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

Abstract

AbstractIn environments with shallow water tables, vegetation may use groundwater to support transpiration (TG). 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 TG 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 TG at plot and whole‐riparian zone scales. Average TG 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 TG was greater when the water table depth was deeper. Furthermore, the pattern of TG with water table depth was not monotonic at all locations. The ratio of TG 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 TG, we explored the number of monitoring wells needed to consistently estimate average TG 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.