671 
i n g units 
i e f - 1 a n d 
use) 
□ 
□ 
□ 
ion hazard 
ma p 
50.000/ 
.000.000) 
-elated to the 
ns (f.e.alluvial 
generally cov - 
tern. 
n multi-seasonal 
iomass densities 
processing and 
; of those ap- 
availability of 
t-benefit of 
account the ex- 
id the requiered 
;ording with 
irpretat ion can 
Feet i ve for many 
j methods. The 
idsat frame is 
computational 
r system can be 
production is 
classification 
s are parameters 
2 risties of the 
at i on with var- 
i, sheet , gully, 
redictions of 
is best know has 
and Smith(l978; 
on the re 1 at i on 
ie characteris- 
/, intensity, 
index expresses 
2 r gy and the 
/ of a rainfall 
(EI,J. Nevertheless, its application shows 
limitations in areas lacking p1uv i ograph i ca1 
records, a fact which is frequent in most of 
the developping countries. To avoid this dif 
ficulty an index obtained by. Arnoldus(l97P) 
was used, i.e. —2, where p=monthly rain 
fall and P=annual riinfall. This index has 
the adventage of employing simple meteorolo 
gical datas and good correlation with the 
values of "R" given by the USLE. To achieve 
such a correlation, the following general 
equation was estab1ished:R=axj- R?b, where 
i = 1 P + 
"a" and "b" are constants derived from region^ 
al climate conditions. This equation was tes 
ted in the United States and in Africa where 
a high correlation with the p1uv i ograph i ca1 
values of "R" was obtained(Arno 1dus,1 978). 
Following this methodology, iso-erosiv i ty 
curves were calculated for the studied area 
(FIGURE 3). Its analysis revealed that in the 
astern part of the studied area the rainfall 
erosivity is relatively weak and increases 
toward the west in parallel with the in 
crease of the orographic precipitations. On 
the intermediate high slope of the eastern 
mountainous area the values fall abruptly, 
thus responding to the diminution of the 
rainfall. In summary, a high correlation 
between pluviosity and rainfall erosivity 
has been verified. 
SO IL ER0DI B I L I TY 
The 
sense 
of the term 
"soi 
il erodibility" is 
different 
from 
the 
one 
o f 
the concept "soil 
eros 
ion" : 
The 
mass 
o f 
soil 
1 loss caused by 
eros 
ion can be 
i n f 1 
uen 
ced 
at a higher deg ree 
by the slope, the coverage or the management 
than by the intrinsic proprieties of the 
soil. Nevertheless, some soils get more 
eroded than others even when all the other 
factors are similar. This difference, due to 
internal proprieties of the soil is called 
"erodibility" (Wischmeier and Smith,1978). 
Among the methods applied for its determina 
tion, the most practical one is the well know 
nomograph included in the Universal Soil Loss 
Equation. For the calculation of the soil 
erodibility of the area, the most representa 
tive types have been sampled and their erodi 
bility calculated on the basis of the informa^ 
tion required by the mentioned nomogram(per- 
centage of silt and very fine sand, percen 
tage of sand, percentage of organic material, 
structure and permeability). 
The values of the soil erodibility in the 
area are shown in FIGURE 4; there is an ob- 
vius relationship with the distribution of 
the original materials and the regional cli 
mate. Thus, it can be seen that in the ex 
treme west the semi-arid conditions and the 
presence of siltstones of the Tertiary deter 
mine a moderate erodibility. On the eastern 
slope of the Aconquija and Cumbres Calcha- 
quies, under wet sub-tropical climate, the 
decrease of the erodibility reflects the pre¿ 
ence of the mountainous wood and its positive 
influence on the structural stability of the 
soils. On the opposite, eastwards, the values 
increase in parallel with the presence of 
loess and inversely with the rainfall diminu 
tion. In summary, the relatively high values 
Of erodibility given by the soils of the area 
must be attributed mainly to the constant 
presence of loessoid material among their 
original materials. 
FIGURE 3 Iso-erodent map ( factor R of USLE) 
of the study area 
FIGURE 4 Regional distribution of soil erodi 
bility average values 
THE SLOPE INFLUENCE 
The two aspects of the slope which have the 
greatest influence on the erosion processes 
are its steepnes and its length. The steep 
ness has an influence in the sense that the 
steeper the slope stronger is the direct ef 
fect of the ra i nfa 1 1 ( sp1 a sh erosion). In the 
same way, the infiltration time will be 
shorter because of the higher runoff speed 
which on turn increases its erosion capacity. 
On the other hand, the longer a slope is, the 
greater are its possibilities to receive rain. 
The kind of influence of the slope s h a p e> on 
erosional processes is not exactly known, 
because the complexity given by the changea 
ble influence of the type of soil and vegetal 
coverage. Nevertheless, it is assumed that 
the concave slopes tend to concentrate the 
runoff and facilitate the gully erosion,, 
while the convex slopes disperse the runoff, 
facilitating the laminar or the rill erosion. 
On the straight slopes predominate either