ASSESSING THE RAIN EROSIVITY AND RAIN DISTRIBUTION IN DIFFERENT AGRO-CLIMATOLOGICAL ZONES IN VENEZUELA

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According to the available data sets, two different procedures were used to calculate (MFI): -In the first procedure the monthly rainfall amounts are averaged over a number of years.
The (MFI) is then calculated from this averaged rainfall data set and reported as (MFI)1.
-In the second procedure the (MFI) is calculated from the monthly rainfall amounts of each individual year and the (MFI) averaged over a number of years.Those long term average values are reported as (MFI)2.
Temporal aspects of the rainfall distribution within a year was defined by the Precipitation Concentration Index (PCI) also based on monthly rainfall amounts and calculated following the two procedures as for determining the (MFI).
From those stations of which R = EI 30 was available an attempt was made to relate R to (MFI).

INTRODUCTION
In Latin America and hence also in Venezuela sufficient rainfall records, especially rain intensity, from automatic rain recorders, are not available to calculate countrywide the rainerosivity index R =E 30 as defined in the Universal Soil Loss Equation (USLE) (Wischmeier and Smith, 1978) with E the kinetic energy and I30 the maximum intensity during 30 minutes of the individual rainstorms.
For a number of experimental sites in Venezuela the R values were calculated on the basis of the relationship between kinetic energy E and intensity I as proposed by Wischmeier and Smith (1978) and by Renard et al (1993).
Attempts were already made in Venezuela to derive the erosivity factor R from more easily determinable rainfall parameters .Guarisma et al (1981) developed a model to determine EI 30 for the tropical zone of Bosque Seco in Venezuela on the basis of daily rainfall amounts P d (mm) : R = EI 30 (MJ.mm.ha -1 .h - ) = -231.7 + 19.3 P d (mm) Paez and Aguero (1986) found a similar realtion for La Paragua (Estado Bolivar, Venezuela) : R=EI 30 (MJ.mm.ha -1 .h - ) = -237.9+ 8.7 P d (mm) Fournier (1960) developed an index (FI) (Fournier Index) to be correlated to sediment loads in rivers (FI) = p²max/P, with pmax the mean monthly rainfall amount of the wettest month of the year and P being the mean annual rainfall amount.
However this (FI)-index has shortcomings as estimator of the rain erosivity index within the USLE.As also low amounts of monthly rainfall can have erosive power, an increase in total rainfall amount should result in an increase of erosivity.It is also not logic that if the maximum monthly rainfall pmax remains the same with the mean annual rainfall increasing, the (FI) is decreasing.
Therefore Arnoldus(1980) modified the (FI) index into a Modified Fournier Index (MFI) considering the rainfall amounts of all months in the year.
(MFI) = Σ p²/P with p: the monthly rainfall amount and P: the annual rainfall amount In an attempt to define temporal aspects of the rainfall distribution within a year, Oliver (1980) proposed the Precipitation Concentration Index (PCI), derived from the Index of Employment Diversification (Gibbs and Martin, 1962) PCI = 100 x Σ p²/P² Where p stands for the rainfall amount of each month of the year and P for the yearly rainfall amount.
The theoretical limits of the (PCI) are obtained as follows: -when the rainfall in each month of the year is the same, the (PCI) equals 8.3 -when all the rainfall of the year occurs in one single month, the (PCI) equals 100 The (PCI) permits grouping of data sets according to the derived value.Oliver (1980) came to the conclusion that a (PCI) of less than 10 suggests a uniform distribution; a value from 11 to 15 denotes a moderate seasonal distribution, a value from 16 to 20 denotes a seasonal distribution.An index above 20 represents strong seasonal effects, with increasing values indicating increasing monthly rainfall concentration.

Meteorological stations and methodologies: Three important agricultural areas in
Venezuela were selected for the study: the 'Llanos Centrales', the 'Llanos Occidentales' and a semiarid zone in Lara State.In this study we include three the meteorological stations in the Llanos Centrales': 'Valle de la Pascua' and 'Los Arbolitos' and 'San Antonio Tama', four stations in the Llanos Occidentales: 'Mesa de Cavacas', 'Santa Bárbara de Barinas', 'Colonia Agricola Turén' and 'Banco de los Cedros', and two stations in the semiarid zone of Lara State: 'Carora Granja' y 'Bobare', of which the coordinates and number of years of data are presented in table 1. -the Modified Fournier Index (MFI), using the two procedures as mentioned above resulting in (MFI)1 en (MFI)2 The (MFI) ranges proposed by CORINE (1992) (cited by Jordán and Bellinfante, 2000), are presented in table 2. -the Precipitation Concentration Index (CPI), is also calculated using the same two procedures as for calculating (MFI).
In the first procedure, the mean monthly rainfall amount is estimated by averaging the monthly rainfall data over a number of years.Afterwards, the (PCI) is calculated from the estimated mean rainfall data set.Those values will be reported as (PCI)1.In the second procedure, the (PCI) values are averaged over a number of years.Those values are reported as The first procedure results in (PCI)1 if an average year if the series of years is long enough.
The second procedure will yield long term average (PCI)2 values of individual years.
In view of estimating the rain erosivity in the three agricultural zones by means of rain characteristics more easily to determine than those required for the R-factor of the USLE, a linear relationship between (MFI)2 and R = EI 30 was found as well as for the Llanos Centrales (figure 7) as for the Llanos Occidentales (figure 8).

Figures 1 ,Figure 1 :
Figures 1, 2 and 3 illustrate the (MFI) values for the individual years for respectively the stations of the Llanos Centrales, of the Llanos Occidentales, and semiarid zone of Lara State

Figure 2 :Figure 3 :
Figure 2: The Modified Fournier Index (MFI) for each individual year of the four stations of the Llanos Occidentales.

Figure 4 .
Figure 4.The Precipitation Concentration Index (PCI) for each individual year of the three stations of the Llanos Centrales.

Figure 5 .
Figure 5.The Precipitation Concentration Index (PCI) for each individual year of the four stations of the Llanos Occidentales.

Figure 6 .
Figure 6.The Precipitation Concentration Index (PCI) for each individual year of the two stations of the semiarid zone in Lara State

Figure 7 .Figure 8 .Figure 9 .
Figure 7. Relationship between the Modified Fournier Index (MFI)2 and the USLE erosivity factor R=EI 30 for the two stations of the Llanos Centrales.

Table 2 :
The (MFI) is classified as follows:

Table 3
reports the mean EI 30, the mean (MFI) and the (PCI) for the nine meteorological stations

Table 3 .
The mean annual (MFI),(PCI)and EI 30 values for nine meteorological values for the three stations of the Llanos Centrales indicate a seasonal distribution of the rainfall within the year and a moderate seasonal distribution for the four stations of the Llanos Occidentales, whereas for the stations of semiarid zone indicate a moderate seasonal distribution in one of those and irregular distribution in the other one.