Climate scenarios for the 21st century: influence on the discharges of the Plata Basin

CAMILLONI, I.; SAURRAL, R.; MEZHER, R.; BARROS, V.

Concordia

Taller; IV Taller Internacional sobre Enfoques Regionales para el Desarrollo y Gestión de Embalses en la Cuenca del Plata; 2005

Most of the Plata River discharge (about 26,000 m3 s-1) originates in the north of the basin, namely over Brazil and Paraguay where the runoff accounts for only 30 % of the mean precipitation volume. Therefore, the percentage changes in discharges generally magnify the corresponding changes in either precipitation or evaporation. In fact, in the last few decades, there were positive trends in the mean annual precipitation over this part of the basin, which resulted in amplified discharge. Since many climate scenarios are projecting several degrees of warming over this region during the course of this century, the resulting enhanced evaporation may also lead to considerable changes in discharge. Since the hydrologic response to 4 or 5°C of warming could be important, a simple model with relative small percent error may be a useful tool to get a first order assessment of the future trends in discharges. Therefore, the objective of this paper is to estimate, as first approximation and by means of a simple hydrologic balance, the potential evolution of total discharges in the Plata Basin. This simple balance adjusts an evaporation formula in such a way that its mean long-term values are compatible with the mean long-term precipitation and discharge values assuming that infiltration is a lot smaller. The formula so adjusted makes possible the monthly calculation of water evaporation by means of temperature and precipitation data. For this region the additional evapo-transpiration potential would be of 100 mm for each degree of temperature rise. An approximate estimate indicates that, for a 2°C (5°C) warming of the surface temperature in this region, the lowering of discharges in the Plata Basin would be in the region of one quarter (two thirds) of its present value. This subject has considerable uncertainties, the most important being the incapability of the climate models to represent the structure of the actual precipitation fields and surface temperature.