Understanding the role of deep root water uptake in Amazon hydroclimate using modeling and observational approaches

Bieri, Carolina A

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https://hdl.handle.net/2142/129445 Copy

Description

Title

Understanding the role of deep root water uptake in Amazon hydroclimate using modeling and observational approaches

Author(s)

Bieri, Carolina A

Issue Date

2025-04-28

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

Dominguez, Francina

Doctoral Committee Chair(s)

Dominguez, Francina

Committee Member(s)

• Jain, Atul
• Fan Reinfelder, Ying
• Proistosescu, Cristian

Department of Study

Atmospheric Sciences

Discipline

Atmospheric Sciences

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Land surface modeling
• hydroclimate
• land-atmosphere interactions
• evapotranspiration
• vegetation optical depth
• rooting depth

Abstract

Plant roots are critical pathways of moisture from the subsurface to the atmosphere. Deep moisture uptake by roots can act as a seasonal buffer mechanism in regions with a well-defined dry season such as the southern Amazon. Here, mature forests maintain transpiration (a critical source of atmospheric moisture in this part of the world) during the dry season. Most existing state-of-the-art earth system models do not have the necessary features to simulate subsurface-to-atmosphere moisture variations during drydowns. These features include groundwater dynamics, a sufficiently deep soil column, dynamic root water uptake (RWU), and a fine model spatial resolution (< 5 km). To address this, we present DynaRoot, a dynamic RWU scheme implemented within the Noah-MultiParameterization (Noah-MP) land surface model, a widely used tool for studying kilometer-scale regional land surface processes. Our modifications to Noah-MP include implementation of DynaRoot, eight additional resolved soil layers reaching a depth of 20 m, and soil properties that vary with depth. DynaRoot is computationally efficient and ideal for regional- or continental-scale climate simulations. We perform four 20-year uncoupled Noah-MP experiments for a region in the southern Amazon basin. Each experiment incrementally adds physical complexity. The experiments include default Noah-MP with free drainage (FD); a case with an activated groundwater scheme that resolves water table variations (GW); a case with eight added soil layers and soil properties that vary with depth (SOIL); and a case with DynaRoot activated (ROOT). Our results show the following: -DynaRoot allows mature forests in upland regions to avoid water stress during dry periods by taking up moisture from the deep vadose zone (where antecedent precipitation is still draining downward). -Valley vegetation can take up moisture from groundwater (while remaining constrained by the water table). -Temporally, we capture a seasonal shift in RWU from shallower soil layers in the wet season to deeper layers in the dry season, particularly over regions with dominant evergreen broadleaf (forest) vegetation. -Compared to the control (FD) case, there is a domain-average increase in transpiration of about 29% during dry months in the ROOT experiment. -Critically, the ROOT experiment performs best in simulating the temporal evolution of dry-season transpiration and evapotranspiration (ET), using a satellite-based ET product as an observational reference. Validation of modeled root depth is challenging due to a lack of in-situ observations. To address this, we describe a method for estimating root depth during drydown periods using observational products. These products include remotely sensed vegetation optical depth (VOD), precipitation, and enhanced vegetation index (EVI), as well as ET from a synthesized eddy covariance product. We compare rooting depth estimates from this method (ZVOD) with modeled rooting depth from the Noah-MP ROOT case (ZNMP) for 20-, 40-, and 60-day drydown events. We analyze results for the entire model domain as well as for two smaller subdomains: one with mostly forested grid points, and the other with mostly non-forested (savanna) grid points. We find that there is general agreement between the two methods for both subdomains. We also find the following: -Forest VOD and ET remain mostly constant throughout drydown events, while savanna VOD and ET decline. However, this decline is steeper in Noah-MP. -Modeled ET values are higher than shown by observational products. Median forest (savanna) ET is 2.5-3.0 (1.25-2.3) mm/day according to the observations-based method; according to Noah-MP it is 2.9-3.45 (1.0-3.0) mm/day. -Median ZVOD and ZNMP during drydown events differ slightly between the observations-based method and Noah-MP. Median ZVOD (ZNMP) is 10-15 m (10 m) for the forest subdomain and 1-3 m (5 m) for the savanna subdomain. -For the full domain, ZNMP is more influenced by water table variations, while ZVOD is more influenced by vegetation type. In addition to validation of the Noah-MP results, the observations-based method provides a measure of rooting depth that can be used when in-situ observations are not available. Future work should focus on further improvements to Noah-MP and application of the observations-based method to a larger domain. Moreover, it is important to explore effects of the DynaRoot uptake scheme on atmospheric variables in a coupled modeling framework.

Graduation Semester

2025-05

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/129445

Copyright and License Information

Copyright 2025 Carolina Bieri

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