Temporal partitioning of water between plants and hillslope flow in a subtropical climate

Abstract

Recent work has suggested that ecohydrological separation may exist between the water sources for recharge and plant water uptake. However, the temporal partitioning of plant transpiration and hillslope flow is still poorly understood. In a growing season, the stable isotopes of precipitation, soil water, groundwater, plant water and hillslope flow in a subtropical climate in Southwest China were determined to assess the compartmentalization of vegetation water use and flow generation. The results suggest that the hillslope flow and plant water have different isotopic characteristics in most cases. The lc-excess values of plants significantly differed from those of the hillslope flow. These different isotopic signatures for plants and the hillslope flow were associated with the different proportions of various water sources in each water pool. Precipitation, the hillslope flow and soil water plot approximately along the local meteoric water line (LMWL), and the studied plant xylem waters plot partly below the LMWL, supporting ecohydrological separation. In this subtropical climate with seasonal droughts, the hydrological separation is temporal and does not occur during the wet season due to the increase in hydrological connectivity. On dry days, the various water sources poorly mix in the subsurface. Thus, the ecohydrological separation between the plant water and hillslope flow water sources varies depending on the rooting depth of plant species and moisture conditions. The implications underlying these findings will be helpful for constructing a process-based ecohydrological model and for understanding the mechanisms underlying the hydrologic interactions between plants and subsurface water flow.