Conversion to pine: Changes in timing and magnitude of high and low flows.
Understanding watershed responses to extreme events is important for assessing potential impacts of floods, droughts, episodic pollution, and other external driving variables on watershed resources. In this study, we combine trend and frequency analyses with paired watershed techniques to evaluate the long-term high- and low-flow data from Coweeta Hydrologic Laboratory in North Carolina, USA in an attempt to quantify and interpret responses to extreme flow events in managed and unmanaged watersheds. Two experimental watersheds were converted from mixed deciduous forest cover to pine (Pinus strobus L.) in 1957 and 1956, respectively and two others were kept untreated to serve as control watersheds. Seventy years of annual streamflow (instantaneous maximum, minimum, and mean) time series data from watersheds 1, 2, 17, and 18 were analyzed with a Mann-Whitney-Pettitt test to identify and compare change points. Mean annual streamflow increased in both watersheds for 10-12 years after harvests, but the more effective factor on the flow series was the growth of pine plantations. Maximum flows of 2-, 5-, and 10-year return periods decreased after planting pine, but there was no difference for larger flow events compared to deciduous forest. For minimum flows, pine stands were more effective compared to maximum flows as minimum flows decreased for all return periods.