Trends in extreme daily rainfall events in La Plata Basin

OLGA C. PENALBA; FEDERICO ARIEL ROBLEDO

Foz do Iguazu, Brasil

Conferencia; 8th International Conference on Southern Hemisphere Meteorology and Oceanography (8ICSHMO); 2006

American Meteorology Society (AMS)

Extreme rainfall events cause significant damage to agriculture, ecology, disruption to human activities, for example. To understand how these changes could influence society and ecosystems, it is useful first to know how these extremes are changing through the years. The objective of this work is to analyze the temporal variability of the frequency of daily precipitation with special attention to interdecadal and interannual variations. This frequency was determined for daily precipitation events of at least 0.1, 2, 5, 10, 30, 50, 80, 100 mm and also their own the 75th, 90th and 95th percentiles. The persistence of rainday and no rainday was also analyzed by assessing the probability of rainday when the day before was a rain day and the probability of no rainday when the day before was a no rainday. This study is perform for different months of the year and also for the annual behavior. The region under study is part of the La Plata basin, and forms the most important agricultural and hydrologic center of Argentina. Twenty five Argentine stations analyzed are located north of 37°S and east of 67°W. Two stations in Uruguay and seven stations in Paraguay and south of Brazil are used in order to improve spatial coverage in the east and  north of Argentina. The data used were processed to obtain consistent homogeneous databases, with less than 10% of months missing for their period of record, with the shorter period 1960-2003 and the longest one 1908-2003. In general, the results show a variability that is highly non-stationary. The most outstanding feature is the difference in the annual frequency before and after the 1960s. The interdecadal variability is particularly well defined in the west and there is a general significant linear trend in the study area.  This interdecadal variations affect the behavior of extreme precipitation in the annual scale and during the months with maximum precipitation in the region. The frequencies calculated in the whole year showed a significant positive trend in some regions of Argentina. The pattern of trends for extreme events has the same spatial behavior, but covering smaller regions. The Argentine and Brazilian stations located around 28ºS and 57ºW  show a significant positive trend in the whole year and during summer and autumn months. Meanwhile in the northeast Argentine stations show a negative trend. The increases in the frequency of events greater than their own percentiles are also evident, indicating that at least a portion of the increase in precipitation is due to an increase in the frequency of extreme events.  The persistence of rainday in the year shows significant positive trend in the stations located in the northeast of Argentina and southeast of Brazil.