Hydrological response to meteorological drought: a case study in La Plata Basin

The quantitative knowledge of the properties of droughts in a region is an important aspect of the planning and management of water resources. This work analyzes the relationship between meteorological droughts and hydrological droughts for a specific location over Uruguay River, belonging to La Plata Basin (LPB), one of the largest basins in the world. The basin generates around 70% of the Gross National Product of Argentina, Brazil, Paraguay and Uruguay, and has a population of over 100 million inhabitants. The LPB is also one of the major producers of hydroelectric power in the world. Therefore, extreme low flows are critical for hydroelectric production and water resources management in the region. The Standardized Precipitation Index (SPI), which is the most widely used drought index, is used to characterize meteorological droughts at different time scales. In order to define the streamflow drought events in the region, we used the Q70 threshold, the flow which is equaled or exceeded for 70% of the time. The Q70 threshold was compared with the SPI considering two approaches: for the whole time series, considering all the months as a continuum, and seasonally, considering the Southern Hemisphere seasons. For comparison we used the Pearson correlation coefficient. In general, different studies indicate that there is a considerable time lag between departures of precipitation and the point at which these deficiencies become evident in surface components of the hydrologic system. Our findings prove that the surface runoff respond to short SPI time scales (2-4 months). The maximum correlation is found on the time scale of 3 months (SPI3). The seasonal comparison shows that the highest correlations were obtained in summer, which is the low flow season. We also made a comparison of the two time series for identified historical drought episodes. In the case of the extreme drought event, the SPI3 was able to anticipate the stages of flow deficit one month in advance. These results could be useful for forecasting and monitoring hydrological drought severity and in developing a drought preparedness plan in the region.