Water-table fluctuations to estimate transpiration from groundwater: What is the area of contributing vegetation?

Groundwater can be an important source of moisture for plants in many environments. However, this flux of water is difficult to observe and quantify. One approach that has been used to estimate groundwater transpiration (TG) is the analysis of water-table fluctuations. These methods have been widely applied in subhumid areas across the globe. However, few studies have used these methods in more humid environments, fewer still in catchments with topographic complexity. A key factor that is lacking in the published methods is the consideration of a specified area of influence. Rather, these approaches are generally applied with an assumption that the point estimates are representative of the pre-defined area of interest. We question how well such an assumption translates to a more topographically complex terrain, where water table depth and forest composition can change over relatively short distances. We compared TG estimated from water table fluctuations to total canopy transpiration (EC) estimated from scaled sap flux measurements and explored scenarios of potential areas of vegetation influencing the water table fluctuations. This study was conducted in the 3-ha riparian zone at the Panola Mountain Research Watershed in the Southeastern U.S. We instrumented 20 trees with sap flow sensors and monitored groundwater levels in 9 wells. We scaled sap flux from the instrumented trees to the whole riparian zone based on species and sapwood area. Riparian zone TG was estimated by taking the daily median TG estimated from all wells and scaling to the riparian zone area. Total canopy transpiration was 241 mm whereas total TG was 455 mm. Hence, the water-table fluctuation method yielded estimates of transpiration of groundwater that were nearly double the amount of total canopy transpiration. Both EC and TG estimates are of similar magnitude as reported in other published studies in the region. We conclude that the seemingly inflated estimates of TG result because the area of vegetation influencing TG may be much larger than just our delineated riparian zonepossibly as much as two to three times that area. This finding calls into question the application of these methods in areas that are topographically complex and host diverse plant communities. The results also highlight an area of research that needs further investigation.