The future of the La Plata Basin hydrological cycle as simulated under different WCRP-CMIP3 dataset emission scenarios

SAURRAL, R.

Foz do Iguazu

Conferencia; 2010 Meeting of the Americas; 2010

The La Plata Basin (LPB) is the fifth largest river basin in the world and second largest in South America, providing fresh water to hundreds of millions of people across five countries. The hydrological modeling helps simulating the main hydrological processes and also allows the scientific community to further understand important topics such as land-atmosphere feedbacks and the potential impacts of climate variability on the hydrology of particular basins. In this paper the hydrology of LPB is analyzed by performing simulations of the water cycle of the main rivers of the basin using a distributed hydrology model forced with observed temperature and precipitation data. Once a good fit is obtained, the model is forced using data from five General Circulation Models (GCMs) for the present climate. It becomes evident that using the data from the GCMs directly as input for the hydrology model leads to very poor simulations, mainly associated to the existence of systematic biases in the representation of the present climate by the GCMs. Then, unbiasing schemes are applied to the data and, after showing the success of these schemes in reducing the errors, new simulations are driven with future socio-economic scenarios (A1B, A2 and B1) to infere the potential changes in the water cycle for the upcoming decades (two future decades are considered: 2030 and 2070). Results suggest a decent consensus on the future changes of temperature (all the GCMs show a warming trend over LPB), but changes in precipitation are less homogeneous. This is crucial, because the hydrological cycle of LPB is highly dependant on the precipitation regime and, thus, small differences in the annual rainfall can lead to substancial differences in the final streamflow at a given basin’s closing point. Then, different solutions are obtained depending on the GCM considered. The ensamble mean of the five GCMs, though, suggests an increase in streamflows during the next decades at all the closing points (i.e., for the Upper Paraná, the Upper Paraguay and the Uruguay river basins). The amount of variation largely depends on the socio-economic scenario considered, with the largest increases associated to scenario A2 (which is the most aggresive in terms of surface warming for the future) and the smallest changes associated to the B1. Then, these results would make one expect a gradual increase in fresh water availability across the region for the next decades. However, caution must be taken when considering these outputs as GCMs are still far from being realistic: in this research, only the systematic bias is removed prior to forcing the hydrology model with GCM outputs, but other modes of variability (such as low frequency -interdecadal- oscillations) were not taken into account in this analysis and precisely these other modes of variability can have an enormous impact on the climate in the future (especially in the next few decades). Furthermore, aspects as land use change were not considered for the future and these can also have large impacts on the final discharges of the main rivers, as many previous works have already shown.