EFFECT OF EL NINO/SOUTHERN OSCILLATION ON RAINFALL EROSIVITY IN SANTA MARIA (RS)

El Nino / Southern Oscillation (ENSO) affects the weather and climate of several locations, causing changes in the atmospheric circulation and climate variables, mainly rainfall, in all regions of Brazil. In southern Brazil, during El Nino years (the positive phase of ENSO), rainfall is often above normal whereas during La Nina years (the negative phase of ENSO), rainfall is often below normal. Some rainfall properties are affected by ENSO, such as frequency, intensity and amount. These properties are important to define erosive rainfall events. Among the methods to determine rainfall erosivity in Rio Grande do Sul State, the EI(30) -erosivity index, is the most commonly used. The objective of this study was to determine and associate the EI(30) erosivity index with the ENSO phenomenon for the region of Santa Maria, RS, Brazil. Daily rainfall data recorded from July 01, 1978 to July 30, 2008, collected in a standard Weather Station in Santa Maria, RS, were used. Individual erosive rainfall events were identified in the records, grouped into El Nino, La Nina and Neutral years, and the EI30 index was calculated. Rainfall events were also grouped into three patterns: Early, Intermediate and Late. Pearson correlation and regression analysis between the EI(30) index and the Nino Oceanic Index (NOD were performed. Regression was tested with the t test in order to quantify the relationship between the two variables, aiming at a possible predictability of the erosion potential of rainfall events due to abnormalities of the Sea Surface Temperature (SST) in the Pacific Ocean. Rainfall erosion potential in Santa Maria is affected by ENSO, so that a greater number of rain fill events have a higher erosion potential during El Nino and neutral years. The variability of the rainfall erosion potential is greater during Neutral years than during years with SST anomalies. The rainfall pattern is changed in years with SST anomalies, so that there is a slight increase in early rains during El Nino years and a slight decrease in late rains during La Nina years, whereas there is a decrease in rainfall events with Intermediate pattern during El Nino and La Nina years compared to Neutral years. The predictive capability of the erosion potential of rainfall events in Santa Maria based on the NOI index is weak or inexistent.