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Title: Steady nonuniform shallow flow within emergent vegetation

Surface flow redistribution on flat ground from crusted bare soil to vegetated patches follow-ing intense rainfall events elevates plant available water above that provided by rainfall. The significance of this surface water redistribution to sustaining vegetation in arid and semiarid regions is undisputed. What is disputed is the quantity and spatial distribution of the redistributed water. In ecohydrological models, such nonuniform flows are described using the Saint-Venant equation (SVE) subject to a Manning roughness coefficient closure. To explore these assumptions in the most idealized setting, flume experiments were conducted using rigid cylinders representing rigid vegetation with varying density. Flow was induced along the streamwise x direction by adjusting the free water surface height H(x) between the upstream and down-stream boundaries mimicking the nonuniformity encountered in nature. In natural settings, such H(x) varia-tions arise due to contrasts in infiltration capacity and ponded depths during storms. The measured H(x) values in the flume were interpreted using the SVE augmented with progressively elaborate approximations to the roughness representation. The simplest approximation employs a friction factor derived from a drag coefficient (C d) for isolated cylinders in a locally (but not globally) uniform flow and upscaled using the rod density that was varied acrossmore » experiments. Comparison between measured and modeled H(x) suggested that such a ‘‘naive’’ approach overpredicts H(x). Blockage was then incorporated into the SVE model calculations but resulted in underestimation of H(x). Biases in modeled H(x) suggest that C d must be varying in x beyond what a local or bulk Reynolds number predicts. Inferred C d(x) from the flume experiments exhibited a near-parabolic shape most peaked in the densest canopy cases. The outcome of such C d(x) variations is then summarized in a bulk resistance formulation that may be beneficial to modeling runon-runoff processes on shallow slopes using SVE.« less
Authors:
 [1] ;  [2] ;  [3] ;  [4]
  1. Wuhan Univ. (China). State Key Lab. of Water Resources and Hydropower Engineering Science; Duke Univ., Durham, NC (United States). Nicholas School of the Environment
  2. Wuhan Univ. (China). State Key Lab. of Water Resources and Hydropower Engineering Science
  3. Univ. of California, Berkeley, CA (United States). Dept. of Civil and Environmental Engineering
  4. Duke Univ., Durham, NC (United States). Nicholas School of the Environment, Dept. of Civil and Environmental Engineering
Publication Date:
Grant/Contract Number:
SC0006967; SC0011461
Type:
Accepted Manuscript
Journal Name:
Water Resources Research
Additional Journal Information:
Journal Volume: 51; Journal Issue: 12; Journal ID: ISSN 0043-1397
Publisher:
American Geophysical Union (AGU)
Research Org:
Duke Univ., Durham, NC (United States)
Sponsoring Org:
USDOE Office of Science (SC), Biological and Environmental Research (BER) (SC-23)
Country of Publication:
United States
Language:
English
Subject:
60 APPLIED LIFE SCIENCES; 54 ENVIRONMENTAL SCIENCES
OSTI Identifier:
1469118
Alternate Identifier(s):
OSTI ID: 1402314

Wang, Wei-Jie, Huai, Wen-Xin, Thompson, Sally, and Katul, Gabriel G. Steady nonuniform shallow flow within emergent vegetation. United States: N. p., Web. doi:10.1002/2015WR017658.
Wang, Wei-Jie, Huai, Wen-Xin, Thompson, Sally, & Katul, Gabriel G. Steady nonuniform shallow flow within emergent vegetation. United States. doi:10.1002/2015WR017658.
Wang, Wei-Jie, Huai, Wen-Xin, Thompson, Sally, and Katul, Gabriel G. 2015. "Steady nonuniform shallow flow within emergent vegetation". United States. doi:10.1002/2015WR017658. https://www.osti.gov/servlets/purl/1469118.
@article{osti_1469118,
title = {Steady nonuniform shallow flow within emergent vegetation},
author = {Wang, Wei-Jie and Huai, Wen-Xin and Thompson, Sally and Katul, Gabriel G.},
abstractNote = {Surface flow redistribution on flat ground from crusted bare soil to vegetated patches follow-ing intense rainfall events elevates plant available water above that provided by rainfall. The significance of this surface water redistribution to sustaining vegetation in arid and semiarid regions is undisputed. What is disputed is the quantity and spatial distribution of the redistributed water. In ecohydrological models, such nonuniform flows are described using the Saint-Venant equation (SVE) subject to a Manning roughness coefficient closure. To explore these assumptions in the most idealized setting, flume experiments were conducted using rigid cylinders representing rigid vegetation with varying density. Flow was induced along the streamwise x direction by adjusting the free water surface height H(x) between the upstream and down-stream boundaries mimicking the nonuniformity encountered in nature. In natural settings, such H(x) varia-tions arise due to contrasts in infiltration capacity and ponded depths during storms. The measured H(x) values in the flume were interpreted using the SVE augmented with progressively elaborate approximations to the roughness representation. The simplest approximation employs a friction factor derived from a drag coefficient (Cd) for isolated cylinders in a locally (but not globally) uniform flow and upscaled using the rod density that was varied across experiments. Comparison between measured and modeled H(x) suggested that such a ‘‘naive’’ approach overpredicts H(x). Blockage was then incorporated into the SVE model calculations but resulted in underestimation of H(x). Biases in modeled H(x) suggest that Cd must be varying in x beyond what a local or bulk Reynolds number predicts. Inferred Cd(x) from the flume experiments exhibited a near-parabolic shape most peaked in the densest canopy cases. The outcome of such Cd(x) variations is then summarized in a bulk resistance formulation that may be beneficial to modeling runon-runoff processes on shallow slopes using SVE.},
doi = {10.1002/2015WR017658},
journal = {Water Resources Research},
number = 12,
volume = 51,
place = {United States},
year = {2015},
month = {12}
}