The Impact of Rainfall and Land Cover Changes on the Flow of a Medium-sized River in the South of Brazil

Abstract

Due to the complexity associated to the atmospheric global circulation models and because they operate globally, with resolutions involving hundreds of kilometers, they are not suitable to represent small-scale processes, such as precipitation, which is a fundamental variable to hydrological modeling studies on medium-sized basins. In addition, even studies based on regional models have showed difficulties in representing precipitation during summer months, mainly due to the higher occurrence of extreme events. Otherwise, hydrological models have a greater chance of success in evaluating the local impact of future scenarios of climate change as well as changing conditions of land use and land cover. This study analyzed the occurrence daily flow rates at two stations located in the Ivaí River Basin in Brazilian state of Paraná. It was examined whether there is a change of trend for flow measurements in recent years compared to the first decades of the data series. Historical rainfall series were analyzed in order to verify if the pattern of rainfall in the basin follows the trends observed for the river flow. In relation to rainfall data it was observed that the average annual rainfall slightly increased and the percentage of rainy days significantly decreased, indicating that the rain events, on average, increased and concentrated in a shorter time, explaining the observed increase in extreme rainfall events (events with rates above 80mm.day-1). A hydrological model was applied in order to verify the role of land cover changes on the observed stream flow trends. The hypothetical scenarios consisted of replacing the current land covered by crops, pasture and original forest. The only significant change in the stream flow was observed for the forest-recovered scenario. In this case, the results indicated a strong reduction in the flow when compared to the current scenario, which is consistent with many recent studies, but not consistent with the trends observed in recent decades in the Ivaí River. This result suggests that a causal relationship between the observed streamflow trends and land cover changes are open to interpretation and need complementary studies.