Assessing the rain erosivity and rain distribution in different agro-climatological zones in Venezuela

Abstract

Evaluating runoff and field soil losses by either empirical or physically based erosion models,
 requires an understanding and assessment of rainfall characteristics affecting the erosion
 (sub)processes.
 In spite of shortcomings and many criticisms on the worldwide use and applicability of
 RUSLE (Revised Soil Loss Equation), this empirical model still gains worldwide popularity
 for estimating erosion on a farmfield or micro-catchment scale.
 One of the limits of the model is the assessment of its climate factor in terms of rain energy
 and maximum storm intensity, both factors combined in the erosivity factor R, although they
 not easily available nor easily determinable. Therefore attempts are made to look for more
 easily determinable rain characteristics such as the monthly precipitation for evaluating the
 rain aggressiveness (or rain erosivity) on a monthly or yearly basis.
 As agro-climatological zones have typical rain distributions and rain concentrations, the
 Modified Fournier Index (MFI) and the Precipitation Concentration Index (PCI), both
 calculated from monthly precipitation values over a number of successive years, will be used
 to characterize zones in Venezuela.
 Monthly rainfall data were collected for at least 10 successive years from nine stations in
 Venezuela: three stations in the Llanos Centrales, four stations in the Llanos Occidentales and
 two stations in the semiarid zone of Lara State. 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 = EI30 was available an attempt was made to relate R to
 (MFI).