SPATIAL AND TEMPORAL VARIABILITY OF RUNOFF AND STREAMFLOW GENERATION WITHIN AND AMONG HEADWATER CATCHMENTS: A COMBINED HYDROMETRIC AND STABLE ISOTOPE APPROACH

The combined influence of topography and vegetation on runoff generation and streamflow in headwater catchments remains unclear. We aim to understand how spatial, hydrological and climate variables affect runoff generation and streamflow at hillslope and watershed scales at the Coweeta Hydrologic Laboratory (CHL) in the southern Appalachian Mountains by analyzing stable isotopes of hydrogen (²H) and oxygen (¹⁸O) coupled with measurements of hydrological variables (stream discharge, soil moisture, shallow groundwater) and landscape variables (upslope accumulated area, vegetation density, slope, aspect). We investigated four small catchments, two of which contained broadleaf deciduous vegetation and two of which contained evergreen coniferous vegetation. Beginning in June 2011, we collected monthly water samples at 25 m intervals along each stream, monthly samples from 24 shallow groundwater wells, and weekly to monthly samples from 10 rain gauges distributed across CHL. Water samples were analyzed for ²H and ¹⁸O using cavity ring-down spectroscopy. During the same time period we recorded shallow groundwater stage at 30 min intervals from each well, and beginning in fall 2011 we collected volumetric soil moisture data at 30 min intervals from multiple depths at 16 landscape positions. Results show high spatial and temporal variability in δ²H and δ¹⁸O within and among streams, but in general we found isotopic enrichment with increasing contributing area along each stream. We used a combination of hydrometric observations and geospatial analyses to understand why stream isotope patterns varied during the year and among watersheds, and we used complementary measurements of δ²H and δ¹⁸O from other pools within the watersheds to understand the movement and mixing of precipitation that precedes runoff formation.This combination of high resolution stable isotope data and hydrometric observations facilitates a clearer understanding of spatial controls on streamflow and their complex interrelationship with hillslope and climatic response in a catchment. In addition, understanding the role of topography and vegetation in these catchments can help scientists and managers better assess the i­­­­mpacts of vegetation activity and disturbance on water supplies downstream of forested headwater catchments.