A key challenge in critical zone science is to understand and predict the interaction between aboveground and belowground eco-hydrologic processes. This study analyzes aboveground and belowground interaction through hydraulic redistribution (HR), a phenomenon that roots serve as preferential pathways for water movement from wet to dry soil layers. By comparing ecosystems in upland (mean groundwater depth > 100m) and riparian (mean groundwater depth ~ 10m) sites, we study how a connection to deep-water resources influence the balance of hydraulic descent (HD, overall downward water movement through roots) relative to hydraulic lift (HL, overall upward water transport through roots), and how this connection to deep-water resources influence the interaction between co-existing vegetation in a water-limited environment. Both study sites are located in southern Arizona: the upland site is in the Santa Rita Experimental Range and the riparian site is located in the San Pedro National Riparian Conservation Area. In both sites, we simulate eco-hydrologic model in conjunction with experimental data to quantify the influence of HR on ecohydrologic processes in order to characterize the competitive and facilitative interaction between co-existing overstory (velvet mesquite) and understory (sacaton bunchgrass) vegetation.
In both sites, we analyze how two vegetation species share and utilize water through HR when groundwater access is absent (in upland site) and present (in riparian site). In upland site, both measured and simulated results show that HD dominates during the wet monsoon season, whereas HL occurs between precipitation events. For 2015 year-long simulation, we find that in the wet season, 13% of precipitation is transferred to deep soil (>1.5 m) through roots, and in the dry season, 9% of this redistributed water is then transported back to shallow soil depths (<0.5 m). HR supports 47% of mesquite transpiration and 9% of understory transpiration. This study demonstrates that in upland site mesquite exhibits a competitive advantage over bunchgrass through HR. By comparing the impact of HR on co-existing vegetation interaction in upland and riparian sites, we evaluate how much the connection to deep-water resources influences the balance of HD to HL and how mesquite HR affects understory function.
Lee, E., Kumar, P., Barron-Gafford, G., Scott, R.L. (2018): Comparing the Impact of Hydraulic Redistribution on Co-existing Vegetation Interaction in Semi-arid Upland and Riparian Sites. Abstract H11A-08 presented at 2018 AGU Fall Meeting, Washington, D.C., 10-14 Dec.