Coupled roles of hydraulic redistribution and sub-surface moisture transport in semiarid ecosystems

Lee, Esther

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https://hdl.handle.net/2142/113958 Copy

Description

Title

Coupled roles of hydraulic redistribution and sub-surface moisture transport in semiarid ecosystems

Author(s)

Lee, Esther

Issue Date

2021-10-15

Director of Research (if dissertation) or Advisor (if thesis)

Kumar, Praveen

Doctoral Committee Chair(s)

Kumar, Praveen

Committee Member(s)

• Valocchi, Albert
• Dominguez, Francina
• Druhan, Jennifer
• Barron-Gafford, Greg

Department of Study

Civil & Environmental Eng

Discipline

Civil Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• hydraulic redistribution
• ecohydrology
• semiarid ecosystem
• modeling

Abstract

In critical zone science, understanding and predicting the interaction between above- and below-ground ecohydrologic processes has been one of the major challenges. Roots play a key role in linking above-ground plant ecophysiological processes and the below-ground processes associated with water, carbon, microbial, and nutrient dynamics. Hydraulic redistribution is a passive process, driven by soil-water potential gradients, where roots serve as preferential pathways for water movement from wet to dry soil layers. Understanding the role of hydraulic redistribution in facilitating the interactions between above- and below-ground ecohydrologic dynamics is crucial. Although studies of hydraulic redistribution have been conducted across a range of vegetation species and climate gradients, a predictive characterization through model and data integration for developing a comprehensive understanding of coupled roles of hydraulic redistribution and subsurface moisture has remained an open challenge. The goals of this research are to: (i) investigate the role of hydraulic redistribution as a mechanism that drives moisture transport and promotes facilitative or competitive interactions between deep-rooted and shallow-rooted coexisting vegetation species; (ii) analyze the dependence of the characteristics of hydraulic redistribution on different attributes of precipitation variability in semiarid ecosystems with and without root-accessible groundwater; and (iii) examine the coupled roles of hydraulic redistribution and lateral moisture transport in below-ground moisture dynamics in riparian environment. This study is conducted as a synthesis of the modeling approach with field measurements to systematically analyze the role of hydraulic redistribution based on a cross-site comparison of the upland and the riparian semiarid ecosystems where deep-rooted trees and shallow-rooted grasses coexist. Based on the model-data integration, we found that under the presence of a root-accessible groundwater, coexisting trees and grasses tend to share the redistributed water and demonstrate facilitative dependence whereas in its absence the relationship is competitive. When compared to the measured sap flow through a taproot of a tree, ecohydrologic simulation provides a close agreement, demonstrating downward moisture transport via tree roots as a response to seasonal precipitation during the North American monsoon. Through this study, we identified a novel hydraulic redistribution pattern where trees transport moisture in a convergent direction from the uppermost layer and the groundwater to the dry middle soil. The occurence of the bi-directional moisture flow within a single root system, termed as convergent hydraulic redistribution, has never been reported in past studies linking moisture transport between plant roots and soil. To examine the linkage between precipitation variability and the corresponding patterns of moisture transport via roots, we introduce four indices that characterize precipitation variability, which include (i) standard deviation of precipitation events, (ii) number of consecutive dry days between precipitation, (iii) total annual precipitation, and (iv) seasonality of precipitation. These are then assessed for their link to seven different attributes of hydraulic redistribution, which include the amounts of moisture transported (a) upward as hydraulic lift, (b) downward as hydraulic descent, and (c) in convergent direction through plant roots. Also (d) total amount of hydraulically redistributed water (HRW), (e) the depth where the HRW is released to the soil, and the amount of HRW (f) used by the coexisting trees and (g) grasses are examined in relation to the precipitation indices. Results based on a multiple linear regression model demonstrate that different characteristics of hydraulic redistribution show dependence on different attributes of precipitation variability, and this dependence varies considerably between the upland and the riparian semiarid environments. For example, the amount of moisture that is redistributed upward is significantly affected by the amount of precipitation when plants lack access to deep moisture sources, whereas it is affected the most by the length of dry periods when trees have access to groundwater. In order to characterize the role of lateral moisture transport induced by stream level in the riparian zone, we use a three-dimensional ecohydrologic model to integrate continuous lateral moisture transport from a river that supplies moisture into the surrounding soil. This lateral moisture flow plays an important role in promoting moisture uptake and release patterns via roots, which alters the belowground moisture matrix over time. Complex patterns of moisture uptake and release in the soil column through convergent hydraulic redistribution and through moisture redistribution to multiple soil depths emerge. This thesis broadens our understanding of the interplay among above-ground ecohydrological processes of plant water exchange with below-ground processes of hydraulic redistribution and subsurface moisture transport. Based on the data-model integration, this work provides a holistic view of moisture exchange in root-soil continuum that is critical in understanding and predicting the response of coexisting vegetation species in semiarid ecosystems to climate variability.

Graduation Semester

2021-12

Type of Resource

Thesis

Permalink

http://hdl.handle.net/2142/113958

Copyright and License Information

2021 Esther Lee

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