Abstract

The Purapel watershed in central Chile has experienced drastic land-use changes since the 1950s, beginning with the conversion of native forests to forest plantations. Controversy exists on the effects of reforestation on sediment production in watersheds. The temporal evolution of stage-discharge curves and sediment production was evaluated and correlated with historic sediment deposition at the Purapel watershed's main gauging station. Annual stage-discharge curves and the evolution of the stage-discharge relationship over time were developed. Trends were generated using the Mann-Kendall statistical test, comparing results with the evolution of sediment downstream. Obtained results denote a positive and significant trend at flows greater than 90 m(3)s(-1)(Z = 2.77). This can be explained by major flows that deepen the bed, which is consistent with a significant and sustained decrease in channel width (Z = -2.51) and the production of sediments (Z = -2.49). Similarly, the average annual number of curves decreased from 1.7 to 1.4. This analysis supports the conclusion that sediment production from the Purapel watershed drastically decreased after the majority of forest plantations were established around 1990, with important implications on how to retain sediments on degraded slopes through large-scale afforestation projects, as well as the necessary politics to make it happen.