Erosivity, hydrological patterns, return period and probability of occurrence of rainfalls at Sao Borja, RS, Brazil.

摘要

The erosivity potential of rainfalls can be estimated by some indexes, among them the EI30, which is given by the product of kinetic energy (E) and rainfall intensity for a 30-minute period (I30). The purpose of this study was to determine rainfall erosivity, hydrological patterns, return period and probability of occurrence of erosive rainfalls in Sao Borja, Rio Grande do Sul, Brazil, based on diary rainfall charts of the FEPAGRO meteorological station, from 1956 to 2003. The duration and accumulated amount of each erosive rainfall were measured, entered in a spreadsheet, digitalized and processed by CHUVEROS software, which calculated the EI30 index and the monthly and annual total erosivity, besides determining the hydrological pattern of each rainfall. Most of the annual erosivity (76%) was concentrated between October and April, in the period of soil tillage, sowing and growth of summer crops. The most notable peak in the erosive potential was observed between March and April (1260-1269 MJ mm/ha/h), when most crops are normally in full growth, while July and August were the months of lowest erosive potential (268-271 MJ mm/ha/h). Of the total erosive rainfalls 47, 25 and 28% had advanced, intermediary and delayed patterns, respectively, while these patterns corresponded to 50, 26 and 24%, respectively, of the mean annual volume of erosive rainfalls and to 53, 25 and 22% of the average annual erosivity. The mean annual index of erosivity in Sao Borja is 9751 MJ mm/ha/h/year. It represents the 'R' Factor of the Universal Soil Loss Equation to be used in Sao Borja and regions with similar rainfall pattern. Linear and potential regressions were obtained to express the mean annual EI30 index by pluviometric records expressed as rainfall coefficient. The mean annual rainfall erosivity index of 9751 MJ mm/ha/h/year (Factor 'R' of USLE for Sao Borja, RS, Brazil), is expected to occur once every 2.2 years, with a 44.9% likelihood.