A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Scaling up from pastures to watersheds: The spatial and temporal structure ofhuman impacts on stream nutrients.Biggs, T.W. 1 , Dunne, T. 2 , Roberts, D.A. 1 , Karen Holems 1 , and Martinelli, L.A. 31. Department of Geography, University of California, Santa Barbara, CA 931062. Donald Bren School of Environmental Science and Management, University ofCalifornia, Santa Barbara, CA 931063. Centro de Energia Nuclear na Agricultura, Piracicaba, Sao Paulo.In past LBA conferences, we reported that land uses besides deforestation, such asurbanization and agricultural intensification, may have significant impacts on streamnutrients compared with conversion of primary forest to pasture. In this presentation, weput our observations of land use and stream chemistry in the spatial and temporal contextof frontier development using a time series of satellite images, census data, and a digitalelevation model to delineate watersheds. With these data, we define the probabilitydistributions of total deforestation extent and of the rates of conversion of primary forestto other land uses for approximately 30,000 watersheds varying in size from 2.5 to100,000 km 2 . The maximum annual and three-year deforestation rates decline rapidlywith watershed size, suggesting that deforestation is a gradual process for watershedslarger than 100 km 2 , and that the relatively short-lived “pulses” of stream nutrientsassociated with vegetation conversion in smaller watersheds are not likely to be observedin watersheds larger than 100 km 2 . Simultaneously, the probability that any givenwatershed contains an urban population increases with watershed size, suggesting a shiftin the relative importance of non-urban and urban sources of nutrient contamination withwatershed size.Contrary to results of studies in small watersheds (< 1km), streams draining nonurbanizedwatersheds (10-1000 km 2 ) with high rates of primary forest conversion topasture do not exhibit the greatest impacts on stream chemistry. Rather, the largestincreases in chloride and nutrient concentrations occur in watersheds with little or nocurrent conversion of primary rainforest to pasture, but that have average clearing agesgreater than 10 years. This pattern, and the high stream chloride signal in watershedswith longer occupation times, suggests that vegetation conversion is not the primaryagent driving anthropogenic impacts on stream nutrients. Other processes, possibly theestablishment and intensification of cattle production, is likely responsible for the impactsin non-urbanized watersheds.Additional stream sampling of larger watersheds (1,000-30,000 km 2 ) shows that thelargest impacts on stream nitrogen occur in watersheds of intermediate drainage areas(700-3000 km 2 ), though a land use signal is still detectable in streams draining areas aslarge as 30,000 km 2 . The watersheds with high stream nutrient impacts contain urbanpopulations, suggesting that urban sources may dominate human impacts on streamnutrients for watersheds larger than ~1000-5000 km 2 .