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A multi-scale analysis of streamflow response to changes in evapotranspiration and soil hydrology in the Blue Ridge Mountains
Citation:
Price, Katie, R. Jackson, AND S. Brantley. A multi-scale analysis of streamflow response to changes in evapotranspiration and soil hydrology in the Blue Ridge Mountains. Presented at American Geophysical Union, San Francisco, CA, December 09 - 12, 2013.
Impact/Purpose:
Presentation given at the 2013 AGU Fall Meeting
Description:
A large amount of research exploring the relationship between watershed forest cover and streamflow quantity has been conducted in the southern Blue Ridge Mountains, particularly in association with the USFS Coweeta Hydrologic Laboratory and the Coweeta LTER. However, a clear answer to the question ‘How does changing tree cover influence runoff?’ has not yet emerged for guidance of policy and management. The southern Blue Ridge is the source of water reaching much of the drought-sensitive Southeastern US, and a firmer understanding of the complexities of this issue is critical for water resources management for millions of people and diverse aquatic habitats. When this question has been explored in mesoscale systems (10s to 100s km2), results indicate that watersheds with greater forest cover have greater baseflow. Associated work has shown that hydraulic conductivities in forest soils are nearly an order of magnitude greater than lawn and pasture soils in this region. Our interpretation has been that in these mesoscale systems, the compaction of soil associated with forest conversion to other land uses has played a bigger role than related changes in evapotranspiration (ET) in shaping watershed dynamics and the overall water budget. Particular influence has been seen in baseflows, we posit, due to reduced infiltration and recharge. However, nearly a century of research in small experimental watersheds at Coweeta has shown that forest ET substantially reduces streamflows, including baseflows, when soils are not substantially altered. At this smaller scale of observations, details of forest composition and species water use variability have been thoroughly considered, while in the mesoscale studies “forest cover” is treated as regionally uniform. Current small-scale work at Coweeta has shown that hemlock decline and subsequent replacement with other species has changed the magnitude and seasonality of ET, which is detectible in streamflow quantity and timing. Here, we attempt to resolve the seemingly conflicting results from experimental watershed and mesoscale studies, and consider the implications for even larger systems more directly linked to policy and management. A singular focus on streamflow quantities ignores broader water quality considerations related to forest management and conversion. We explore the idea that the pronounced control of precipitation variability on streamflow variability in this region confounds the inference of the relative importance of other influences, such as ET and soil hydraulics, particularly at moderate levels of disturbance. We also consider the complexities of heterogeneous land use and geomorphology, which are inevitably encountered in larger watersheds. Finally, we suggest preliminary guidance and future research approaches to provide information to policy and management on the sensitivity of various systems to forest removal or species conversion, across a range of spatial scales.
