MEASURING, UNDERSTANDING AND MODELING ECOHYDROLOGICAL SEPARATION

My dissertation sought to answer some of the fundamental questions on how subsurface water may be partitioned between root water uptake and streamflow. I explored a phenomenon called ecohydrological separation – plants using water of a character different from the mobile water found in soils, groundwater and streams. The generality of ecohydrological separation, however, remained wanting; and, possible controls in both space and time was elusive. I began with testing the generality of ecohydrological separation, first at two sites in the tropics with contrasting moisture conditions, and then at the global scale. Using a global database of water stable isotopes, I then quantified the degree of groundwater use by vegetation. Finally, I unscrambled the possible process controls behind the partitioning of subsurface water between root water uptake, groundwater recharge, and streamflow generation by conducting controlled drought-rewetting experiments in a tropical mesocosm. Key results of these research efforts were: (1) ecohydrological separation was widespread across biomes of the world, providing clues to fundamental controls; (2) groundwater use by vegetation globally was not as widespread as increasingly assumed in the literature; and, (3) transpiration flux was older than groundwater recharge flux, supporting a perceptual model whereby transpiration and groundwater recharge fluxes were sourced from separate storage volumes and sampled at markedly different average sampling flux. Because determining the ages and sources of water that supply transpiration and groundwater recharge was a major challenge in ecohydrology, these findings are ground-breaking. Indeed, I was the first to measure and quantify what was referred heretofore as the “missing exit age” of transpiration. The mechanisms underlying the phenomenological manifestations of ecohydrological separation, as explored and uncovered in my dissertation, have direct implications for how we measure and model the transport of water, nutrients, and pollutants at various scales in space and time