672 
laminar or gully erosion, depending on the 
characteristics of the gradient, the length 
and form of the transversal profile. The eval 
uation in extended areas of the influence of 
the slope arises difficulties .derivated from 
the irreality of expressing the characteris 
tics of the relief by means of average values. 
On the other hand, the calculation of the 
slope mean values in small areas and their 
extrapolation for more extended regions im 
plies an overestimation of its influence, 
since the more extended is the area studied, 
the more important is the participation of 
the areas with minor gradients (Janssen,1 983) . 
With regard to studied region, considering 
that many landforms seems be morphometrical 
ly identical for their common origin and evo 
lution, some ranks of slope values corre 
sponding to the morphogenetic units of major 
recurrence were established by means of aeri 
al photo-i nterpretat i on and field measuring. 
Consequently, the LS values of each mapping 
unit reflects the relative presence of the 
shapes included in it. 
In the regional distribution of the steep 
ness and slope length Values, it can be seen 
(FIGURE 5) that these values generally coin 
cide with the main geomorpho1og i ca1 units. 
So, the highest values are found in the moun 
tainous area of the West, decreasing toward 
the east in the alluvial plain at the bottom 
of the mountain. 
FIGURE 5 Regional slope influence 
THE COVERAGE FACTOR 
It is well know that the vegetal coverage pro 
tects the soil from the direct impact of the 
rainfall, facilitates the storage of the wa 
ter and the biological activity, and reduces 
the dripping. Nevertheless, its importance 
increases in the tropical and sub-tropical 
regions because of the aggressiveness of the 
climate factors. In the studied area, the 
evaluation of the vegetal coverage as a fac 
tor of erosion risk is complicated by the 
marked seasonal cl imate contrasts and the 
wide extension and varieties of physiognomies 
and crops types. The criteria applied to 
obtain the coverage values (factor 11 C" of the 
USLE) were the following ones: 
a) Determination of the area occupied by each 
culture type and vegetal physiognomy within 
the area. 
b) Setting of the extrems of the natural veg 
etation development (dry and wet season). 
c) Definition of the calendar of the differ 
ent vegetative cycle stages of the main crops 
in the area. 
d) Representative coverage measurements of 
the different types of natural vegetation 
growing during the dry and the wet season. 
e) Representative coverage measurements of 
different crops in three stages of its veg 
etative cycle (establishment, development, 
maturing). 
f) Calculation of the factor "C" of the USLE 
for each type of crop and natural vegetation. 
g) Calculation of weighted averages of "C" 
values, corresponding to different coverage 
types, in each mapping unit. 
The evaluation registred by of the area 
occupied by each culture was made on the 
basis of historical records given by techni 
cal agencies. The determination of the per 
centage of land occupied by the diverse cov 
erage types in each mapping unit was made 
through the interpretation of Landsat pic 
tures taken during the dry and the wet sea 
sons, aerial photos, and ground controls. 
The coverage measurements and calculation of 
the "C" values were carried out after the 
criteria recommended by the Manual 357 of 
the United States Departament forAgricu1ture. 
On the map of FIGURE 6 it can be seen that 
there is a high correlation between the soil 
occupation, the cl imate and the coverage val 
ues. The eastern slope of the mountainous 
region, covered with subtropical wood, pos 
sesses the greatest coverage (C<0.1). To 
ward the west, the presence of deciduous for 
est and summit grass, which can be consid 
ered as a indication for minor precipitation 
and temperature, show up a slightly coverage 
diminution (0.11-0.15). In the central and 
eastern part of the area, the Chaco forest, 
though affectéd by degradation processes, 
shows moderate coverage values (0.16-0.20) 
in comparison with the ones of the main crops. 
FIGURE 6 Coverage values of the main crops 
and natural vegetation units in the region 
In this sense, the citrus plantations in the 
piedmont plain shows lower coverage values 
(0.21-0.25) despite of lesser exploitation, 
which are similar to the ones of the sorghum 
in the ondulated eastern plain. Finally, in