Space-time Variability of Baseflow in Headwater Streams of the Southern Appalachians

Abstract:

How hillslope shape and structure affect the spatial and temporal variability of streamflow is a key question in catchment hydrology. In particular, the role of hillslope structure and arrangement in mediating the longitudinal variability of streamflow in first-order headwater catchments remains poorly understood. Here we assess the spatio-temporal variability of streamflow under baseflow conditions in forested headwater catchments characterized by different topography and vegetation. We studied the relationship between hillslope structure and baseflow along a series of first-order streams using a combination of in-stream water isotope sampling, stream gauging, and geospatial analysis. We studied two pairs of small (< 15 ha) forested catchments at the Coweeta Hydrologic Laboratory (CHL), a long-term ecological research (LTER) site in the southern Appalachian Mountains. The catchments comprise different combinations of aspect (north and south) and vegetation type (deciduous and evergreen), and they contain hillslopes covering a range of sizes. We sampled water along first-order streams, from shallow hillslope wells, and from rainfall collectors distributed across CHL. These monthly samples, collected from June 2011 through June 2013, were analyzed for stable isotopes of water (¹⁸O and ²H) using laser spectrometry. During the same period, we quantified longitudinal changes in stream discharge at different baseflow states for each catchment and developed discharge versus contributing area relationships along streams within each continuously gauged catchment. We found relationships between stream water isotopic composition, landscape variables, and hillslope discharge, which was estimated as the difference in streamflow between consecutive measurement locations. Results showed that landscape heterogeneity exerts a strong control over longitudinal variability of baseflow by modulating the timing and volume of lateral flow along the stream gradient. However, the strength of relationships changed with seasons and flow states, suggesting that seasonality and climate interact with hillslope structure and arrangement to either enhance or dampen the influence of landscape heterogeneity on streamflow in headwater catchments.