CONICET | Buscador de Institutos y Recursos Humanos

The Comahue region is located in Central Andes Mountains in Argentina, between 36º and 42ºS. The region is crossed by three major rivers: the Limay, Neuquén and Negro. It is one of the major river systems in energy terms because a main portion of the hydro-electric energy used in the country is generated there, due to the presence of dams: El Chocón, Alicurá and Piedra del Aguila in Limay river basin and Portezuelo, Los Barreales y El Chañar in Neuquén river basin. It drains an area of 140,000 km2 and covers almost the whole of the province of Neuquén and the provinces of Buenos Aires and Rio Negro. The various uses of water are for example, hydroelectric generation for the national grid and water supply for the development of local subsistence economies. The operation of dams is highly dependent on rainfall, as it is the variable that mainly regulates the flow of rivers. In this area the annual cycle of rainfall is characterized by a maximum in winter (April to August). The significant interannual rainfall variability generates great uncertainty, since a relatively good seasonal rainfall forecast is needed to perform efficiently the operation of dams [1] [2]. During the period 2000-2012 only 4 of the 13 years have been recorded rainfall above average in representative stations of Limay and Neuquen River Basins. These values are indicative that the region is going through a period of rain below normal which affects the optimal operation of dams. In this context the study of the relationship between the precipitation and flow and secondly the ability to predict with some anticipation these events, become relevant. Current forecasts of seasonal rainfall using dynamic modeling have serious shortcomings because of, among other things, the low spatial resolution. That is why in this context, the development of statistical rainfall forecasting techniques specially adapted to the study region is addressed in this proposal. The Limay river basin was carefully studied [3] and in this case a detailed statistical analysis was performed for Neuquen river basin using precipitation and flow data for 1980-2007 period. The water year begins in March with a maximum in June, associated with rainfall and another in October probably derived from snow-break. A significant relationship between interannual variability of mean and maximum flow and rainfall during the hydrological year was detected. General features of rainy season with excess or deficits are analyzed using standardized precipitation index for 9 month period (SPI) in Neuquen River basins. SPI corresponding to the rainy season (from April to September) has no significant low frequency trend but a significant cycle of 14,3 years, the more severe excess (SPI greater than 2) has a return period of 25 years meanwhile the most severe droughts (SPI less than -2) has return a period of 10 years. Wet and dry years were classified using SPI. In order to establish the existence of previous circulation patterns associated with interannual SPI variability, the composite fields of wet and dry years are compared. There is a tendency for wet (dry) periods to take place in El Niño (La Niña) years; when there is positive anomalies of precipitable water over the basin; when the zonal flow over the Pacific Ocean is weakened (intensified) and when there is negative pressure anomalies in the southern of the country and Antarctic sea. Some prediction schemes, using multiple linear regression, were performed. The model derived using forward stepwise method explained 42% of the SPI variance and retained two predictors related to circulation over the Pacific Ocean: one of them shows the relevance of the intensity of zonal flow in mid-latitudes and the other the influence of low pressure near the area. The cross-validation used to prove model efficiency showed a correlation of 0,41 between observed and estimated SPI; a probability of detection of wet (dry) years of 80% (65%) and a false alarm relation of 25% in both cases.