Effects of Large-Scale Changes in Land Cover and Climate Variability in the Discharge of the Tocantins River

At the small scale (< 10 km2), studies that relate changes in land cover with changes in river discharge are abundant. Usually, these studies indicate that deforestation causes an increase in the annual mean discharge, and an increase in the wet season discharge and a decrease in the dry season discharge. However, previous studies that evaluated the effects of changes in land cover in large river basins (> 100 km2) could not find similar relationships. Here we analyze a 50-year long time series of discharge of the Tocantins River at Porto Nacional (175,360 km2), as well as precipitation over this drainage area, during a period where substantial changes in land cover happen in the basin (1949-1998). Initially, we compare one period with little changes in land cover (period 1 - 1949-1968) with another with more intense changes in land cover (period 2 - 1979-1998). Based on agricultural census data, we estimate that, in the middle of period 1 (1960), about 30% of the basin was used as natural pasture, while in the end of period 2 (1995), about 60% of the basin land was used as cropland and pastures. Our analysis indicates that: (a) annual mean precipitation over the basin is not different between period 1 and period 2, at the 95% level of significance (l.s.); (b) annual mean discharge in period 2 is greater than in period 1, at the 97% l.s.; (c) wet season precipitation in period 1 is not significantly different from the wet season precipitation in period 2, at the 95% l.s.; (d) wet season discharge in period 2 is greater than in period 1, at the 99.9% l.s. Our study indicates that, in this basin and for the period considered, the large-scale effects of land cover change are consistent with the effects observed at the small scale. Finally, we compare the effects of climate variability with the effects of changes in land use, by selecting the five wettest and five driest years in each 20-year period. We find that the variability of discharge of the Tocantins River is more affected by climate variability than by land cover change.