Water Use Patterns of a Riparian Forest in a Humid Subtropical Catchment in the Southeastern United States

The role of groundwater in sustaining plant transpiration has been studied for nearly a century. However, the body of literature investigating plant uptake of groundwater has largely been focused on arid and semiarid climates, with few examples from more humid locations. In this dissertation, I attempt to contribute a rigorous evaluation of groundwater transpiration (TG) from a humid riparian forest to fill in this knowledge gap. In chapter two, I explored the groundwater use by a riparian forest using techniques that exploit diurnal water table fluctuations from groundwater wells. Specifically, I investigated the spaciotemporal variability of TG using nine groundwater wells in a small headwater catchment in the Piedmont of Georgia. Results indicated a relatively high degree of variability, 3.30 ± 1.05 mm d-1 but lacking a consistent spatial pattern. Furthermore, groundwater derived transpiration was approximately 22 % of the average baseflow discharge over the growing season; indicating that even in humid regions plant transpiration can be a substantial component of the seasonal water budget. In chapter three, I incorporated an independent estimate of canopy transpiration (EC) to better constrain the estimates of TG from chapter two. This was motivated by wanting to partition the total water used by the riparian forest into groundwater and soil water sources. However, the results were not as anticipated. For the 2019 growing season TG was 455 mm, approximately twice as much as EC (241mm). This instead highlighted a methodological issue that had not been previously addressed. The formulae for estimating TG lacks a defined area of influence; however, in the present study I estimated this area of influence would have be 2 – 10 times larger than the delineated riparian zone for the fluxes to balance. In chapter four I explored the response of a riparian forest to a rapid onset flash drought. Results indicated that there was not watershed wide response of the forest canopy to the drought. However, at individual trees water use patterns did suggest a drought response, one that was dominated by an increase in reverse sap flow suggesting hydraulic redistribution was occurring to compensate for the excessively dry soils