Extreme Droughts in La Plata Basin. The Climate of the Region is Going Drier?

JUAN ANTONIO RIVERA; OLGA PENALBA

Foz do Iguaçu

Conferencia; AGU 2010 The Meeting of the Americas; 2010

American Geophysical Union

During the second half of the twentieth century, a positive trend in annual precipitation was observed in Southeastern South America. Instrumental and historical records of Pampean lakes (from 28° to 38°S) show that long dry intervals characterized the first 75 years of the 20th century, whereas positive hydrological balances have occurred after the 1970s. This annual precipitation increase might be accompanied by greater interannual variability in precipitation, which could increase the risk of droughts and their consequent negative impacts. The aim of this paper is to analyze the spatio-temporal variability of droughts in La Plata Basin (LPB) and to determine the occurrence of the most extreme drought on record from a precipitation standpoint alone. We used historical records of daily rainfall stations distributed along LPB, which in some cases begin during the first decades of the twentieth century. Based on these data, we used three indices to assess precipitation deficits in the region: the annual amount of dry days (AADD), considering a dry day as a day without precipitation; the Lack Of Rain Index (LORI), which is built using a 12-month moving average to the monthly anomalies of dry days; and the Standardized Precipitation Index (SPI), defined as the number of standard deviations that the observed cumulative rainfalls at a 12 months time scale would deviate from the long-term mean. Through the AADD and the LORI we found trends to a lower amount of dry days, which was reflected in increases in precipitation at different time scales. An examination of the decadal variability in the AADD shows significant cycles in the range of 10-30 years, and variations in the interannual scale, depending on the region considered. Although there were trends toward a decrease in the annual amount of dry days, in some regions these behaviors have begun to reverse. This was verified through the accumulation of the LORI during the periods of 2008, 2007-2008 y 2004-2008, in which it was observed a higher amount of dry days. By means of the SPI, the year of the most extreme drought was determined for every pluviometric station analyzed. This was obtained by determining the minimum absolute value of the index over the total measurement period. Many of the stations with records starting in the 1920s and 1930s, which are located in the Central region of Argentina, had their most extreme drought during the 1930s. This could indicate that LPB experienced its worst drought during this period. In turn, for a large number of stations located in the eastern portion of the domain, which start their records between 1950 and 1970, it was determined the occurrence of its extreme drought during the 2000s. This is consistent with the results outlined above and allow us to ask if is this behavior a signal to a permanent return to dry conditions in the region or is it part of a long-term variability? Only continuous monitoring of water conditions and the development of appropriate decadal forecast models will discern whether there is the beginning of a prolonged dry period or if the rainfall will grow over the coming decades in the region, as stated in Fourth Assessment Report of the IPCC.