Evaluating the spatial and temporal variability of groundwater uptake by riparian vegetation in a humid southeastern US catchment

In environments with shallow water tables, vegetation may use groundwater to support transpiration (T_G). This process has been carefully studied in some arid climates but rarely in humid climates—even those with severe droughts and seasonal water deficits. As such, the role of T_G in humid-catchment hydrology is poorly constrained. We analysed water table fluctuations from nine monitoring wells along three transects in a second-order forested catchment to estimate T_G at plot and whole-riparian zone scales. Average T_G estimated around all well locations ranged from 1.06 to 4.95 mm d⁻¹ and did not change systematically as a function of distance from stream channel or with plot-scale tree basal area. Counter to some previous studies, we found that T_G was greater when the water table depth was deeper. Furthermore, the pattern of T_G with water table depth was not monotonic at all locations. The ratio of T_G to potential evapotranspiration tended to increase over the growing season, reflecting the progressive decrease in soil moisture storage and a greater reliance by vegetation on groundwater. Due to the lack of consistent spatial patterns in T_G, we explored the number of monitoring wells needed to consistently estimate average T_G within the 95% confidence bounds of the true mean. Based on this analysis, six or more wells were needed to consistently fall within the 95% confidence interval of the true mean. While this is based on the observed variability at a single site, it provides information for others considering this approach in similar upland forested catchments in humid regions.