The human factor in seasonal streamflows across natural and managed watersheds of North America

There is considerable debate on the effect that humans have on streamflow. While annual streamflow trends are driven predominantly by climate, human activity works to both amplify and dampen the seasonal trends of managed streamflows across North America. While it is established that climate change and human activities (for example, urbanization, dams) alter streamflows, there exists considerable uncertainty regarding the relative magnitude of their contributions. Most studies have focused on annual flows and found trends to be dominated by climate. Here we compare trends in seasonal flow totals for 315 natural and 1,957 managed watersheds across North America over 60 years (1950-2009). We find an amplification of seasonal flow trends in 44% of the managed watersheds, while 48% of the watersheds exhibit flow dampening. The magnitudes of amplification (20-167%) and dampening (5-52%) are substantial and vary seasonally. Multivariate models reveal that while rainfall, slope and forest cover are the key drivers of seasonal trends in natural watersheds, canals, impervious areas and dam storage dominate the responses in managed watersheds. Our findings of human-driven seasonal flow alterations highlight the need to develop adaptation strategies that mitigate the associated negative impacts.

Automated summary

There is ongoing debate about the impact of human activities on streamflow. While climate is the main driver of annual streamflow trends, human activities can amplify or dampen seasonal trends in managed streamflows in North America. The magnitude of these impacts and their relative contribution are still uncertain. This study compares trends in seasonal flow totals for natural and managed watersheds in North America over a 60-year period. The results show that human activities can amplify or dampen seasonal flow trends in a substantial number of managed watersheds, and the magnitude of these changes varies seasonally. Rainfall, slope, and forest cover are key drivers of seasonal trends in natural watersheds, while canals, impervious areas, and dam storage dominate the responses in managed watersheds. These findings emphasize the importance of developing adaptation strategies to mitigate the negative impacts of human-driven alterations in seasonal flow.