Abstract:

Studies that relate changes in land cover with changes in river discharge at the small scale (< 1 km2) are abundant. These studies generally indicate that deforestation causes an increase in the annual mean discharge. However, previous studies that evaluated the effects of changes in land cover in large river basins (> 100 km2) usually have not found similar relationships. Here we analyse a 50-year long time series of discharge of a tropical river, 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 occurred in the basin (1949-1998). Based on agricultural census data, we estimate that, in 1960, about 30% of the basin was used for agriculture. Previous work indicates that by 1995, agriculture had increased substantially, with about 49% of the basin land used as cropland and pastures. 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). Our analysis indicates that, while precipitation over the basin is not statistically different between period 1 and period 2 (£\=0.05), annual mean discharge in period 2 is 24% greater than in period 1 (P < 0.02), and the rainy season discharge is greater by 28% (P < 0.01). Further analyses present additional evidence that the change in vegetation cover altered the hydrological response of this region. As the pressure for changes in land cover in that region increase, one can expect important changes in the hydrologic regime of the Tocantins River.