Rainfall characteristics may determine most of the harmful effects of soil erosion. Intensity variation during a rainfall is one of these important rainfall characteristics. However, there is a lack of information about rainfall patterns that occur in Brazil. This research had as objectives the determination of natural erosive rainfalls patterns verified in Santa Maria (RS), and the estimation of rainfall return period for the three patterns proposed. Daily rainfall charts were obtained from the Research Center of Forestry and Soil Conservation, of FEPAGRO, from 1963 to 1989 and from 1991 to 1992. Rainfalls were classified in advanced, intermediary and delayed patterns, according to their maximum intensity peak position and afterwards, the rainfall return period for each rainfall pattern was estimated. Some interesting characteristics were calculated, such as the mean intensity peaks, duration of rainfalls and peaks with intensity over 100 mm h-1. Out of the 1193 natural erosive rainfalls analyzed, the advanced, intermediary and delayed patterns had 551, 295 and 347 rainfalls, corresponding to 46,2; 24,7 and 29,1% of the total number of rainfalls, respectively. This characterizes the advanced pattern as the most frequent rainfall pattern in Santa Maria. From the 79 rainfalls with intensity peak greater than 100 mm h-1, the advanced, intermediary and delayed pattern had 45, 18 and 16 rainfalls, corresponding to 8,2; 6,1 and 4,6% of the total number of rainfalls. The mean duration of rainfalls with peaks greater than 100 mm h-1 was of 8 hours, and mean peak duration was 6 minutes and 50 seconds. For the estimation of return periods, the maximum rainfall amount in 10, 20, 30, 60, 120, 240 and 480 minutes during the 29 years studied were identified, for the three rainfall patterns defined before. The maximum intensity values found for the specified duration were submitted to a computer program, and the extreme distribution type I had the best fit to the data. The return periods of 2, 5, 10, 20 50 and 100 years for the studied rainfall duration were calculated and intensity-duration-frequency curves were built for the three above-mentioned rainfall patterns.
return period; rainfall intensity; profile of precipitation