Source Water Variability to Soils of Large, Forested Floodplains

Forested wetlands are prominent features across floodplains of the southeastern United States. The importance of flooding in controlling community structure has been well documented, but the role of seasonal water limitation has been under studied, even though it may become more ecologically important due to intensified river management and climate change. In particular, the source of soil moisture for tree growth during the late summer dry period is relatively unknown. Therefore, in this study we investigated source water to the root zone using stable isotopes of water to trace origin to precipitation, surface flood water, and shallow groundwater at the end of the growing season in the floodplains of the Tensas, Boeuf, and Sabine rivers in Louisiana and Texas. Preliminary analysis show that the spatial variability of soil water deuterium depth profiles decreases throughout the growing season as the floodplain dries. This indicates higher spatial variability in source water sources during the early portion of the growing season and an increasing reliance on spatially homogeneous sources (i.e. precipitation) during the latter half of the growing season. In addition, soil water deuterium depth profiles at the lowest sites with the highest water table levels have the lowest temporal variability during the growing season. This suggests that root zone soil moisture of these sites is dominated by shallow groundwater and associated capillary rise relative to higher elevation sites. Ongoing analyses of 2018 field data and isotopic modeling aim to clarify the relative importance of precipitation, surface flood waters, and shallow groundwater late in the growing season.