Is it deforestation affecting the discharge of the major Amazon Rivers?
Trancoso, INPA, email@example.com
Filho, INPA, firstname.lastname@example.org
Tomasella, CPTEC - INPE, email@example.com
A large part of the world’s tropical deforestation is presently concentrated along the southern and eastern margins of the Amazon basin, an area known as the “deforestation arc”. This region might play a fundamental hydrological role since it includes the headwaters of three large southern tributaries of the Amazon, the Xingu, Tapajós and Madeira rivers. We looked for trends in the hydrological cycle in 30 watersheds of these three rivers. Historical records for discharge and precipitation since the 1970’s, when rapid deforestation began in the Amazon Basin, were obtained from Brazil’s Hydrological database (Hidroweb). Watersheds areas were delineated using a Digital Elevation Model (SRTM 30). Seasonal Mann-Kendall, a non-parametric statistical test, was chosen due to the skewness of the time series, and Sen-slope estimator was applied to obtain trend magnitudes. Watershed size ranged from 4,409 to 1,324,195 km2, and deforestation levels were 0.68 % to 60.55 %. Although a discharge increase was expected as a consequence of deforestation (evapotranspiration reduction and overland flow increase), we actually observed discharge decrease at 40% of river gauge stations. Even in watersheds with more than 50% deforested area, no discharge increase was detected. We did not find any relation between watershed deforestation levels and streamflow changes. Precipitation trends were similar to streamflow, with decreases observed in 40% of catchments surveyed. Simple regression between discharge and precipitation revealed a positive relationship (R2= 0.449; p= 0.006). Despite small-scale experiments having demonstrated that deforestation leads to an increase in streamflow, this effect is not replicated in large Amazon watersheds. We conclude that is difficult to detect a deforestation signal in the hydrology of large tropical basins over the noise of natural climate trends and a scale effect. Furthermore, little is known about the hydrological impacts of forest fragmentation and open land mosaics, which are the prevailing land cover in the headwaters of the three rivers.