Although mathematical models tend to suggest the
contrary, the results presented here do not allow us to
reject the idea of dynamic equilibrium in the region.
DENISE B. FERREIRA
Assessment of erosional impact of land use change
during the xx- Century in the inner Alentejo (Portugal)
Centro de Estudos Geograficos, Cidade Universitaria,
1699 Lisboa, Portugal
The main objective of the research presented here is the
understanding of the various dynamic processes (natural
and anthropogenic) responsible for the soil degradation in
the inner Alentejo region. The guideline for the investigation
is the assessment of the impact of land use and agricultural
practices changes since the last decade of the
XIX th Century on soil erosion at a regional scale, in relation
to climate. Part of the results obtained in the councils
of Moura e Barrancos (basin of the Guadiana River) are
presented. These councils were chosen to be included in
an information system on the montado's transformations in
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