Impact of hydraulic redistribution on multispecies vegetation water use in a semi-arid savanna ecosystem: An experimental and modeling synthesis

Authors: LEE, E. - University Of Illinois; KUMAR, P. - University Of Illinois; BARON-GAFFORD, G. - University Of Arizona; HENDRYX, S. - University Of Arizona; SANCHEZ-COHEN, E.P. - University De Granada; MINOR, R. - University Of Arizona; COLELLA, T. - University Of Arizona; Scott, Russell - Russ

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/9/2018
Publication Date: 6/28/2018
Publication URL: https://handle.nal.usda.gov/10113/6471223
Citation: Lee, E., Kumar, P., Baron-Gafford, G., Hendryx, S., Sanchez-Cohen, E., Minor, R., Colella, T., Scott, R.L. 2018. Impact of hydraulic redistribution on multispecies vegetation water use in a semi-arid savanna ecosystem: An experimental and modeling synthesis. Water Resources Research. 54:4009-4027. https://doi.org/10.1029/2017WR021006.
DOI: https://doi.org/10.1029/2017WR021006

Interpretive Summary: A key challenge for the disciplines of ecology and hydrology is to determine how water, soil and plant below-ground processes interact with above-ground processes. Facilitating this connection is hydraulic redistribution, a phenomenon by which roots serve as preferential pathways for water movement from wet to dry soil layers. We used a computer simulation model in conjunction with experimental data to quantify the influence of hydraulic redistribution on ecohydrologic processes, such as transpiration, soil evaporation, and soil moisture dynamics in order to characterize the competitive and facilitative interaction between trees and grasses in a semiarid savanna of southwestern U.S. Both measured and simulated results show that downward hydraulic redistribution dominates during the wet monsoon season, whereas upward hydraulic distribution occurs between precipitation events. For the year-long simulation, about 17% of precipitation is absorbed as soil moisture, with the rest of the precipitation returning to the atmosphere as evapotranspiration. In the wet season, 13% of precipitation is transferred to deep soil (>1.5m) through mesquite roots, and in the dry season, 9% of this redistributed water is then transported back to shallow soil depths (<0.5m). Hydraulic redistribution supports about 47% of mesquite transpiration and 9% of understory transpiration. Through modeling and experimental synthesis, this study demonstrates that in this savanna ecosystem, mesquite trees exhibit a competitive advantage over understory bunchgrass through hydraulic redistribution.

Technical Abstract: A major challenge in critical zone science is to understand and predict the interaction between above- and below-ground ecohydrologic processes. A process that facilitates this connection is hydraulic redistribution, a phenomenon by which roots serve as preferential pathways for water movement from wet to dry soil layers. We use a multi-layer canopy model in conjunction with experimental data to quantify the influence of hydraulic redistribution on ecohydrologic processes, such as transpiration, soil evaporation, and soil moisture dynamics in order to characterize the competitive and facilitative interaction between overstory mesquite trees and understory bunchgrasses in a semiarid savanna. Both measured and simulated results show that hydraulic descent dominates sap flux during the wet monsoon season, whereas hydraulic lift occurs between precipitation events. For the year-long simulation, about 17% of precipitation is absorbed as soil moisture, with the rest of the precipitation returning to the atmosphere as evapotranspiration. In the wet season, 13% of precipitation is transferred to deep soil (>1.5m) through mesquite roots, and in the dry season, 9% of this redistributed water is then transported back to shallow soil depths (<0.5m). Assuming water supplied through hydraulic redistribution is well-mixed with moisture transported directly through the soil matrix and supports vegetation evapotranspiration, hydraulic redistribution supports about 47% of mesquite transpiration and 9% of understory transpiration. Through modeling and experimental synthesis, this study demonstrates that in semiarid mesquite savanna ecosystems of the southwestern U.S., mesquite trees exhibit a competitive advantage over understory bunchgrass through hydraulic redistribution.