Controls on the isotopic composition of soil water in fine-grained floodplains

Abstract:

Precipitation, river water, and groundwater can all potentially contribute to floodplain soil moisture stores. However, there has been little investigation into which waters replenish soil moisture in fine-grained floodplains and if their contributions vary seasonally. We used stable isotopes to identify the sources of soil moisture in three floodplain forests in Louisiana and Texas, USA over a two year period. Overall, results indicated that precipitation and groundwater are the most important sources of recharge for soil moisture, but there was high spatial heterogeneity. Unsaturated-zone water isotopes were above the local meteoric water line, especially late in the growing season. While it is possible that uncertainty in the pore water – vapor equilibrium fractionation factor used with the direct equilibrium technique may explain this observation, the structure of the data indicates that a previously unaccounted-for process may be at work. Specifically, we hypothesize that a lateral distillation process, in which water infiltrating along root channels evaporates and condenses onto distal soil surfaces, could have resulted in depleted soil water isotopes. We also hypothesize that this process may be more pronounced in smectitic soils due to the common occurrence of disequilibrium between macropores and the soil matrix in structured soils.